A new strategy for the selective determination of d-amino acids: Enzymatic and chemical modifications for pre-column derivatization

A new strategy for the selective determination of d-amino acids (DAAs) employing a pre-column derivatization was designed with concepts based on both enzymatic and chemical modifications. Selective determination of DAAs was accomplished by following: DAA was enzymatically modified with d-amino acid oxidase (DAAO: EC 1.4.3.3) to form an alpha-keto acid. Subsequently, resulting alpha-keto acid was detected by high-performance liquid chromatography (HPLC) after chemical modification with o-phenylenediamine (PDA) in the presence of 2-mercaptoethanol (2ME) to give the corresponding quinoxalinol derivative (PDA-alpha-keto acid derivative). After optimizing the pre-column derivatization and HPLC separation, five peaks corresponding to DAAs (d-alanine, d-leucine, d-methionine, d-phenylalanine, d-valine (as the standard mixture of DAAs in this paper) were separately eluted and monitored by means of a conventional HPLC system with a gradient elution on octadecyl silica gel (ODS) column and a fluorescence detector (Ex.: 341 nm, Em.: 413 nm), respectively. It was confirmed that the present method was incapable of detecting l-amino acids (LAA) when a sample solution consisting of both LAAs and DAAs was examined. The linearity of the peak-area responses to their concentration range of DAAs from 10 to 500 μM is 0.994–1.000, and their detection limits were 0.2–1 μM (signal/noise = 3). When this method was applied to a methanolic extract of short-necked clams, Ruditapes philippinarum (in Japanese, Asari), a big peak, corresponding to d-alanine was detected, corresponding to 2.9 mg/g d-alanine. In this paper, we present an example of pre-column derivatization method that was newly configured to take into account both the biological and chemical properties of the substances in question.