Fluorescent properties of amino acids labeled with ortho-aminobenzoic acid

ortho-Aminobenzoic acid (Abz) has been used as a convenient fluorescent donor group in internally quenched fluorescent peptides, which are employed as substrates for several proteolytic enzymes. As Abz is usually bound to the N-amino terminal of these peptides, it is of interest to investigate the Abz group fluorescent properties bound to different amino acids. We report in this article the optical absorption and fluorescent properties, in aqueous media, of Abz bound to the alpha-amino group of Ala, Gly, Leu, Ile, Val, Pro, Phe, Arg, Glu, Met, Asn, Tyr, and Trp, with monomethylamidated alpha-carboxyl group. In order to explore the origin of the drastic reduction of Abz attached to Nalpha amino group of prolyl-peptides, we also examined the fluorescence properties of Abz-NHCH3, Abz-N(CH3)2, and Abz-pyrrolidine. Molecular dynamics simulation and NMR data indicated a lack of periplanarity of the Abz-dimethylamide, which could be the origin of low fluorescence quantum yield of Abz-prolyl-peptides.