Synthesis and Characterization of a Series of Orthogonally Protected l-Carnosine Derivatives

L-Carnosine (-alanyl-l-histidine) is an endogenous dipeptide that has been recognized for its broad spectrum of beneficial biological activities. However, the therapeutic utility of molecule has been hampered by its instability in human plasma (half-life in human serum<5min) due to the presence of carnosinase enzyme that catalyzes its hydrolysis into the respective individual amino acids. While a large number of carnosine derivatives have been synthesized to optimize its overall pharmacokinetic profile, reports that provide molecular evidence as to how the dipeptide interacts with its biological target are scarce. Therefore, many questions are yet to be answered concerning the pharmacophoric regions in carnosine and its significance to the molecule's diverse biological activities. In this study, we set out to construct a small library of carnosine analogues that can be used in assessing the influence of the various functional groups of the dipeptide on its important biological properties. Orthogonal protection/deprotection of selected functional groups led to the exposure of amino group at the N-terminus, the carboxyl group at the C-terminus, and the imidazole ring of histidine. To examine the significance of the imidazole group in preventing the aggregation of the -amyloid plaques, histidine was replaced by phenylalanine and a series of -Ala-Phe analogues was generated. To study the influence of the length of the carbon chain in -Ala on the -amyloid aggregation, a series of Gly-His analogues was synthesized. A series of Gly-Phe was also constructed and will be used as negative control in future -amyloid plaque assembly experiments. The synthesized carnosine derivatives were characterized by NMR (proton, carbon, and H-1-H-1 COSY), and mass spectroscopy.