Dr Samuel Furse » amphiphilic

 How Hot is Hot? A Burning Question About a Hot Condiment Thursday, Jan 19 2012 

Plenty of people like a good hot curry. I am not one of them, but I think that most people have met, or know, someone who likes wolfing down the hottest curry in the house as a matter of pride. I do know someone who likes to munch on the same kind of hot curry but exquisitely slowly. Either way, it is safe to assume that pretty much anyone who has had a strong curry, and either enjoyed it or not, will remember the flavour forever.

There has even been a certain amount of study on this topic: some time ago, the compound capsaicin (Figure 1) was identified as the cause of the hotness. Several related compounds have also been identified, some of which are ‘hotter’ than others. This led to the desire to measure the ‘hotness’, resulting in the Scoville Heat Unit, and the Scoville Scale.

There is also the well-known opportunity for a schadenfreude with curry flavours. As it can be a strong flavour, when someone bites on something unexpectedly teeming with chopped jalepeños, the shock on their face is palpable. However, this shock can also be turned on its head with respect to public order. Recent anti-capitalist protests in America have given rise to some disturbing images of people sprayed with a capsaicin formulation (commonly known as pepper spray) either intentionally, or apparently not.

While these are shocking, and the mental and physical distress caused by the use of this ‘riot-control agent’ are readily understood, other factors are also important. The use of pepper spray as a weapon of self-defence, against a rapist or criminally violent attacker for example, seems not unreasonable. However, the link between pepper spray and deaths in people exposed to it who also have compromised respiratory function, increases the interest in managing the use of pepper spray, both politically and scientifically.

Figure 1. The structure of capsaicin, showing a more polar section (left) and a more lipophilic one (right), giving rise to a comparison with lipid structures.

One way of taking things further is to understand the science behind what is happening when pepper spray is used. A judgement can then be made about safety and appropriate conditions for use. The structure of capsaicin (Figure 1) suggests that it has a lot in common with what we know about lipid structure – a relatively polar (hydrophilic) section as one end, and a lipophilic hydrocarbon chain as the other. However, it is not just the lipid-like properties of capsaicin and its related compounds that give rise to the effect we remember so readily – after all, we eat lipids of one sort or another in almost every mouthful and most do not have the same effect as a vindaloo on our taste buds. This hot sensation is due to an effect of the capsaicin on nerves that feel heat (thermoception) and pain (nociception).  Recent work has suggested that there is a direct impact on the activity of calcium channels in nerves and earlier work has found that such exposure was responsible for permanent damage to the cells involved.This is quite sobering when looked at from a riot-control angle. If a chemical is able to cause innervation, as measured by pain, it is arguable that it is a drug. If it is a drug, strict licensing laws would apply. This also influences the dose(s) that can be used legally. Perhaps we need to reflect on the use of pepper spray of indiscriminate dose, as a crowd control agent?

 

 What is a Lipid? Tuesday, Oct 25 2011 

For most people who have heard it, the word lipid sounds like the scientific word for fat. Although scientists tend not to use the word fat, and do tend to use the word lipid, the two do not quite match-up. When we say fat in the context of food, we think of cooking or olive oil, butter, grease and possibly cream. Perhaps confusingly, all of these have lipids in them, but none of them are pure lipids.

The confusion about these strange molecules is probably related to the fact that they are hard to pin down. There is virtually an art form in isolating them. Only in the last 50 years have we really started to understand them, or their role in biology. The thing that makes lipids what they are, is that they are what is known as amphiphilic. This is from the Greek ‘amphi-‘, meaning ‘both’, and ‘‑philic’, meaning ‘loving’. Although this sounds like an intellectual’s way of referring to a bisexual person, here the ‘both’ refers to water and grease. As we know, water and grease do not mix: if we pour oil onto water, the two just sit there. Lipids are special because they can dissolve in both of them. This property can even be used as a way of mixing grease and water together. Homogenous mixtures of grease and water with another agent are known as emulsions. Examples include milk, cake batter and margarine. The emulsion as a construct is useful in the manufacture of foods, giving rise to a plethora of agents that are able to perform this role, namely emulsifiers. Although in theory all lipids can be emulsifiers, not emulsifiers all are lipids.

We can see the roots of the amphiphilic behaviour in the structure of the lipid molecules. Moreover, structural analysis of molecules can help us determine whether or not a molecule is a lipid at all, and if so, which sort. Figure 1 shows an ordinary lipid with the head (water-loving) and tail (grease-loving) regions marked out.

Figure 1.  Phosphatidylcholine (Lecithin). The red section represents the hydrophilic (water-loving) section, whereas the blue represents the greasy-loving section.

The structural approach has been used by several institutions in recent years, including Lipid Maps.  This has given rise to a broader definition of the term lipid.  Thus, a variety of compounds not traditionally referred to as lipids are included, even if they are not measurably amphiphilic. However, when these ‘sort-of lipids’ are inserted into lipid systems whose physical behaviour is well understood, their influence on the character of the system is measurable.  This influence of the added component is also described as concentration-dependent, i.e., the more of the molecule that is put in (the higher the concentration), the stronger the given effect.
Some of the most amazing and unexpected behaviour of lipids occurs when they are exposed to water.  As part of the molecule likes water, and part of it does not, you can imagine that this situation is not going to be simple for the poor little lipid.  We as scientists describe the stresses and strains in systems like these according to the laws of thermodynamics.  Those of a certain generation may like to remind themselves of the Flanders and Swann song at this point.  Either way, we have put an unsuspecting lipid into a system with water and so what does it do?  Generally, lipids will self-assemble.  The exact manner in which this occurs varies between types of lipid, but the principle is observable across all of them.

The principle of self-assembly can be explained in terms of two competing priorities.  Thermodynamics means that the grease-loving (lipophilic) part of the lipid molecule wants to be shielded from the water, but at the same time, the water-loving (hydrophilic) part wants to be in touch with the water.  As our lipid cannot bear to disobey the laws of thermodynamics (who would?), it arranges itself in order to compromise between these competing forces.  This compromise can also be described in terms of energy.  In order to adopt a position in which the greasy section is exposed to water requires a lot of energy.  When the energy that is available to the system is insufficient for this, exposing the greasy section of the lipid to the water is described as energetically unfavourable.  This is the thermodynamic law I refer to and is also known as the hydrophobic effect.  The latter term, as you might well guess, is from the Greek ‘hydro-‘ meaning ‘water’ and ‘‑phobic’ meaning ‘hating’.

The hydrophobic effect may sound obscure, but the forces involved in it are what hold the membranes of cells together.  This means it has a huge and unsung part to play in understanding biology.