Dual-channel Raman investigation of porphyrin binding to anti-porphyrin antibody

Dual-channel Raman spectroscopy was used to monitor changes in the structure and ligand field of Ni-mesoporphyrin (Ni-mP) that accompany binding to a catalytic antibody produced against N-methyl mesoporphyrin. By using 406.7 nm excitation from a Kr[sup +] laser, resonance Raman spectra were collected simultaneously for the antibody-bound Ni-mesoporphyrin (ab/Ni-mP) and for Ni-mP in a detergent (cetylpyridium bromide) solution. Additionally, absorption changes in the Soret band of the Ni-mP associated with the antibody suggest that Ni-mP is monomeric. These changes are similar to those observed for Ni-mP in detergent solution but contrast with aggregated species observed in non-detergent solution. An analysis of specific Raman modes indicates an increased population of ruffled conformers in the ab/Ni-mP complex over the planar form, which is dominant for Ni-mP in the micelles. The Raman data suggest the possibility of a distribution of conformers within the site(s) occupied. Additionally, Raman spectra are presented for iron mesoporphyrin bound to the hepten-binding specific fragment (F[sub ab]) of the catalytic antibody.