Recent developments in separation methods for enantiomeric ratio determination of amino acids specifically involved in cataract and Alzheimer’s disease

Abstract The racemization of amino acids provides another example of how the analytical sciences sit at the confluence of several disciplines. Over the past 60 years a quantity of articles has been published on amino acid racemization in various fields such as anthropology, geology, paleontology, archaeology, history, forensic science, medicine, pharmaceutical analysis and food science. The degree of postmortem racemization of amino acids from their L-to their D-configuration, for example, has long been used in the relative and quantitative dating of long-lived proteins which link archaeology and bioanalysis. Since the 1970s, the presence of d -amino acids in these long-lived proteins has also been reported to be involved in aging-related diseases. Since then, a large number of articles have published with a constant improvement of the analytical methods culminating in their chiral separation and this review will focus on analysis of amino acid racemization in Alzheimer's disease and cataracts published between 2015 and 2020. The reader will herein find a review, including articles older than 2015 when of utmost importance, of analytical approaches toward explaining the involvement of amino acid racemization in Alzheimer's disease and cataract. A second section is devoted to chiral separation methods: “indirect” involving formation of covalent diastereoisomers, through the use of a chiral derivatizing agent (CDA) and “direct” involving transient formation of diastereoisomers. Methods employing a chiral stationary phase (CSP) are presented and these CSPs extensively described. Capillary electrophoretic methods and multidimensional liquid and supercritical fluid chromatography are also reviewed. A third section is devoted to the recent application of these methods for determination of the D/L ratio, including studies where the localization of racemization in specific peptides has been studied: various biological samples will be reviewed, with sections on sample preparation and application of these methods to Alzheimer's disease and to studies of cataracts. A brief conclusion will summarize the main points and future trends of this topic.