Comparison of the stability and UV and fluorescence characteristics of the o-phthaldialdehyde/3-mercaptopropionic acid and o-phthaldialdehyde/N-acetyl-L-cysteine reagents and those of their amino acid derivatives

Derivatization conditions of amino acids with the o-phthaldialdehyde (OPA)/3-mercaptopropionic acid (MPA) and with the OPA/N-acetyl-L-cysteine (NAG) reagents have been exhaustively reviewed and investigated. The existence and extent of the self fluorescence of both reagents have been demonstrated and measured. The stability of the OPA/MPA and OPA/NAC isoindoles have been determined and compared to each other, from the same working solution, at pH=9.4 and at pH=7.2, by UV and fluorescence photometry. Data obtained proved that only slight differences can be expected applying the OPA/MPA or the OPA/NAC reagents in terms of the corresponding molar absorptivity, isoindole's stability and concentration linearity values. The relative molar fluorescence intensities of the OPA/MPA derivatives proved to be higher by relatively 10-55%, in comparison to the corresponding OPA/NAC ones. Stabilized background fluorescence can be obtained with both reagents prepared at least 90 min before use and stored in a refrigerator. Reproducibility values determined in the linear concentration ranges for both derivatives were much better applying UV photometry in comparison to the fluorescence intensity measurements. (C) 1998 Elsevier Science B.V.