Eat up your crusts !

Eat up your crusts!

Me and my wife were going to cook a nice joint of roast beef and show you how to do this roast dinner but sadly I overcooked the joint a little. So as a result I am going to tell you something about the chemistry of how meat cooks. During cooking a complex set of chemical reactions alter the food creating a complex palette of new chemicals which contribute to the sensual experience of eating the food.

One of the important reactions which occurs during cooking is the Maillard reaction; this is a reaction which involves carbohydrates and the amino acids from the protein in meat. Firstly the aldehyde form of a sugar such as glucose reacts with the amino group of an amino acid to form a thing called an imine. This imine then can undergo a series of reactions; please do not be afraid if it looks complex.

Figure 1. Formation of the imine followed by the Amadori reaction.

The reactions forming the imine are closely related to what we teach first years in Chemical and Biological Engineering. I hold the view that most organic chemistry can be understood after a relatively small number of reactions are understood, what a person needs to do is to understand the organic reactions, recognise them when they attempt to go incognito and understand how to combine them into sequences rather than just memorizing thousands of examples of reactions.

My advice right now to anyone about to start the organic part of the first year course and older students who are nervous of organic chemistry is “Don’t Panic!”. When you see some scary looking organic chemistry it is important to take a deep breath, relax for a moment and see if it is some variation on an existing reaction you have seen.

The ketosamine then can do a wide range of things, for instance it can form on acid treatment 5-hydroxymethylfuran-2-carboxaldehyde (HMF) with great ease, the HMF is an important substance which can react further. It is also important for bee keepers to ensure that the sugar they feed to their bees has a very low level of furans, this is because the furans are very toxic to bees. Here is the reaction scheme showing how HMF forms under acidic conditions from the ketosamine.

Figure 2. Formation of hydroxymethyl furan aldehyde from an Amadori product

Now you might ask why I am telling you about the Maillard chemistry so close to Christmas, the reason is that many of my favourite English Christmas foods are the way that they are because of the Maillard chemistry, for example a Christmas pudding is cooked with rasins. A paper by F. Karadeniz, R.W. Durst and R.E. Wrolstad, Journal of Agricultural and Food Chemistry, 2000, 48, 5343 explains how Maillard chemistry occurs during the sun drying of grapes into raisins. On a more everyday note I am about to do something which will make British children hate me, I am about to become worse than the Child Catcher in Chitty Chitty Bang Bang (Most British children hate eating their crusts). Here is a slice of bread.

Figure 3. A slice of Bread

I have a new reason for telling children to eat up their crusts! Because of the Maillard chemistry the crust on bread contains a chemical (Pronyl-L-lysine) which protects your body against cancer. For details see M. Lindenmeier and T. Hofmann, Journal of Agricultural and Food Chemistry, 2004, 52, 350. This paper was interesting to me, while it is known that Maillard chemistry can form some carcinogens in food, here was a substance which protects the lower end of the digestive system from cancer. An amino acid named lysine has a primary amine in the side group, this can react with the carbohydrates to form an exotic amino acid named pronyl-lysine which is able to react with carcinogenic hydrazines to form a very stable looking pyrazole which should inactivate the hydrazines.

Figure 4. The hydrazine catching reaction of the pronyl-lystine

I suspect thatthe chemistry is rather similar to that which I used years ago in Aberdeen when I used to make to make pyrazoles; I used to add with care hydrazine to solutions of 1,3-diketones in alcohol. I say with care are it was always a very exothermic reaction which sometimes made my alcohol reach boiling point. Here is one of the beauties of organic chemistry; the same reactions appear again and again but in different contexts. We can work backwards in our brains from the pronyl group to find that we need the sugar based fragment known as 3,4-dihydroxyhex-e-ene-2,5-dione (Acetylformoin). Below I have shown how it can form from glucose.

Figure 5. The formation of acetylformoin from the Amadori product

What happens next is that it reacts with the dangling amino group of the lysine to form a five membered heterocycle. What can happen is that the open chain form of acetylformoin can react with the amino group to form an imine. This imine can then close the ring to form the wonderful pronyl group.

Now I hope that some of you have started to think a little more about organic chemistry, I would like to stress that all the chemistry I have written about can be explained with first and second year courses taught at Chalmers. While thinking about the chemistry of my Christmas dinner I have introduced to you some carbonyl chemistry and some heterocyclic chemistry, I hope I have given you a taste or flavour of what organic chemistry is about. After having done all this organic chemistry I have worked up quite an appetite which brings me onto the subject of what I have for you. The movement of electricity in electrical circuits canbe described mathematically using the laws of physics. In organic chemistry we have electrons whose movement can be explained with the curved arrow. This mechanistic tool helps us to make sense of the vast world of organic chemistry; I want to set before you a banquet of organic chemistry.

Crystals and the home made nuclear reactor

Dear Reader,

I feel that nature has not taken its course yet with the Swedish home made nuclear reactor but it is high time I wrote again on the subject of crystals. So lets do both at once !

