This Week in Chemistry – Mars’ Past Oxygen Atmosphere, and Hydrogen Fuel from Desalination

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This Week in Chemistry – Mars’ Past Oxygen Atmosphere, and Hydrogen Fuel from Desalination

Here’s the weekly summary of both new chemistry research and studies that have been in the news. This week features news on evidence for a past oxygen atmosphere on Mars, a new theoretical explanation for the density difference of water and ice, and more. As always, links to further articles and original research papers are provided below, as well as further studies of interest not included in the graphic.

Note: links to studies behind a journal paywall are indicated with (£). Studies without this symbol are open access, and can be accessed and read for free. 

Featured Stories

Manganese oxides on Mars hint at previous atmospheric oxygen: [Article] [Study]

Model hints at prebiotic chemistry on Saturn’s moon Titan: [Article] [Study (£)]

Using liquid metal wheels to drive miniature vehicles: [Article] [Study (£)]

New theoretical explanations for water and ice density difference: [Article] [Study]

Solar-powered desalination produces hydrogen fuel: [Article] [Study (£)]

Other Stories This Week

Formation of the browning pigment melanin decoded: [Article] [Study (£)]

Catalyst creates peroxide reagents from thin air: [Article] [Study (£)]

Encapsulated sunscreen sticks to the skin without being absorbed: [Article] [Study (£)]

Nanoparticles in dusty streets contribute to air pollution: [Article] [Study]

Prodding a molecule switches between its tautomers: [Article] [Study]

Engineered bacteria produce silver nanoparticles: [Article] [Study (£)]

The Chemistry of Fireworks

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The colours in fireworks stem from a wide variety of metal compounds – particularly metal salts. ‘Salt’ as a word conjures up images of the normal table salt you probably use every day; whilst this is one type of salt (sodium chloride), in chemistry ‘salt’ refers to any compound that contains metal and non-metal atoms ionically bonded together. So, how do these compounds give the huge range of colours, and what else is needed to produce fireworks?

The most important component of a firework is, of course, the gunpowder, or ‘black powder’ as it is also known. It was discovered by chance by Chinese alchemists, who were in actuality more concerned with discovering the elixir of life than blowing things up; they found that a combination of honey, sulfur and saltpetre (potassium nitrate) would suddenly erupt into flame upon heating.

The combination of sulfur and potassium nitrate was later joined by charcoal in the place of honey – the sulfur and charcoal act as fuels in the reaction, whilst the potassium nitrate works as an oxidising agent. Modern black powder has a saltpetre to charcoal to sulfur weight ratio of 75:15:10; this ratio has remained unchanged since around 1781.

The combustion of black powder doesn’t take place as a single reaction and so the products can be rather complicated. The closest thing to a representative equation for the process is shown below, with charcoal referred to by its empirical formula:

6 KNO₃ + C₇H₄O + 2 S → K₂CO₃ + K₂SO₄ + K₂S + 4 CO₂ + 2 CO + 2 H₂O + 3 N₂

Variation in pellet size of the gunpowder and the amount of moisture can be used to significantly increase the burning time for the purposes of pyrotechnics.

As well as gunpowder, fireworks will contain a ‘binder’ – used to hold the components together, and also to reduce the sensitivity to both shock and impact. Generally they will take the form of an organic compound, often dextrin, which can then act as a fuel after ignition. An oxidising agent is also necessary to produce the oxygen required to burn the mixture; these are usually nitrate, chlorates, or perchlorates.

The ‘stars’ contained within the rocket body contain the metal powders or salts that give the firework its colour. They will often be coated in gunpowder to aid in ignition. The heat given off by the combustion reaction causes electrons in the metal atoms to be excited to higher energy levels. These excited states are unstable, so the electron quickly returns to its original energy (or ground state), emitting excess energy as light. Different metals will have a different energy gap between their ground and excited states, leading to the emission of different colours. This is the exact same reason that different metals give different flame tests, allowing us to distinguish between them. The colours emitted by different metals are shown in the graphic at the top of the page.

