Origin of the homochirality of biomolecules

Abstract

Molecules built up from a given set of atoms may differ in their three-dimensional structure. They may have one or more asymmetric centres that serve as reference points for the steric distribution of the atoms. Carbon atoms, common to all biomolecules, are often such centres. For example, the Cα atom between the carboxyl and amino groups in amino acids is an asymmetric centre: looking ON ward (i.e. from the carbOxyl to the amiNo group, with the Cα oriented so that it is above the carboxyl and amino groups) the radical characterizing the amino acid may be to the right (D-molecules) or to the left (L-molecules). Nineteen of the 20 amino acids occurring in proteins have such a structure (the exception is glycine, where the radical is a hydrogen atom). These pairs of molecules cannot be brought into coincidence with their own mirror image, as is the situation with our hands. The phenomenon has therefore been named handedness, or chirality, from the Greek word cheir, meaning hand. The two forms of the chiral molecules are called enantiomers or antipodes. They differ in rotating the plane of the polarized light either to the right or to the left. The sense of rotation depends on the wavelength of the measuring light, but at a given wavelength it is always opposite for a pair of enantiomers. Chirality may also occur when achiral molecules form chiral substances during crystallization (for example, quartz forms D-quartz or Lquartz). A detailed theoretical treatment of molecular chirality is given by Barron (1991).