Файл: Pye D. Polarised light in science and nature (IOP)(133s).pdf

Figure 5.2. Two hypothetical three-dimensional molecules with a carbon atom C joined to four different atoms or groups of atoms, A, B, D and E. Although they have the same formula, it is impossible to superimpose them and each resembles the mirror image of the other. The components A, B, D and E may be anything from a single hydrogen atom to a long, complex chain provided that they are all different. Each of these ‘chiral’ molecules will rotate polarised light in opposite directions, even in free solution, although all their other properties are identical.

reflected back through the solution, it is again rotated in the same sense but, as it is now travelling the opposite way, the original rotation becomes cancelled out and the light is unaffected after the double passage. This is in marked distinction to the cumulative rotatory effect of magnetic fields in the Faraday effect (chapter 4).

When a molecule has more than one atom that is linked to four different groups, then there are generally more than two possible configurations: in principle, two such atoms give four possibilities and every extra carbon atom doubles the number of possibilities. Figure 5.3 shows that tartaric acid has two asymmetrical carbon atoms (shown in heavy type and with a central dot) but, as their connections are identical, two of the possibilities turn out to be the same. So there are actually three tartaric acid molecules: one is laevorotatory at both ends (l– l), one is dextrorotatory at both ends (d– d) and an equal quantity of these produces an optically inactive racemic mixture (l– l + d– d). The third form, also discovered by Pasteur and called mesotartaric acid, is (l) at

Figure 5.3. Three possible configurations of tartaric acid molecules projected into two dimensions. The laevorotatory (l– l) form is the mirror image of the dextrorotatory (d– d) form. The third (d– l) form, called mesotartaric acid, is (d) at one end and (l) at the other: it can be superimposed on its own mirror image if it is also turned lengthwise and it is therefore optically inactive and can be regarded as ‘ambidextrous’.

one end and (d) at the other; there is only one such form because if each end is changed the molecule can be turned end for end to be the same as before. This (l– d) molecule is optically inactive because the two ends effectively counteract each other.

Another way of looking at it is to say this ‘ambidextrous’ molecule is not asymmetrically chiral: it can be superimposed upon its own mirror image and therefore cannot be optically active. When tartaric acid is synthesised in the laboratory, all three forms are normally produced because the ends of the molecules are usually generated randomly. This mixture is optically inactive because the racemic mixture of (l– l) and (d– d) balances out and the meso (l– d) form is itself inactive. Under heat treatment, as mentioned earlier, some of the (d– d) form that is found naturally in grape juice becomes converted to the other two forms until all optical activity is eventually lost. Some other chiral substances will do this spontaneously in a process called autoracemisation.

Since the tartaric acid molecule can exist in three forms, then so can many of its salts—in which hydrogen ions on either end are replaced by other ions such as sodium or ammonium. If both ends are replaced by the

same kind of ion, as in disodium tartrate, then three forms are possible: left, right and ambidextrous. But when the two ends have different replacements, then four different forms are possible, since (l– d) is not equivalent to (d– l) when the first and second groups are different, and there are also the (d– d) and (l– l) forms. In principle, this is the case with sodium ammonium tartrate which Pasteur studied, but this complication did not arise in his case. The two ‘quasi-ambidextrous’ forms can only be prepared from ambidextrous mesotartaric acid and this does not occur in natural grape juice. So Pasteur was only dealing with the left-handed (l– l) and right-handed (d– d) forms.

Optical activity is also a property of many sugars and these form a splendid basis for demonstrating the phenomenon with the simple apparatus shown in figure 5.4. A glass cylinder with a flat bottom stands over a polaroid and is capped by a second polaroid that is free to rotate. A radial pointer is fixed to the upper cap to show its orientation and the whole apparatus can be mounted over a hole in a cardboard mask so that an overhead projector can show it to an audience (it helps if the projector lens can be raised so that it focuses at about the middle of the tube).

With water in the tube, the two polaroids are crossed so that no light passes, and the position of the pointer is noted (perhaps on a simple dial). Then the water is replaced by a strong solution of cane sugar (sucrose); light now comes through and the upper polaroid must be rotated to the right (clockwise) in order to get extinction once again. The sucrose is seen to be dextrorotatory. This apparatus is a simple form of a polarimeter, also called a saccharimeter in the sugar refining industry where it is routinely used to measure the strength of sugar solutions. The new extinction is not sharp but changes colour over a small range without going black, due to the dependence of rotation on wavelength, where blue light is generally rotated more than red light. It can be made sharper and more convincing if a piece of coloured cellophane is added as a filter. Although blue shows the greatest rotation, green is almost as (and sometimes rather more) effective and is usually brighter.