It’s the metal atom present in the compound that’s important, then – but some compounds are better than others. Hygroscopic compounds (those that attract and hold water) aren’t much use in fireworks, as they can render the mixture damp and hard to burn. Some colours are also notoriously hard to produce. The copper containing compounds tend to be unstable at higher temperatures, and if it reaches these temperatures, it breaks apart, preventing the blue colouration from being exhibited. For this reason, it’s often said that you can judge the quality of a fireworks display on the quality of the blue fireworks! Purple is also quite hard to produce, as it involves the use of blue-causing compounds in combination with red-causing ones.

The Chemistry of Bell Peppers – Colour and Aroma

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The Chemistry of Bell Peppers – Colour and Aroma

Bell peppers come in a range of hues, from fresh greens to vibrant reds. Chemical pigments are behind these, but what changes to cause peppers to travel through this spectrum of colours? Here we take a look at the compounds behind the colours (as well as some pepper aroma chemistry) – and find that peppers have some extraordinary chemistry to thank for some of their hues.

Peppers start off green, which unsurprisingly is due to the presence of chlorophyll pigments. These are vital for photosynthesis in plants, and actually come in two subtly different forms, chlorophyll a and chlorophyll b. As the pepper ripens, these chlorophyll pigments start to decompose, and other types of pigments start to take their place.

All of the different colours of peppers that follow green are due to the presence of carotenoid pigments. Small amounts of these pigments are present even in green peppers, but their synthesis is increased as the pepper ripens. Yellow peppers owe their colouration primarily to violaxanthin, though a number of other yellow-orange pigments, including lutein and beta-carotene, also play their part. Lutein also contributes to the yellow colour of egg yolks, whereas beta-carotene is well known as the compound behind the bright orange colour of carrots.

The chemistry really starts getting interesting when we get to red peppers. The red colouration is due to the production of the carotenoids capsanthin and capsorubin, both of which are found near-exclusively in red peppers as most other plants are not able to synthesis them. Taking a closer look at what’s going on to create them at a chemical level makes it even more amazing that peppers are able to.

Synthesis of capsanthin and capsorubin, also known as paprika ketones, requires a type of chemical reaction known as a pinacol rearrangement. This reaction usually requires a concentrated acid catalyst, but peppers can carry it out in near-neutral conditions and at room temperature with the help of hitherto unidentified enzymes. That the humble bell pepper can do with these enzymes what chemists require specific lab conditions to achieve is pretty impressive.

Changes in colour aren’t the only effects of pepper ripening. It also affects the compounds that give peppers their aroma. In green peppers, a significant contributor to aroma is 2-methoxy-3-isobutylpyrazine, more commonly referred to as ‘bell pepper pyrazine’. This compound, as the name suggests, has an smell that is very characteristic of green peppers. It has an incredibly low odour threshold, meaning that its smell can be detected below the part per trillion level.

Other compounds are present too, and make minor contributions to the pepper aroma. These include ‘green’-smelling compounds such as cucumber-like and grass-like aldehydes. The concentration of most of these volatile compounds actually falls as the pepper ripens, with a few notable exceptions. The concentrations of both (E)-2-hexenal and (E)-2-hexanol, associated with sweet and fruity aromas, increase with ripening. This helps to explain the subtle difference between the aromas of peppers of different colours.

A New Approach to Chemical Synthesis

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A New Approach to Chemical Synthesis

MIT chemists have devised a new way to synthesize a complex molecular structure that is shared by a group of fungal compounds with potential as anticancer agents. Known as communesins, these compounds have shown particular promise against leukemia cells but may be able to kill other cancer cells as well.

The new synthesis strategy, described in the Journal of the American Chemical Society, should enable researchers to generate large enough quantities of these compounds to run more tests of their anticancer activity. It should also allow scientists to produce designed variants of the naturally occurring communesins, which may be even more potent.

“This is just the foundation,” says Mohammad Movassaghi, an MIT professor of chemistry and the paper’s senior author. “We’ve laid the foundation for implementation of this strategy to access other variations, both natural and nonnatural.”

