INTERPRETATION OF MULTIPLE Q(0,0) BANDS IN THE ABSORPTION-SPECTRUM OFMG-MESOPORPHYRIN EMBEDDED IN HORSERADISH-PEROXIDASE

Mg-mesoporphyrin horseradish peroxidase (MgMP-HRP) and MgMP-HRP comple xed with naphtohydroxamic acid (NHA) have been studied by fluorescence line narrowing (FLN) and pressure tuning spectral hole burning (SHE) techniques. In each sample, the low temperature absorption spectra sho w more than one transition in the origin range of the Q band. Comparis ons with broad-band fluorescence spectra and FLN studies suggest that the multiple band feature originates from the presence of different co nfigurations of the metal-porphyrin that are subject to Q(x)-Q(y) spli tting within the protein cavity, This suggestion is supported by press ure tuning SHE studies, In the uncomplexed as well as in the NHA-compl exed form of MgMP-HRP, irradiation in the Q band produces photoproduct bands, which has been attributed to a species with smaller Q(x)-Q(y) splitting, In an amorphous matrix, on the other hand, only one form of MgMP could be found, and no splitting could be observed. The binding of NHA does not significantly alter the bulk parameters of the protein matrix, but it reduces the structural variety in the configuration of MgMP to a single form with a more distorted structure and thus with a n enlarged Q(x)-Q(y) splitting.