Racemisation of Amino Acids: From Synthetic Challenge to Biological Significance.

Racemisation has traditionally been regarded as an undesirable side reaction that must be minimised or eliminated during chemical synthesis. However, racemisation is also a naturally occurring process that amino acids undergo as part of normal biological and evolutionary dynamics. Increasing evidence suggests that this stereochemical conversion plays important roles in biological systems and has been used to investigate cellular growth, ageing and protein turnover. While maintaining stereochemical purity is essential during synthetic processes, often achieved through milder coupling conditions and controlled reaction environments, it is important to recognise that racemisation can occur even under relatively mild, physiological conditions, without the need for elevated temperatures or extreme pH values. This review examines racemisation from both synthetic and biological perspectives, together with its biological applications. Particular attention is given to spontaneous racemisation as a contributing factor in ageing, where disruption of chiral balance and the accumulation of irreversible post-translational modifications (PTMs) are frequently observed. We discuss the mechanisms and factors that trigger this stereochemical conversion, strategies for minimising it during synthesis, and the potential benefits of understanding and exploiting racemisation. Applications such as age estimation in forensic science and insights into neuronal development are also highlighted.