Communesins are a unique family of polycyclic and complex naturally occurring alkaloids. One of the major hurdles to synthesizing communesins in the lab using this new strategy is a chemical reaction in which two large, bulky molecules must be joined together in a step known as heterodimerization.

Movassaghi’s lab, which has been working on this type of synthesis for several years, was inspired by the way related compounds are produced in nature. The details of the natural synthesis are not fully known, but it is believed that it also involves a heterodimerization step. In fungi, there is evidence that an enzyme catalyzes this reaction.

Without an enzyme, the heterodimerization required to produce communesins is difficult to carry out because it requires forming a bond between two carbon atoms that are each already bonded to four other atoms, some of which have additional bulky groups attached to them. This makes it challenging to bring the two molecules close enough for them to fuse together.

To overcome this, Movassaghi’s lab developed an approach in which they transform the two carbon atoms into carbon radicals (carbon atoms with one unpaired electron). To create these radicals, the researchers first attach each of the targeted carbon atoms to a nitrogen atom, and these two nitrogen atoms bind to each other.

When the researchers shine certain wavelengths of light on the reactants, it causes the two atoms of nitrogen to break away as nitrogen gas, leaving behind two very reactive carbon radicals that join together almost immediately.

“If you break the carbon-nitrogen bond, the intermediate has a very short lifetime. We predict it to be roughly on the order of picoseconds,” Movassaghi says. “Dinitrogen pops out and now you have two radicals in very close proximity.”

Once the heterodimer is formed, three more chemical steps are required, including the transfer of a nitrogen-containing chemical group from one carbon atom to another.

“Just heterodimerizing is only half the battle,” Movassaghi says. “There were two major challenges in this successful synthesis. One was how do you get to a heterodimer, and once you fuse the two halves together, how do you guide the rearrangement to match the structure that you find in nature?”

In this study, the MIT team prepared a key precursor that was converted to the compound known as communesin F in only five steps. The critical heterodimer rearrangement step proceeded to yield 82 percent of the desired heptacyclic communesin structure.

Scott Miller, a professor of chemistry at Yale University, describes the new approach as “a masterful synthesis.”

“The strategy is incredibly ambitious and reflects a sophisticated assessment of the plausible biosynthetic precursor. This is really very clever, since these pathways are typically not known at the level of complete understanding, so outstanding intuition and creativity are required,” says Miller, who was not involved in the research.

This strategy can also be used to produce related communesins, including variants not found in nature.

“Nature has likely evolved these compounds for chemical defense or signaling between different organisms, but if we’re thinking about their potential for treatment of human disease, we may need to access nonnatural derivatives,” Movassaghi says. “Our ability to go in with pinpoint accuracy and make structural variations to these complex alkaloids is going to be helpful in enabling the thorough evaluation of these compounds and related derivatives.”

The study was conducted by graduate student Matthew Pompeo and former postdocs Stephen Lathrop and Wen-Tau Chang. The project was funded by the National Institutes of Health and the National Science Foundation.

Drug Candidate Shrinks Tumor When Delivered by Plant Virus Nanoparticle

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Drug Candidate Shrinks Tumor When Delivered by Plant Virus Nanoparticle

Source: Case Western Reserve University News Release

In a pair of firsts, researchers at Case Western Reserve University and Massachusetts Institute of Technology have shown that the drug candidate phenanthriplatin can be more effective than an approved drug in vivo, and that a plant-virus-based carrier successfully delivers a drug in vivo.

Triple-negative breast cancer tumors of mice treated with the phenanthriplatin -carrying nanoparticles were four times smaller than those treated either with cisplatin, a common and related chemotherapy drug, or free phenanthriplatin injected intravenously into circulation.

The scientists believe the work, reported in the journal ACS Nano, is a promising step toward clinical trials.

"We may have found the perfect carrier for this particular drug candidate," said Nicole Steinmetz, an assistant professor of biomedical engineering at Case Western Reserve, who has spent 10 years studying the use of plant viruses for medical purposes.

She teamed with Stephen J. Lippard, Arthur Amos Noyes Professor of chemistry at MIT, and an expert in biological interactions involving platinum-based chemotherapies.

