Molecular Orientation of Neurotensin and Its Single-Site Mutants on a Colloidal Silver Surface: SERS Studies

In this report, we describe a comprehensive systematic surface-enhanced Raman spectroscopy (SERS) study of the structures of neurotensin (NT), which is known to stimulate the growth of human tumors, in an aqueous solution of nanometer-sized colloidal silver particles under various environmental conditions, including excitation wavelengths (488.0, 514.5, and 785.0 nm), peptide concentrations (10 –4 –10 –6 M), pH levels of the solutions (from pH 2 to 11), H 2 O/D 2 O solvent exchange conditions, and structural mutations. The investigated mutated-NT analogues contain a natural (pig and frog NTs) and single-site synthetic ([Gln 4 ]NT, [Trp 11 ]NT, and [ d -Tyr 11 ]NT) backbone and/or side-chain modifications, which induce striking biological in vitro effects. On the basis of the analyses of the spectral profiles, specific conclusions were drawn with respect to the peptide geometry and changes in the geometry that occurred when the adsorption conditions were varied. In addition, the findings for adsorbed NT at different pH of silver sol were fully supported by a generalized two-dimensional correlation analysis (G2DCA).