Glucose (grape sugar) is also dextrorotatory, but fructose (fruit sugar) is laevorotatory so the upper polaroid must be turned to the left (anticlockwise) in order to restore extinction. For this reason glucose (a d sugar) is often called dextrose and fructose (an l sugar) is also called laevulose. Like characters in a Russian novel, many sugars have three names! Now the sucrose molecule is exactly like a glucose molecule and a fructose molecule joined together. In the presence of a specific enzyme called sucrase (or invertase), or when boiled with a little acid,

Figure 5.4. A simple polarimeter for demonstrating the optical activity of sugar solutions. A glass column of solution is capped at each end by polaroids (shown black) that can be rotated to extinguish the passage of light from below. When sugar solutions are present, the setting of the caps must be changed. Sucrose and glucose are d sugars, twisting the direction of polarisation clockwise; fructose is an l sugar that twists the vibration anticlockwise. A cardboard mask helps when the apparatus is shown on an overhead projector and if possible the projector should be focused about halfway up the tube. A radial wire pointer may then be brought down to read against a transparent scale, illuminated by a second window in the mask as shown on the right. Extinction is sharper if a green filter is placed under the column.

the sucrose combines with a molecule of water and splits into its two components:

C12H22O11 +H2O = C6H12O6 + C6H12O6 .

(sucrose) (glucose) (fructose)

Although glucose and fructose have the same basic formula, their internal structures are different (figure 5.5) and they do not form an inactive racemic mixture. Fructose is inherently much more optically active than glucose (as shown by the numbers in table 5.1), so the equal mixture turns polarised light to the left. The right-turning sucrose is said to have been inverted or changed into ‘invert sugar’. This is often done in wine and beer making as it is then more readily fermented. Invert sugar can be bought ready to use but yeasts produce their own sucrase so

Figure 5.5. The structural formulae for fructose and glucose projected into two dimensions. Heavy type and a central dot show the asymmetrical carbon atoms, each connected to three different atoms or groups, of which fructose has three while glucose has four. In principle, there can be eight forms of fructose and 16 forms of glucose but only one of each occurs naturally.

they can manage quite well, only a little more slowly, to ferment sucrose after inverting it themselves. For a simple demonstration, domestic cane sugar can be boiled for some minutes with a little citric acid (or vinegar etc) and examined in the polarimeter before and after. Glucose is readily available and fructose can be bought at health food shops since it is very sweet but less harmful to diabetics than the usual culinary sucrose.

Figure 5.5 shows that fructose has three asymmetrical carbon atoms and therefore can exist in eight different configurations, while glucose has four asymmetrical carbons giving 16 possibilities. Half the forms of each sugar will be dextrotatory and half laevorotatory but only one form of each is ever found in nature. Actually, when glucose is first dissolved in water it shows a high optical activity that then gradually declines to less than half (from +112 to +52.7) over a period of time. Fructose also changes but to a lesser extent (from −132 to −92.7). In both cases this is because the molecules change gradually by spontaneous rearrangement of internal linkages from the form they have in the crystals to a somewhat

Table 5.1. The specific optical activity (as measured under specified conditions) for sugars mentioned in the text. A positive value indicates rotation to the right, a negative one to the left.

less active form in free solution. This behaviour explains why two values are often given for their optical activities in reference tables, including table 5.1. The molecular structures and therefore the optical activities can also be markedly influenced by the physical conditions of the solutions, such as temperature.

All other natural sugars also occur in only one of their possible chiral forms. Some are (l) sugars and some are (d) sugars but they can all be derived from a very simple sugar called d-glyceraldehyde which has the formula C3H6O3 and just one asymmetrical carbon atom. No natural sugars that can be derived from l-glyceraldehyde are known. For this reason natural sugars are said to be right-handed D-sugars (with a capital D), from the dextrorotatory series, regardless of their own (l) or (d) optical activity. The corresponding L-sugars do not occur naturally.

Amino acids are also very important components of all living systems since they are the building blocks of all proteins including enzymes. Of the 20 amino acids that can combine to form proteins, 19 are optically active (one is non-chiral) and all are related to l-glyceraldehyde; they are therefore said to be all left-handed or L-amino acids (with a capital L). The corresponding D-amino acids are virtually absent from nature. Furthermore, the complex proteins formed by combinations of amino acids have a variety of characteristic shapes. The common alpha-helix, when formed from L-amino acids is always righthanded (like a right handed screw—figure 5.6) so that many structural elements in our bodies are of only one chiral form. Even DNA, which carries the genetic code, is coiled into a right-handed double helix because it is composed of D-sugars and L-nucleotides (figure 5.6).

