Raman and surface-enhanced Raman spectroscopy investigation of vasopressin analogues containing 1-aminocyclohexane-1-carboxylic acid residue

In this work, Raman spectroscopy (RS) was employed to characterize molecular structures of (Arg(8)]vasopressin (AVP) and its [Acc(2),D-Arg(8)]AVP, [Acc(3)]AVP, and (Cpa(1)Acc(3)]AVP analogues. The RS band assignments have been proposed. To determine the mechanism of adsorption of the above-mentioned compounds adsorbed on a colloidal silver surface, surface-enhanced Raman spectra (SERS) were measured. The SERS spectra were used to determine relative proximity of the adsorbed functional groups of the investigated peptides and their orientation on the silver surface. The AVP and (Acc(3))AVP SERS spectra (Acc: 1-aminocyclohexane-1-carboxylic acid) show that the L-tyrosine (Tyr) lies far from the metal surface, whereas the (Cpa(1) ,Acc(3))AVP spectrum (Cpa: 1-mercaptocyclohexaneacetic acid) provides evidence that Tyr interacts with the silver surface. These results suggest the binding of the Tyr-ionized phenolic group might be responsible for the selectivity of the analogues. We show that the aromatic ring of L-phenylalanine (Phe) of AVP and [Acc(2),D-Arg(8)]AVP interacts with the silver surface. The strength of this interaction is considerably weaker for [Acc(2),D-Arg(8)]AVP than for AVP. This might be due either to a longer distance between the Phe ring and the silver surface, or to the almost perpendicular orientation of the Phe ring towards the surface. The carbonyl group of the L-glutamine acid (Gin) or L-asparagine acid (Asn) of AVP, (Acc(2),D-Arg(8))AVP, and [Acc(3)]AVP is strongly bound to the silver surface. We have also found that all peptides adsorb on the silver surface via sulfur atoms of the disulfide bridge, adopting a "GGG" conformation, except [Cpa(1),Acc(3)]AVP, which accepts a "TGG" geometry. (c) 2006 Wiley Periodicals, Inc.