Platinum-based drugs are used to treat more than half of cancer patients receiving chemotherapy. Two of the most commonly used drugs are cisplatin and carboplatin. They form bifunctional cross-links with DNA in cancer cells, which block the DNA from transcribing genes and result in cell death, Lippard explained.

Despite widespread use, cisplatin has been shown to cure only testicular cancer, and many cancers have or develop immunity to the drug.

Lippard's lab altered cisplatin by replacing a chloride ion with phenanthridine and found that the new molecule also binds to DNA. Instead of forming cross-links, however, phenanthriplatin binds to a single site but still blocks transcription.

In fact, his lab found that phenanthriplatin is up to 40 times more potent than traditional platins when tested directly against cancer cells of lung, breast, bone and other tissues. The molecule also appears to avoid defense mechanisms that convey resistance.

But when injected into mouse models of cancer, the drug candidate performed no better than standard platins.

Lippard realized phenanthriplatin wasn't reaching its target. He had a drug delivery problem.

He found a potential solution while visiting Case Western Reserve's campus and heard Steinmetz explain her work investigating tobacco mosaic virus (TMV) for drug delivery more than a year ago.

"I envisioned that TMV would be the perfect vehicle," Lippard said. "So we had a beer and formed a collaboration."

The long, thin tobacco mosaic virus nanoparticles are naturals for delivering the drug candidate into tumors, said Steinmetz, who was appointed by the Case Western Reserve School of Medicine.

The virus particles, which won't infect humans, are hollow. A central tube about 4 nanometers in diameter runs the length of the shell and the lining carries a negative charge.

Phenanthriplatin is about 1 nanometer across and, when treated with silver nitrate, has a strong positive charge. It readily enters and binds to the central lining.

The elongated shape of the nanoparticle causes it to tumble along the margins of blood vessels, remain unnoticed by immune cells and pass through the leaky vasculature of tumors and accumulate inside. Little healthy tissue is exposed to the toxic drug.

Inside tumors, the nanoparticles gather inside the lysosomal compartments of cancer cells, where they are, in essence, digested. The pH is much lower than in the circulating blood, Steinmetz explained. The shell deteriorates and releases phenanthriplatin.

The shell is broken down into proteins and cleared through metabolic or natural cellular processes within a day while the drug candidate starts blocking transcription, leading to greater amounts of cell death through apoptosis than cross-linking platins.

The researchers say delivery of the phenanthriplatin into the tumor led to its improved performance over cisplatin or free phenanthriplatin.

Lippard and Steinmetz continue to collaborate, investigating use of this system to deliver other drugs or drug candidates, use in other types of cancers, the addition of agents on the exterior of the shell to increase accumulation inside tumors and more.

Ocean Acidification and Chemical Signalling

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Ocean Acidification and Chemical Signalling

Ocean acidification is one of the concerns surrounding the increasing amounts of carbon dioxide in the atmosphere. Some of this carbon dioxide can dissolve in seawater, and when it does so, it increases the seawater’s acidity over long time scales. University of Hull chemists found that this increase in acidity could have effects on molecules used by marine organisms as chemical ‘cues’ for, amongst other things, egg ventilation, hatching, and settlement.

The Chemistry of Maple Syrup

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The Chemistry of Maple Syrup

Here’s a brief look at some of the different chemicals in maple syrup.

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Aromatic Chemistry Reactions Map

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Aromatic Chemistry Reactions Map

 Here’s a reaction map looking at reactions that allow you to vary the substituents on a benzene ring. This was a far larger undertaking than expected; the bulk of the work on the organic reaction map was done in the space of a day, whereas this one is probably pushing towards three days – suffice to say that there were a lot of reactions that could’ve been included!

As far as the content goes, this map (hopefully) details the majority of basic reactions of aromatic compounds. It certainly includes all the reactions necessary for most A Level courses, and more besides. The many possible reactions, and the fact that I really don’t like arrows crossing over in a tangled mess if I can help it, means that it’s by no means exhaustive, but it should be conclusive enough for casual reference purposes.

