FT-Raman and surface enhanced Raman spectra (SERS) of leucine and isoleucine, the only isomer in proteinic amino acids, on the silver colloidal substrate are recorded. The vibrational and enhanced peaks are assigned; the Raman shifts that stem from different vibrational model in the molecular inner structure, and the variations of SERS at different pH values are analyzed. One methyl’s different connection to the main chains of the isomer amino acid resulted in different vibrational model in inner molecule, expressed in Raman spectra: ρ(CH₃) and δ_as(CH₃) of leucine are at 962, 945, 924 and 1454, 1408 cm^－1, by the comparation, isoleucine’s are at 922 and 1448, 1420, 1394 cm^－1. Vibrations involved in C—CO, C—C, H—O…H and lattice vibration mode are identical. In saturated solution’s Raman spectra and SERS, all the different shifts are expressed more clearly than those in solid state’s Raman spectra. The adsorption models of the isomer amino acid on the silver surface are speculated.

Key words: leucine, isoleucine, FT-Raman, FT-SERS, pH value