VIBRATIONAL OPTICAL ACTIVITY IN BIOMOLECULES

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Vibrational optical activity (VOA), the differential interaction of a chiral molecule with left and right circularly polarized radiation during vibrational excitation, measured for absorption (vibrational circular dichroism, VCD) and for Raman scattering (Raman optical activity, ROA), will be further developed and applied as a biospectroscopic technique. Upgrades in both VCD and ROA instrumentation will permit measurement of VOA over a wider range of frequencies and at lower sample concentrations, and implementation of ab initio molecular orbital formalisms for calculation of VCD and ROA intensities will enhance and refine the interpretation of VOA spectra, with a goal of establishing VOA as a principal technique in biomolecular spectroscopy that can complement electronic CD, NMR, FTIR and x-ray crystallography studies. Applications of VOA to small to medium sized native and model biomolecules will provide unique information on active solution conformations. Specifically, we propose the following: (1) Measurement of VCD spectra in the hydrogen stretching regions will be improved and the acquisition of VCD spectra will be extended into the near-infrared region of overtone and combination bands by upgrade of our dispersive VCD spectrometer to include step-scan FT-IR technology. We will continue to emphasize measurement of hydrogen stretching modes due to their simple localized nature and their use as probes of local conformation and intramolecular associations and hydrogen- bonding. (2) Our ROA instrument will be upgraded and optimized for biological applications by installing a back-thinned CCD array detector and replacing the spectrograph with one based on holographic dispersion and imaging technology. These improvements will permit the exploration of resonance ROA and surface-enhanced ROA, and extend the measurement of ROA to more dilute solutions. (3) Interpretation of VCD and ROA spectra will emphasize both empirical correlation and ab initio molecular orbital calculation of frequencies, optimized geometries and VOA intensities, which will be enhanced by implementation of the nuclear-velocity perturbation velocity-gauge a priori formalism for VCD, the localized origin gauge ab initio model for VCD and new sum-over-states ab initio formalism for ROA. The display of maps of vibrationally generated current (for VCD) and induced current (for ROA) will be developed to gain insight into the mechanisms for large VOA intensity. (4) Specific applications of VOA to provide information on solution conformations of biomolecules, which involve collaborations world-wide for sample synthesis and complementary measurements, will include ROA and VCD investigations of small bioactive and model peptides that form beta-turns, VCD of the immunosuppressive cyclic peptide cyclosporin with a variety of solvents and added cations, the ROA of peptides that form 3(10)-helices and beta- ribbon spirals, and VOA studies on small chiral pharmaceutical molecules such as AZT and sensory molecules such as aspartame. Studies of the VCD of a variety of ligands bound to heme proteins will be continued to characterize the chiral heme binding site. Resonance ROA will be developed by using bilirubin complexed with chiral agents such as quinine and albumin.