Adsorption of S—S Containing Proteins on a Colloidal Silver Surface Studied by Surface-Enhanced Raman Spectroscopy

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Abstract
We present a Raman and surface-enhanced Raman scattering (SERS) study of the following proteins containing S–S group(s): α-chymotrypsin (α-CHT), insulin, lysozyme, oxytocin (OXT), Streptomyces subtilisin inhibitor (SSI), and trypsin inhibitor (STI). The SERS study is performed in order to understand the adsorption mechanism of the above-mentioned proteins on a colloidal silver surface. The SERS spectra presented here show bands associated mainly with aromatic amino acid vibrations. In addition, two distinct vibrations of the –C–S–S–C– fragment are observed in the Raman and SERS spectra, i.e., v(SS) and v(CS). The enhancement of the v(SS) vibration in the SERS spectra yields evidence that the intact disulfide bridge(s) is (are) located near the silver surface. This finding is supported by the presence of the v(CS) mode(s). The presence of vs(COO−) and v(C–COO−) in the SERS spectra in the 1384–1399 cm−1 and 909–939 cm−1 regions, respectively, indicate that the negatively charged COO− groups (aspartic and glutamic acids) assist in the binding on the positively charged silver surface. The Raman amide I and III bands observed in the 1621–1633 and 1261–1289 cm−1 ranges, respectively, indicate that the α-helical conformation is favored for binding to the surface over the random coil or β-sheet conformations. In addition, the presence of the imino group of Trp and/or His indicates that these amino acid residues may also bind to the silver sol.

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