If you’re also a chemist and can spot something horribly wrong, then please let me know – similarly if you have any suggestions for something you think should have been included but hasn’t.

This Week in Chemistry – Espresso Machine Extraction & Chiral Molecules in Space

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This Week in Chemistry – Espresso Machine Extraction & Chiral Molecules in Space

Here’s the weekly summary of both new chemistry research and studies that have been in the news. This week features news on the first discovery of a chiral compound in space, how to improve the flavour of your morning coffee, and more. As always, links to further articles and original research papers are provided below, as well as further studies of interest not included in the graphic

The Chemistry of the Euro 2016 Football

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The Chemistry of the Euro 2016 Football

This year’s European Championships are now well under way in France, and there’s already been some great football on show. It might surprise you to learn that some chemistry has also been taking centre-stage! The Euro 2016 ball is a triumph of materials science, and in this post we take a look at some of the chemicals that make it up.

A number of chemical materials are used in the manufacture of the Euro 2016 football. The majority of these materials are polymers; these are very long molecules built up from many smaller component molecules. A simple, everyday example is polyethene, used to make some plastic bags. Different classes of polymers are used to achieve particular properties for the ball.

Footballs consist of three main component parts: the covering (the outermost layer), the lining, and the bladder. Of course, these will be designed in a manner that provides the most favourable aerodynamic properties for the ball – however, that’s veering dangerously into physics territory. None of these properties would be achievable without chemistry providing the materials required, so here’s a breakdown of the different types of polymers used in each component part of the ball.

Covering

The covering of the ball is made of six polyurethane panels, which are thermally bonded together. This covering is important to protect the ball, and to prevent it from absorbing too much water – the water absorption of the ball has been improved from the World Cup ball of two years ago, which had a water absorption of just 0.2%. This makes the ball much lighter than the leather-coated balls used in the past. Some balls may also have a polyurethane foam layer underneath the covering.

Polyurethanes are built up from compounds called isocyantes and polyols. The middle parts of these molecules can be varied to give different polyurethanes with differing properties. Polyurethanes have a wide range of applications, including foam in seating, adhesives, synthetic fibres and even skateboard wheels.

Cheaper footballs may use PVC (polyvinyl chloride) instead of polyurethane for the coating. They may also be stitched together, rather than thermally bonded. This stitching will be made from another class of polymers called polyesters; on higher end balls this stitching may be reinforced with Kevlar.

Lining

Underneath the covering layer, the ball will have several layers of lining. These are present to improve the bounce and strength of the ball. In the Euro 2016 ball, these are made from another class of polymers, polyamides, more commonly referred to as nylon. Polyesters can also be utilised for this purpose. 

Nylon and polyesters are also commonly used components in the manufacture of football shirts, as well as other clothing. Nylon is additionally used in parachutes, ropes and fishing nets, whilst polyesters can be found in bed sheets, carpets and plastic bottles. 

Bladder

The bladder is the part of the football that holds the air. In the Euro 2016 ball, this is made from butyl rubber, but it can also be made from latex. Both have their benefits: butyl rubber retains the air for a longer period of time, whilst latex provides better surface tension. Butyl rubber can also be found in the valve through which air can be pumped into the ball, where it aids air retention. Silicone valves can also be used.

Most modern chewing gum also uses food grade butyl rubber to give the gum its elasticity. Unfortunately, it also contributes the unwanted stickiness of gum. It can also be found in the inner tubing of tyres.

This is just a peek into the world of polymers – any plastics you use on a day-to-day basis are composed from polymers, as well as your clothing, and many other everyday items. Without synthetic polymers, Euro 2016 would be kicking off today with a much more rudimentary ball!

The Chemistry of Foxgloves – Poison & Medicine

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The Chemistry of Foxgloves – Poison & Medicine

The vibrancy of foxgloves belies their poisonous nature – ingesting even a small amount of the plant can cause unpleasant effects, and in some cases death. However, the same compounds that make it poisonous can also have medicinal uses. This graphic takes a look at them in detail.

