SURFACE-ENHANCED RAMAN SPECTROSCOPY AND DFT CALCULATION OF NON-NATURAL -AMINO ACIDS

T. Iliescu1, D. Maniu1, V. Chiş1, F. D. Irimie2, Cs. Paizs2, M. Toşa2

1Physics Faculty, Babeş-bolyai University, M. Kogălniceanu 1, 400084 Cluj-Napoca, Romania
2Chemistry and Chemical Enginering Faculty, Babeş-Bolyai University, Cluj-Napoca Romania

Although of less importance than their -analogues, -amino acids are also present in peptides and different heterocycles, and their free forms and derivatives exhibit interesting pharmacological effects. Beside their own pharmacological activity, the -amino acids can be used as building blocks for the preperation of modified (non-natural)analogues of biological active peptides, By insertion of -amino acid in the place of an -amino acid of naturally occurring pharmacological active peptides, the activity or the effects can be modified [1]. 
Non-natural -amino acids are new category of -amino acids which were not yet studied by spectroscopic point of view. 
In this paper FT-Raman and NIR surface-enhanced Raman (SER) spectroscopies have been applied to the vibrational characterization of no-natural -amino acids, 3-amino-3-(furan-2yl)-propionic acid and 3-amino-[(5-benzothiazole-2yl)-furan-2yl]-propionic acid. Semiempirical and density-functional theory (DFT) calculation on both amino acids in their zwintterionic forms have been performed in order to find optimized structure and to compute the vibrational spectra. The NIR SER spectra in silver hydrosol and silver-coated paper have been recorded, compared and analysed. Good SER spectra were obtained at the pH values where dipolar ion structure are present providing the chemisorption of -amino acids molecules on the silver surface via positively charged NH3+ group. The carboxilate anion of both -amino acids are parellel oriented, whereas the plane of rings is oriented perpendicular to the silver surface.

1. F. Fülop, Chem. Rev., 101, 2181, (2001)