Speak Easy Competition - Nicolle C. Habermehl

 

            Have you ever wondered why it is possible to wear either one of a pair of socks on one foot, yet only one of a pair of shoes will fit on that foot? The reason for this is that your feet and shoes are chiral objects, whereas socks are achiral. A chiral object is one that is not super-imposable on its mirror-image; the word chiral is from the Greek 'cheir' meaning 'hand' because, like your feet and shoes, you cannot superimpose your left hand on your right hand. Your two feet are enantiomorphic and whilst they will behave the same way in an achiral environment (such as wearing socks), they behave differently when in a chiral environment (wearing shoes).

            Chirality is of great importance in chemistry and biology. In everyday life you experience chirality, from smell and taste to the effects of drugs. Some fascinating examples are limonene (an oil found in citrus fruits), where the right-handed form smells of oranges whilst the left-handed form smells of lemons; aspartame (an artificial sweetener) has only one form that is sweet to the taste, the others being bitter; and L-DOPA (the drug used for treatment of Parkinson's disease) is left-handed only - the right-handed form has no therapeutic benefit. In fact, the amino acids found in essentially all living things on Earth are homochiral (all the same hand) and there is great speculation as to the origin of this homochirality. It is even possible that somewhere in the universe there is a mirror-image Earth!

            When a compound is a helix, the chirality of that helix is called the helicity (or screw-sense), that is, whether it twists clockwise as you look down the centre of the helix (a right-handed helix, given the term P for plus), or counter-clockwise (left-handed helix, M for minus). Such chirality is found in propellers, spiral staircases, pasta spirals, and DNA. DNA is a right-handed double helix - there are two helical strands of DNA bases attached to sugar-phosphate groups wrapped around a central axis.

            My work involves preparing helical compounds called helicates, then separating the right-handed form (P) from the left-handed form (M). Since 1987, when interest in helicates was renewed, there have been hundreds of papers dealing with the formation of helicates, but only a handful concerning the separation into the right- and left-handed forms. The process of separating the P and M forms (the enantiomers) is called resolution.

Helicates are inorganic compounds - they consist of metal ions positioned along a central axis and wrapped around these metals are strands of a definite length called ligands. Typically, there are two or three metal ions in a helicate, depending on the length of the ligand, and two to four ligand strands, depending on the type of metal.

 

 

The right-handed (P) and left-handed (M) forms of a helicate consisting of two metal centres and two ligand strands. The hatched line in the centre indicates a mirror.

 

            I synthesize helicates that contain two metal atoms (either copper or silver) with two ligand strands. Such a helicate is termed a dinuclear, double-stranded helicate as it has two metal centres and two ligand strands. These helicates always form a 1:1 mixture of P and M (referred to as a racemic mixture as there are equal amounts of both hands). Unlike organic compounds such as limonene and L-DOPA, my helicates can change their handedness. This is because the bond between the metal atom and the ligand is comparatively weak. The process of converting from one hand to the other is called racemization, and the time it takes for this to occur can be measured. I determined that racemization was slow enough that it should be possible to separate the two hands of the helicate. The next step was to determine how to separate the two hands.

            When you put on a pair of gloves, your right hand fits in the right-handed glove; it isn't comfortable to wear your left glove on your right hand. In a similar way, if you add a right-handed compound to a racemic compound (a 1:1 mixture of right- and left-handed forms), then one of the two pairings (called diastereomers) is preferred over the other. I add a right-handed propeller-shaped compound to my mixture of P and M helicates in solution. Usually, one combination or diastereomer will separate out as a solid leaving the other pairing dissolved, but my compounds undergo a remarkable process called the Pfeiffer effect. What happens is that the right-handed propeller prefers to combine with the right-handed helicate in solution, so one half of the mixture is happy. The other half of the mixture, right-hand propeller-left-hand helicate, is less favoured, but because the helicate can racemize (M can convert to P), the left-handed form converts to the right-handed form after a few days. Thus, I end up with essentially 100% of the right-hand helicate with the right-hand propeller.

The propeller can easily be removed, leaving the helicate that is now all of one hand. This is important as one hand can have different effects than the other in a chiral environment, just as the two hands of limonene smell different.

            Meanwhile, I'll keep waiting for confirmation of mirror-image Earth, where it is probable that the majority of people will be left-handed and Australians will drive on the right-hand side of the road.

 

 

M. C. Escher's "Bond of Union": the woman's head is a left-handed helix whilst the man's head is a right-handed helix.