Abstract
A general labelling method is presented which allows the determination of the number of guanidine groups (related to arginine and homoarginine in peptides and proteins) by means of mass spectrometry. It implies a guanidine‐selective derivatization step with 2,3‐butanedione and an arylboronic acid under aqueous, alkaline conditions (pH 8–10). The reaction mixture is then directly analysed by electrospray ionization mass spectrometry without further sample pretreatment. Other amino acids are not affected by this reaction although it is demonstrated that lysine side‐chains may be unambiguously identified when they are converted to homoarginine prior to derivatization. Guanidine functionalities, as e.g. in the amino acid arginine, are easily identified by the characteristic mass shift between underivatized and derivatized analyte. The tagging procedure is straightforward and selective for guanidine groups. The influence of several experimental parameters, especially the pH of the solution and the choice of reagents, is examined and the method is applied to various arginine‐containing peptides and to lysozyme as a representative protein. Possible applications of this technique and its limitations are discussed. Copyright © 2003 John Wiley & Sons, Ltd.