I commented on how I thought that it was a bad idea to try to use sulphuric acid to dissolve up the radium which is in solid form. I suspect that the radium in a radioactive source or on the surface of ye olde glow in the dark clock will be as the insoluble radium sulphate.

Much of radium chemistry is shrouded in darkness when compared with other metals, for example only four crystal structures have ever been published which contain radium. One of the key gaps in our knowledge is radium sulphate; we will assume for a moment that radium sulphate is isostructural to barium sulphate. The word isostructural is a big technical sounding word which means that the basic structure is the same, but the exact distances between the atoms in the unit cell might differ.

For example calcium fluoride and uranium dioxide are isostructural, the fractional coordinates of the uranium / oxygen atoms match those for calcium and fluorine atoms. But the size of the cubic unit cells are different. But lets get back to our barium and radium chemistry.

I think that the radium will have a lower solubility in sulphuric acid than it will in tap water. Tap water is normally low in sulphates; this lack of sulphate will become clear in a moment.

For many poorly soluble metal salts a thing called a solubility product is known.

This is often written as Ksp.

For barium sulphate Ksp = [Ba²⁺][SO₄^2-]

[Ba²⁺] and [SO₄^2-] are the concentrations of barium and sulphate in the solution.

Now those of you who paid attention in your GCSE maths lessons should understand that when barium sulphate is placed in pure water and stirred (until it reaches equilibrium) that

[Ba²⁺] = (Ksp)^½

But when the barium sulphate is placed in 0.01 sulphuric acid, then the concentration of the barium will be given by a new equation.

[Ba²⁺] = Ksp / [SO₄^2-]

It should be clear to you that by increasing the sulphate concentration that the equilibrium concentration of the barium will go down. It is very likely that the radium will behave the same way as the barium; Marie Curie isolated the radium from uranium ore together with the barium fraction. As I said yesterday for public safety reasons I will not tell you how she converted the barium / radium fraction into a water soluble form. If you are keen to know, please do not ask me about that chemical step as refusal often offends! If you want to know about other bits of chemistry then feel free to ask.

But now we have thought about solubilities lets look at the solid.

The unit cell of barium sulphate is 8.884 by 5.458 by 7.153 Å and it has atoms with the following fractional coordinates.

Ba	0.1846	0.2500	0.1581
Ba	0.6846	0.2500	0.3419
Ba	0.3154	0.7500	0.6581
Ba	0.8154	0.7500	0.8419
S	0.0630	0.2500	0.6914
S	0.5630	0.2500	0.8086
S	0.4370	0.7500	0.1914
S	0.9370	0.7500	0.3086
O	0.0814	0.0298	0.8190
O	0.1808	0.2500	0.5515
O	0.0814	0.4702	0.8190
O	0.9122	0.2500	0.6062
O	0.4122	0.2500	0.8938
O	0.5814	0.4702	0.6810
O	0.5814	0.0298	0.6810
O	0.6808	0.2500	0.9485
O	0.9186	0.9702	0.1810
O	0.8192	0.7500	0.4485
O	0.9186	0.5298	0.1810
O	0.0878	0.7500	0.3938
O	0.5878	0.75	0.1062
O	0.4186	0.5298	0.319
O	0.4186	0.9702	0.319
O	0.3192	0.75	0.0515

If you build a unit cell with these atoms then I think you need a prize from your teacher! I am not sure how it will apply to those of us who either left school twenty years ago or used a copy of ORTEP or some other computational aid.

For those of you who are not motivated to draw or build a unit cell here is a unit cell for BaSO₄.

A unit cell of barium sulphate, barium is in green, sulphur in orange and oxygen in red

Now the unit cell for strontium sulphate is a 8.377 by 5.350 by 6.873 Å box, all the atoms have the same fractional coordinates except the bariums are now strontiums. I suspect that radium sulphate has the same structure as barium sulphate and that the cell will be slightly bigger than that of barium sulphate. The fluorides of calcium, strontium, barium and radium all have the same fluorite structure, but the unit cells differ in size. Here is a table of the lengths of the sides of the unit cells of the fluorides.

Element	Length of unit cell (Å)
Ca	5.450
Sr	5.800
Ba	6.196
Ra	6.381

Sadly magnesium and beryllium has a different structure so we can not compare it to these other alkaline earth fluorides. Well I suspect that I have given you something to think about for a while.

The most dangerous words in science and modern life

Dear Reader, I feel that I have a moral duty to warn you about a widespread and nasty threat to your wellbeing.

It is those things which “everyone knows”, when I hear the phrase “everyone knows” I often cringe. When you think about it at length many times that “everyone knows” is used close to

1. A common fallacy

2. Something which is well-known to some people, who then assume all others will know about it.

In good time I will address both of these problems.

Chairs

Dear Reader,

Here is something interesting, it is a pair of chairs which change colour. The artist does not say how he did it but I think one possible way would be to use an electrochromic dye such as polyaniline.

http://www.tofsen.se/articles/34/two-chairs-changing-appearance-in-9-was-6-if