SITE‐SELECTION and SHPOLSKII SPECTROSCOPY OF MODEL PHOTOSYNTHETIC SYSTEMS

Abstract

Abstract— Low temperature ( T = 10 K) site‐selection fluorescence and excitation spectra of both isolated pheophorbide‐ a molecules (I PHEO) and pheophorbide covalently bonded to the synthetic l ‐lysyl‐ l ‐alanyl‐ l .‐alanine polypeptide (B PHEO) were measured in dimethylformamide (DMF) matrices. The fluorescence spectra display sharp vibronic lines at different wavenumbers for different dye laser excitation wavelengths, superimposed on a broad band background. Spectral analysis provides frequencies of normal vibrations (FNV) and site distribution functions (SDF). The FNV and SDF of I PHEO in DMF have been found to show good agreement with other chlorophyll‐like molecules. A covalent binding between the polypeptide and PHEO molecules produces only slight changes in FNV but significant broadenings and shifts to lower energies in SDF. The fluorescence spectra of I PHEO in a typical Shpolskii matrix ( n ‐octane) were also measured and found to exhibit sharp non‐shifting vibronic lines for all wavelengths and kinds of excitation. A general model explaining both site‐selection and Shpolskii spectroscopy is presented.