pH dependent surface enhanced Raman study of Phe+Ag complex and DFT calculations for spectral analysis

Abstract

Surface enhanced Raman spectra of Phenylalanine (Phe)

pH dependent SERS and solvation studies of tyrosine adsorbed on silver colloidal nano particles combined with DFT calculations

“…The variation of the linewidth of the SERS band at $1359 cm À1 with pH can be explained by arguments similar to those presented in our recent study [11] for phenylalanine (Phe): (a) similar to the concentration fluctuation model, we propose that the random motion of solute molecules throughout the volume of the colloidal solution causes pH to fluctuate around a mean value of pH in a Gaussian manner. This explanation is also supported by the fact that during our experiment, the pH of the Tyr + Ag complex did not stabilize, even after 15-20 min [11]. (b) In addition to pH fluctuations, two other effects seem to play important roles in causing the variation of linewidth: (i) the transition of tyrosine from one ionic form to another with increasing pH results in a higher mobility [24] due to the decrease in the intermolecular interactions owing to the increased abundance of the zwitterionic form of the Tyr molecule.…”

“…This type of variation of linewidth with the relative concentrations of solute and solvent has been explained in terms of fluctuation around a uniform concentration within a microvolume of the solution [21][22][23]. The variation of the linewidth of the SERS band at $1359 cm À1 with pH can be explained by arguments similar to those presented in our recent study [11] for phenylalanine (Phe): (a) similar to the concentration fluctuation model, we propose that the random motion of solute molecules throughout the volume of the colloidal solution causes pH to fluctuate around a mean value of pH in a Gaussian manner. This explanation is also supported by the fact that during our experiment, the pH of the Tyr + Ag complex did not stabilize, even after 15-20 min [11].…”

“…This essentially shortens the bond length and leads to a blue shift. The blue shift of the SERS band at $1359 cm À1 with increasing pH can be explained in the following two ways: (i) as the pH of a Tyr solution changes, the charge states of the amino acid change, as argued in our earlier study for Phe [11]. The different ionic forms of Tyr molecules lead to a net electrostatic repulsion between them, which in turn increases the force constant for the corresponding mode, causing a blue shift.…”

“…In an earlier study [11], we reported that the different charge states of the Phenylalanine [29,30]. However, the other two configurations, O/O (V) and /OH (VI) show salt-bridge solvation (SB) geometry.…”

“…The analysis of spectral profiles of SERS bands provides information about spectral parameters, such as: Raman shift, changes in line width and variation in Raman intensities. Shifts in vibrational wavenumbers and relative intensities in the spectra of adsorbed molecular species compared to spectra in solution provide information about the average conformation of different parts of the adsorbed molecules to the surface [11]. However, linewidth changes provide information about the dynamics of the adsorbate molecules.…”