The mantra ‘the dose makes the poison’ is oft-repeated in the field of toxicology; the foxglove perhaps provides one of the best examples of how true this is. The compounds in foxglove that lend it both its toxicity and medicinal use are called cardiac glycosides. A glycoside is a molecule which contains a steroid portion bonded to a sugar portion. The glycosides in foxgloves are found in higher concentrations in the leaves, but they’re still found in all other parts of the plant as well.

Ingestion of a small amount of parts of a foxglove can cause symptoms including nausea, vomiting, and diarrhoea. Though it may seem like an unlikely turn of events, the leaves of foxgloves can easily be confused with other edible plants – there’s a case of a man mistaking the leaves for that of another plant, and brewing a herbal tea from them. Larger amounts can result in death; although cases of this are rare, they have been documented.

Despite its toxicity, foxglove has actually been used in medicine for a number of centuries. Back then, it was used as a treatment for ‘dropsy’, what we now recognise as edema, an excess of fluid collecting in the tissues of various parts of the body. We now also know that this condition is often a side-effect of heart problems.

The use of foxglove for treating dropsy was trialled by an English doctor, William Withering. In the course of recording and eventually publishing his findings he found that foxglove extract was an effective method for treating dropsy and heart failure – though he also discovered it could have unpleasant effects if given in too high a dose. One of the more curious of these is a yellowing of the vision.

The foxglove extract, the key constituents of which are the cardiac glycosides digoxin and digitoxin, is known as digitalis after the Latin name for the plant. After Withering’s work, it became a common treatment for heart issues, including heart failure. Unusually for a drug that has persisted from antiquity to the present day, digoxin is still extracted from foxgloves, as it’s difficult for chemists to synthesise it in a cost-effective and efficient manner.

So how does digoxin exert its beneficial effects, and why is the line between its ability to heal and harm so fine? To answer this, we need more insight into how it affects the body. Though its exact mechanism of action is still unclear, it’s thought that it affects the sodium-potassium ion pumps in heart cells. These usually remove sodium ions from the cells. Digoxin stops sodium being removed from the cells, which has a knock on effect of causing the concentration of calcium ions in the cells to rise. This, in turn, interferes with the electrical signals that keep the heart beating, causing its pumping to become more forceful but slower.

This ability to interfere with and slow the heart rate makes digoxin useful for treating heart arrhythmias. It can still be used to treat heart failure, though its use for this has declined with the advent of other drugs. Its therapeutic range (the range in which it exerts beneficial medicinal effects) is very close to its toxic level – the point at which unpleasant effects start to be seen. In excess, it slows the heart rate so much that the brain becomes starved of oxygen; the body’s reflex response is to try and increase the heart rate, and this eventually results in a heart attack.

Finally, there are, of course, plenty of cases of people using foxglove and digitalis for more nefarious means. ‘Nature’s Poisons’ cites the case of a German doctor who murdered his girlfriend by administering the poison to his girlfriend rectally (!). Charles Cullen, New Jersey’s most prolific serial killer, also used digoxin to kill a number of his victims.

(It goes without saying, but do be wary of foxgloves, particularly if you’re into foraging. You need to ingest very little to experience toxic effects; if you notice symptoms, or think you or someone you know might have accidentally ingested some part of a foxglove, seek medical attention immediately.)

The Chemistry of Rhubarb

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The Chemistry of Rhubarb

Field-grown rhubarb will shortly be coming into season and appearing in supermarkets in the UK, so it seems like a good time to take a look at the chemistry behind this odd-looking vegetable. It’s mostly used in pies and desserts, but it’s only the stalks of the plant that we eat – and there’s a reason for that. This graphic takes a look at why, and also looks at the chemical compounds that contribute to the colour and the laxative effect of rhubarb.

Firstly, let’s consider the poisonous nature of rhubarb leaves. It’s generally thought that this is due to the presence of a chemical compound called oxalic acid. This compound doesn’t just occur in rhubarb – it also occurs in lower amounts in spinach, broccoli, cauliflower, and brussels sprouts. Obviously, we eat those pretty regularly just fine; in vindication of the old adage ‘the dose makes the poison’, it’s the higher concentration in rhubarb leaves that poses the problem.

