Surface-enhanced resonance raman scattering spectroscopy applied to phytochrome and its model compounds. 1. Biliverdin photoisomers

The application of surface-enhanced resonance Raman scattering (SERRS) spectroscopy to the analysis of the configuration of biliverdin dimethyl ester (BVDE) is reported. SERRS spectra obtained by adsorption of the compounds onto an electrochemically roughened silver electrode and recorded at 77 K were intense and free of significant photodegradation. The similarity of the SERRS and resonance Raman (RR) spectra obtained under identical conditions suggests that no perturbation of the electronic structure of the BVDE occurs upon interaction with the silver surface, and that the distribution of conformers comprising the BVDE solution is not changed. SERRS spectra of the deuterated and monoprotonated Z, Z, Z isomer are also presented. To investigate the influence of configuration upon the Raman spectrum we have synthesized and purified the E, Z, Z and Z, Z, E isomers of BVDE. Excellent SERRS spectra were obtained from the solutions of the compounds eluted directly from the TLC plates. Comparison of these isomers to the more stable Z, Z, Z isomer reveals differences in frequencies and relative intensities associated with the change in molecular geometry upon isomerization. Most significant is the change in intensity of bands observed at 1255 and at 1245 cm⁻¹, a band that has been previously correlated to changes in conformation of the closely related phycocyanin chromophore.