Raman Optical Activity of Small Peptides and Terpene Molecules

Raman optical activity (ROA), the circular intensity difference between right and left circularly polarized incident and/or Raman scattered radiation, provides a unique conformational probe of biological molecules in aqueous solution.1,2 In the classical measurement of ROA, the polarization of the incident laser radiation is modulated between the two circular polarization states, keeping the scattered linear polarization fixed, resulting in ICP-ROA. More recent methods developed in our laboratory include modulation only of the Raman scattered radiation, denoted SCP-ROA, or dual circular polarization modulation of both the incident laser and the Raman scattered radiation, either in- or out-of-phase, denoted DCPI- and DCPII-ROA, respectively. The depolarized backscattering DCPI-ROA measurement has been found to be particularly advantageous for biological samples. One emphasis of our recent research is to explore the sensitivity of ROA to locallized vs. longer range vibrational modes and to local vs. long-range stereochemistry. In addition, comparison of results from the various forms of measurement of ROA provides information on the relative importance of magnetic dipole vs. electric quadrupole mechanisms for ROA intensity. Two classes of molecules have been surveyed. Studies of simple peptides provide the opportunity to investigate the degree of localization of ROA features on specific residues and the effects of vibrational coupling among residues.3