  In this article, we have compared the metal-amino acid interactions in Tyr-Ag
and Phe-Ag complexes through pH dependent SERS measurements. By analyzing the
variation in relative intensities of SERS bands with the pH of the amino acid
solution, we have obtained the orientation and conformation of the amino acid
molecules on the Ag surface. The results obtained from our experimental studies
are supported by the the energy minimized strucutres and the observed charge
distributions in different terminals of the molecules. This, in a way, shows
that SERS measurements not only exhibit the interaction of the amino acid
molecules with Ag clusters but also demonstrate their orientation around it. We
have addressed a long standing query on whether the amine group is directly
attached with Ag surface along with the carboxylate group and $\pi$-electrons
in these systems. In addition, pH dependent optical absorption and transmission
electron microscopy measurements have been performed to understand the required
conditions for the appearance of the SERS spectra in the light of the
aggregation of metal particles and the number of hot sites in the sol. Our
results confirm that the formation of hot sites in the sol plays a direct role
to form a stable Ag-ligand complex. Furthermore, the interaction kinetics of
metal-amino acid complexes have been analyzed via both Raman and absorption
measurements.