Rhubarb leaves have a comparatively high oxalic acid content of around 0.5 grams per 100 grams of leaves. This is present in the form of oxalic acid, and also in the form of calcium and potassium oxalate salts, and is at a level much higher than that found in other portions of the plant such as the stem. The suggested lethal dose of oxalic acid is in the region of 15-30 grams, meaning you’d have to eat a fair few kilograms of the leaves to reach this dose, but lower doses can still cause nausea and vomiting.

This was discovered to the detriment of the British government in World War I, when, due to food shortages, they advocated eating rhubarb leaves. Of course, this led to cases of poisoning, and at least one death is reported in the literature. The only other study on a death due to oxalic acid poisoning was back in 1960, so the specifics of the mechanism of the poisoning are a little hazy. However, it’s known that, in the body, oxalic acid binds to calcium ions, producing calcium oxalate. Calcium oxalate is insoluble, and as such accrues in the kidneys as kidney stones.

It’s not quite as clear as oxalic acid or oxalates being the culprit, though. Some critics have pointed out that no traces of oxalates were found in post-mortem examinations of those who supposedly died from poisoning after eating rhubarb leaves, and it’s also been suggested that there may be another, as yet unidentified chemical component in the leaves of rhubarb which contributes to their toxicity. Compounds known as anthraquinone glycosides have been suggested as potential candidates, but as yet no specific compound has been identified.

This leads nicely on to a discussion of some of the other compounds found in rhubarb stems, which include anthraquinones. They’re contributors to the colour of rhubarb, although not major contributors – that part is played by compounds called anthocyanins, common causes of colour in plants. The major anthocyanin in rhubarb is cyanidin-3-glucoside. A range of anthraquinones are also present, including emodin (orange), chrysophanol (yellow), physcion (red-orange), and rhein (red). Besides their colour contribution, these compounds and their derivatives also give rhubarb a laxative effect.

The compounds of interest as far as these effects go are the sennosides, derivatives of anthraquinones. During digestion, these compounds are hydrolysed into a number of smaller molecules, including rheinanthrone. It’s rheinanthrone that is thought to be the primary compound behind rhubarb’s laxative effect. Sennosides are also found in the senna plant (hence the name), and are commonly used in laxative medications. They’re included in the World Health Organisation’s list of the essential medicines.

Compounds from rhubarb have also been examined for other potential medical uses. In particular, the anthroquinones have been researched as potential anticancer compounds, with both emodin and aloe-emodin having been shown to exhibit anti-tumour properties.

The Chemistry of Blood

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The Chemistry of Blood

Fake blood is an essential accessory if you’re going for a gory halloween look with your costume this year. There’s a lot of chemistry in the substance it sets out to mimic; we can explain the colour, smell, and different types of blood with the aid of chemistry (and some biology thrown in for good measure).Today’s graphic takes a look at each in turn.

The chemistry behind blood’s colour is perhaps that we’re all most familiar with. Most of us are aware that blood contains haemoglobin; this is the protein, found within red blood cells, that enables our blood to carry oxygen to our cells. It also helps carry some carbon dioxide back to the lungs, in the form of carbaminohaemoglobin, though the majority of carbon dioxide is carried in the blood as bicarbonate ions.

The red colouration of blood is due to the sub-units of the haemoglobin protein. Each of the four sub-unit consists of a protein chain which is bound to a haem group. It is these haem groups, which contain bound iron atoms, that cause blood’s dark red colour. Their structure of alternating double and single bonds absorbs light of particular wavelengths, causing us to see it as red. Red isn’t actually the only possible blood colour; some animals can have green, blue, or even purple blood, due the the use of different oxygen-carrying proteins.

A common misconception about our blood is that deoxygenated blood (that which flows back from our cells through out veins) is blue. Veins do appear blue when we look at them through our skin, so it’s perfectly understandable that a lot of people think that this is the case; also, pick up any biology textbook, and the likelihood is that in a diagram depicting blood vessels the veins will be denoted with a blue colouration.