[The handedness of a helix causes much confusion due to

Figure 5.6. Left: the shape of the right-handed alpha helix, which is a common configuration in proteins and forms naturally from L-amino acids; the mirror-image left-handed helix would need D-amino acids which do not occur naturally. Right: the double helix of DNA is also right-handed due to the chirality of its constituents.

its ambiguity: if it is clockwise or ‘right-handed’ when viewed from behind and traced away from the observer, as with most fixing screws, then it is anticlockwise or ‘lefthanded’ when viewed from in front and traced towards the observer. Different conventions are used in different fields or even in different countries: the rear view convention seems to be commoner although botanists use the front-view convention to describe twining plants—a 16th century book on the cultivation of hops (regarded as twining to the right) said they should be trained round their poles ‘alwayes according to the course of the sunne’, although this advice would obviously be wrong in the southern hemisphere where the sun’s course appears reversed. Even illustrations, which might seem to

be unambiguous, may actually add confusion: the alpha and double helices are often depicted the wrong way round, suggesting that the artist was confused by the instructions given. This book uses the rear-view convention for helices except for optical rotation where there appears to be universal acceptance of the front-view convention (as seen by the user of a polarimeter) so the latter is used here for this topic only.]

So all life ‘as we know it’ is based upon D-sugars and L-amino acids and the reason for this is an outstanding puzzle. Several theories have been proposed to explain how this came about but so far none has proved entirely convincing. A recent idea is that it might have been connected to the processes of radioactive decay which are inherently asymmetrical and produce particles that spin in only one direction. It has recently been found that left-spinning electrons can bias the formation of chiral crystals of sodium chlorate (see later), which are otherwise produced from solution in equal numbers. But maybe the handedness of biological molecules was originally due to some completely random event when life was only beginning on earth.

It is much easier to understand why only one form should take precedence over the coexistence of both. Food is broken down by enzymes, which are proteins whose overall shapes are important for their function because they must ‘fit’ onto the molecules they work on. Their overall shapes are determined by the shapes of their constituent amino acids. Enzymes composed of D-amino acids would themselves be mirror images of the ones made of L-amino acids that we find today, and they would probably not work well if at all because they would not fit their substrates in food, which are themselves chiral. So life based on L-sugars and D-amino acids is perfectly conceivable but would be incompatible with the life forms here on earth.

At the beginning of Through the Looking-Glass, Alice speculates on the presence of milk for her kitten in Looking-glass House, but she decides that looking-glass milk might not be good to drink. In 1872 Lewis Carroll could hardly have known that, because of chirality, the mirror-image constituents of looking-glass milk probably would be indigestible and of little use to real-world kittens. One can also imagine deep-space explorers of the future arriving at a beautiful planet, just like earth in most respects, but being unable to replenish their stores because on that planet life is based (by chance?) on L-sugars and D-amino acids and food substances there would be of little or no nutritional value to

earthmen. (Strangely perhaps, stereoisomers may smell quite different from each other.) If one imagines that an enzyme fits a part of the protein it attacks like a glove fitting on a hand, then feeding proteins made of D- amino acids to any creature from Earth would be like giving a supply of right-handed gloves to a person with only a left hand: they might be beautifully made and all just the right size, but completely useless! Of course if the radioactive decay theory for the origin of chirality is correct, then all life everywhere in the Universe would have the same handedness and so be compatible after all. This is another example of an apparently simply observation (the rotation of polarisation by some solutions) leading to conclusions that are very profound indeed.

At present there is much interest in the development of chiral pharmaceuticals, or drugs whose molecules are all of the same handedness. Many natural chemical agents within the body are chiral and match the receptor sites at which they act. So any drugs intended to act at these sites must also be chiral. If a racemic mixture is administered, then at best the ‘unwanted’ stereoisomer is inactive and simply represents a waste of manufacturing potential. But in many cases the ‘wrong-handed’ molecules cause side effects in quite different areas. The most infamous example is that of thalidomide which was used to alleviate morning sickness during early pregnancy. This drug is chiral and only one stereoisomer is effective; unfortunately the other stereoisomer was found, too late, to cause very severe defects in embryonic development, leading to babies with greatly reduced or missing limbs. Now other uses have been found for the ‘good’ isomer and there is pressure to produce it in a pure form so that it can be administered safely—although spontaneous racemisation may be a danger in some cases.

When chiral chemicals are synthesised from non-chiral components, both isomers are produced giving a racemic mixture. Selective production of one stereoisomer can be achieved in several ways, all essentially originated by Pasteur, that are now being exploited industrially, for example by ChiroTech of Cambridge. Some molecules can be synthesised in only one chiral form by using enzymes, either in vitro or by the activity of living micro-organisms such as bacteria or yeasts; since enzymes are chiral, any of their chiral products will be all of one stereoisomer. Other asymmetrical catalysts are now available to allow asymmetrical syntheses. Single stereoisomers can also be extracted from racemic mixtures by selective crystallisation. A saturated solution is seeded with tiny crystals of one stereoisomer and the resulting crystals,