The two enantiomers of a compound often have profoundly different biological properties and thus their liability to racemisation in aqueous solutions is an important piece of information. The authors reviewed the available data concerning the process of racemisation in vivo, in the presence of biological molecules (e.g., racemase enzymes, serum albumin, cofactors and derivatives) and under purely chemical but aqueous conditions (acid, base and other aqueous systems). Mechanistic studies are described critically in light of reported kinetic data. The types of experimental measurement that can be used to effectively determine rate constants of racemisation in various conditions are discussed and the data they provide is summarised. The proposed origins of enzymatic racemisation are presented and suggest ways to promote the process that are different from processes taking place in bulk water. Experimental and computational studies that provide understanding and quantitative predictions of racemisation risk are also presented.

When chemistry meets biology, as in the image with a child stumbling upon the enantiomers of cetirizine in the sea, stereochemistry often governs the outcome. In aqueous environments, the process of racemisation will therefore be important and can be facilitated in a range of ways including by base, acid or enzymatic catalysis. The limited information concerning racemisation under each of these conditions is surveyed and critiqued. Tools to guide prediction of racemisation risk are presented. For more details, see Review article by A. Ballard et al. on page 3661 ff.