Surface enhanced Raman scattering (SERS) is potentially tool in the characterization of biomolecules such as amino
acids, complicated peptides and proteins, and even tissues or living cells. Amino acids and short peptides contain
different functional groups. Therefore, they are suitable for the investigations of the competitive-interactions of these
functional groups with colloidal silver surfaces. In this paper, Normal Raman and SERS of amino acids Leucine and
Isoleucine and short peptide Leu-Leu were measured on the silver colloidal substrate. Raman shifts that stem from
different vibrational mode in the molecular inner structure, and the variations of SERS of the samples were analyzed in
this study. The results show that different connection of one methyl to the main chains of the isomer amino acids resulted
in different vibration modes in the Normal Raman spectra of Leucine and Isoleucine. In the SERS spectra of the isomer
amino acids, all frequency shifts are expressed more differently than those in Normal Raman spectra of solid state.
Orientation of this isomer amino acids, as well as specific-competitive interactions of their functional groups with the
colloidal silver surface, were speculated by detailed spectral analysis of the obtained SERS spectra. In addition, the
dipeptide Leu-Leu, as the corresponding homodipeptide of Leucine, was also measured adsorbed on the colloidal silver
surface. The SERS spectrum of Leu-Leu is different from its corresponding amino acid Leucine but both of them are
adsorbed on the silver surface through the carboxylate moiety.