MODELING THE HEME VIBRATIONAL-SPECTRUM - NORMAL-MODE ANALYSIS OF NICKEL(II) ETIOPORPHYRIN-I FROM RESONANCE RAMAN, FT-RAMAN, AND INFRARED-SPECTRA OF MULTIPLE ISOTOPOMERS

Nearly complete vibrational assignments have been obtained for a heme model, nickel etioporphyrin-I (NiEPI), using variable-wavelength reson ance Raman (RR), and FT-Raman (FT-R), as well as infrared (IR) spectro scopy, on a series of isotopomers labeled at positions in the skeleton (N-15, beta(13)C, meso-d(4), N-15-meso-d(4)) and in the peripheral su bstituents (methyl-d(12), ethyl-d(8), and ethyl-d(12)). The vibrationa l bands are assigned to the porphyrin skeletal and substituent modes o n the basis of the mode description scheme developed for nickel octaet hylporphyrin (NiOEP) with the aid of a normal-mode analysis of NiEPI, explicitly including the peripheral substituents, i.e., the methyl and ethyl groups. The previously reported NiOEP force field was refined t o account for the observed isotope shifts of NiEPI isotopomers. An imp ortant result is the requirement of relatively large, long-range force constants for methine bridge bonds on opposite sides of the porphyrin ring. These 1-8 and 1-9 interaction force constants are required to r eproduce the frequencies and isotope shifts of six C-alpha-C-m stretch ing modes and especially to predict the relative order of the two high est-frequency E(u) modes, nu(C-alpha-C-m) (nu(38), similar to 1570 cm( -1)) and nu(C-beta-C-beta) (nu(37), similar to 1600 cm(-1)). Most of t he substituent (methyl and ethyl) vibrations are located in the RR and IR spectra. Strong RR enhancement of some substituent modes can be at tributed to hyperconjugative interaction of the aliphatic groups with the porphyrin al, orbital, as well as vibrational mixing of substituen t modes with the nearby skeletal modes. (C) 1996 John Wiley & Sons, In c.