Whereas oxygenated haemoglobin is a bright red, deoxygenated haemoglobin is a darker red colour – but not blue! The reason that blood appears blue when we look at our veins through our skin is due to the interaction of light with both our blood and the skin covering the blood vessels. Red light can penetrate more deeply into our tissues than blue light, and since deoxygenated blood absorbs more red light than oxygenated blood, our veins tend to look blue as a result.

Haemoglobin can also help us explain the colour change we see in blood when it’s removed from the body. If you’ve ever had a nose bleed, you’ll have probably noticed that any blood you stem with a tissue turns a dark brown colour as it dries. This is due to the oxidation of the iron atoms in the haemoglobin subunits, from iron (II) to iron (III), producing methaemoglobin which is a dark brown colour.

If you’ve ever, say, accidentally bitten your tongue, you’ll have also noticed that blood has a somewhat metallic taste. This is in part due to the presence of the iron in haemoglobin; it can also react with fat molecules to produce a range of compounds that help to produce a metallic flavour.

The compounds created include oct-1-en-3-one, which is described as having a mushroom-like, metallic odour. This is also the compound behind the metallic smell you can detect on your skin after touching metal objects – so in these cases it’s not the metal you’re smelling, but chemical breakdown products of molecules in your own skin.

Blood itself smells metallic all on its own. Researchers have determined that a particular compound in blood that contributes this faint metallic odour, trans-4,5-epoxy-(E)-2-decenal, is also an important compound detected by predators. A study last year identified the compound, then ran a number of tests with different predators where they soaked logs in the compound, as well as soaking other logs in actual blood, fruit-essence, and an almost odourless compound. They found that the predators were attracted to the log soaked intrans-4,5-epoxy-(E)-2-decenal as much as that soaked in actual blood.

Though all of our blood is coloured by haemoglobin, and blood from different people will produce the same metallic smell, there are still differences in blood from one person to the next. We commonly refer to these differences as blood types. There are actually lots of different blood types (35 are recognised by the International Society of Blood Transfusion) but there are essentially two classifications that we usually refer to.

The first of these classifications is the ABO system. A person can have type A, type B, type AB or type O blood. This classification is determined by the presence of antigens, which are structures found on the surface of red blood cells. They are either sugars or proteins, and the types of antigens present in a person’s blood determines their blood type.

Type A blood has A antigens on the red blood cells; type B blood has B antigens. Type AB blood has both A and B antigens, whereas type O blood has neither. Our own blood antigens are ignored by our own immune system; however, if, during a transfusion, we receive blood containing an antigen not found in our own blood, it can trigger an adverse immune reaction.

Our blood also contains antibodies; these are proteins in blood plasma that help to fight infection. In most transfusions, it is only the red blood cells that are transferred from the donor’s blood to the recipient. If these red blood cells have antigens that match to an antibody in the recipient’s blood, the antigens bind to the antibodies, and the adverse reaction is set into motion. This is why people with some blood types can only receive blood from certain other blood types.

O group blood is known as the universal donor, because, since its red blood cells don’t contain A or B antigens, it can be safely given to recipients with any blood type. Similarly, group AB blood is known as the universal acceptor, because it doesn’t contain A or B antibodies, so no reaction will be triggered even if blood with A or B antigens is given.

Blood type can also be termed as positive (e.g. A+) or negative (A-). This refers to the presence or absence of Rh antigens on the red blood cells, and is also something that must be considered during blood transfusions. Rh positive blood cannot be given to Rh negative recipients, as the recipient can develop Rh antibodies which can then attack the donated blood. People with Rh positive blood can receive blood from either Rh positive or Rh negative donors.

This is all a little bit confusing to get your head around, so here’s a handy diagram which shows which blood types can be given to patients with a particular blood type.

To conclude (and point out the obvious), of course the fake blood we’ll all be using for Halloween chemically doesn’t have a whole lot in common with actual blood. The red colouration usually comes from red food colouring, and some gloopiness is added via the addition of some form of sugar syrup. There are also numerous suggested recipes for making your own online which use cornflour to thicken it up and give it a more congealed look.