OPTICAL FINE-STRUCTURE INVESTIGATION OF PORPHYRIN PROTEIN INTERACTIONS - MAGNESIUM AND METAL-FREE MYOGLOBIN

Fluorescence line-narrowing spectroscopy was used to evaluate the true 0,0 band shape from the population distribution of the 0,0 spectral b and and to determine the excited-state vibrational spectrum of the por phyrin in Mg protoporphyrin substituted myoglobin (Mg-Mb) and protopor phyrin substituted myoglobin (HH-Mb). The population distribution func tion for Mg-Mb showed a bimodal distribution, with approximately equal contributions for the two components. The 0,0 energies were centered at 16 716 and 16 873 cm-1 and the widths were 106 and 66 cm-1 for the lower and higher energy forms, respectively. The population distributi on for HH-Mb also showed two main components, but the contribution of the second component at lower frequency was less than for the predomin ant form centered at 16 127 cm-1 with width of 72 cm-1. The bimodal di stribution is discussed in terms of possible ligational effects or tau tomeric equilibrium. Evaluation of the energy-selected excitation spec tra for Mg-Mb and HH-Mb yields the vibrational frequencies of the exci ted singlet state porphyrin in the range 100-1550 cm-1. Within the acc uracy of the measurement, the vibrational frequencies for Mg-Mb were c onstant within the inhomogeneous distribution, indicating that changes in the electric field in different protein substates, rather than tor sional differences in the porphyrin, lead to the inhomogeneity in the spectrum. The excited-state vibrations show a 2-20-cm-1 downshift rela tive to the ground-state levels and comparison with resonance Raman sp ectra allows tentative assignments to be made.