Structures and Bonding on a Colloidal Silver Surface of the Various Length Carboxyl Terminal Fragments of Bombesin

Abstract

Raman (RS) and surface-enhanced Raman scattering spectra (SERS) were measured for various length carboxyl terminal fragments (X-14 of amino acid sequence) of bombesin ( BN): BN13-14, BN12-14, BN11-14, BN10-14, BN9-14, and BN8-14 in silver colloidal solutions. Density functional theory (DFT) calculations of Raman wavenumbers and intensities with extended basis sets (B3LYP/6-31++G**) were performed with the aim of providing the definitive band allocations to the normal coordinates. The proposed band assignment is consistent with the assignment for similar compounds reported in the literature. The nonadsorbed and adsorbed molecular structures were deducted by detailed spectral analysis of the RS and SERS spectra, respectively. This analysis also allowed us to propose the particular surface geometry and orientation of these peptides on silver surface, and their specific interaction with the surface. For example, a SERS spectrum of BN8-14 indicates that the interaction of a thioether atom and Trp8 with the silver surface is favorable and may dictate the orientation and conformation of adsorbed peptide. One of the most prominent and common features in all of the fragments' SERS spectra is a approximately 692 cm (-1) band due to nu(C-S) accompanied by two or three bands of different C-S conformers for all, except BN8-14, which suggests that all of the above-mentioned compounds adsorb on the silver surface through the thioether atom and that the attachment of Trp8 produces limitation in a number of possible C-S conformers adopted on this surface. Our results also show clearly that His12 and CO do not interact with the colloid surface, which supports our earlier results.

Structural properties of bombesin‐like peptides revealed by surface‐enhanced Raman scattering on roughened silver electrodes

“…41,43 Although in the case of these peptides adsorbed on the colloidal silver surface their -NH 2 group interacts with the silver nanoparticles. 42,44 …”

“…41,43 In each case, bands associated with Trp 8 dominated the SERS spectra, thus confirming previously obtained results. [41][42][43][44] Therefore, it may be deduced that this residue was involved in substrate binding to the Ag surface, although it occurred in a different manner for each peptide. In addition, the SERS spectra (see Figure 3) differed substantially from the corresponding FT-RS spectra (see Figure 2).…”

“…Also, the observed SERS signals for various length carboxyl terminal fragments (X-14 of the amino acid sequence) of BN: BN [13][14] [8][9][10][11][12][13][14] in silver colloidal solutions were compared with the contribution of the structural components of these fragments to interact with the receptors. 44 To further explore the actions of BN and structurally similar peptides on a silver surface, we have examined the ability of BN-like peptides (phyllolitorin and its [Leu 8 ] analogue from Phyllomedusa sauvagei ([Leu 8 ]-phyllolitorin, L-phenylalanine (Phe) at position 8 of phyllolitorin substituted by Lleucine (Leu 8 )), NMB, NMC, and PG-L) to bind to an electrochemically roughened silver electrode surface. Given the structural similarity between these peptides and their receptors, we hypothesized that small alternations in both their amino acid composition and tertiary structures may be important for their different adsorption mechanisms on the silver roughened electrode surface.…”

“…The former SERS signal was due to no hydrogen-bonded carboxyl groups being present, while the latter band at 1689 cm 21 corresponded to the strongly hydrogen-bonded carboxyl group or to the m(C¼ ¼O) A 1 m(C A À ÀN A ) mode. 44 Additionally, the strong FT-RS bands with the maxims spanned between 1442 and 1431 cm 21 (shown in Figure 2), assigned to the deformations of the CH 2 and CH 3 groups, shifted by several wavenumbers to higher values in their corresponding SERS spectra (see Figure 3 and Table II ]phyllolitorin, and PG-L was the lack of an amide I band near 1670 cm 21 (the b-sheet structure); this is usually the most prominent spectral feature in a peptide's spectrum. The absence of the amide I band may indicate that only side chain groups were enhanced.…”

Nociceptin and its natural and specifically‐modified fragments: Structural studies

“…SERS has been shown to be a powerful tool for structural analysis of metal-dependent changes in amino acids, dipeptides, [17][18][19][20][21] peptides, [22][23][24][25][26][27][28] and proteins. 29 In SERS, the molecule of interest is attached to a nanoscopically structured metallic surface (metal nano-resonator), which provides an increase in the observed Raman scattering by many orders of magnitude (up to 10 14 -10 15 ), allowing detection limits to approach the single-molecule level.…”