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SYNOPSISNearly complete vibrational assignments have been obtained for a heme model, nickel etioporphyrin-I ( NiEPI) , using variable-wavelength resonance Raman (RR) , and FTRaman (FT-R), as well as infrared (IR) spectroscopy, on a series of isotopomers labeled at positions in the skeleton ( 15N, ,8-13C, meso-d4, 15N-meso-d4) and in the peripheral substituents ( methyl-d12, ethyl-d8, and ethyl-d12). The vibrational bands are assigned to the porphyrin skeletal and substituent modes on the basis of the mode description scheme developed for nickel octaethylporphyrin ( 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 to account for the observed isotope shifts of NiEPI isotopomers. An important 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 reproduce the frequencies and isotope shifts of six C,-C, stretching modes and especially to predict the relative order of the two highest-frequency E, modes, v( C,-C,) ( v 3 ' , -1570 cm-') and v( Co-Co) ( -1600 cm-') . Most of the substituent (methyl and ethyl) vibrations are located in the RR and IR spectra. Strong RR enhancement of some substituent modes can be attributed to hyperconjugative interaction of the aliphatic groups with the porphyrin a,, orbital, as well as vibrational mixing of substituent modes with the nearby skeletal modes. 0 1996 John Wiley & Sons, Inc. I NTRODUCTI 0 NT h e potential of resonance Raman (RR) spectroscopy as a probe of structure and function in heme proteins'-' motivates a continuing effort t o elucidate t h e vibrational modes of t h e heme chromop h~r e .~-'~ Excitation into t h e intense visible absorption bands in both the electronically allowed B-state and vibronically active Q-transitions of hemes a n d metalloporphyrins affords a rich array of spectral features. Some of these bands have been found t o convey information about t h e spin * To whom all correspondence should be addressed. Biospectroscopy, Vol. 1,395-412 (1995)

A framework is developed for modeling the resonance Raman (RR) intensities of metalloporphyrins, with a view toward rationalizing the enhancement patterns observed in the spectra of heme proteins. The geometry of the S 2 excited state of nickel(II) porphine is computed using INDO/1s methods, and the structural changes resulting from S 0 -S 2 photoexcitation are projected onto the ground-state normal modes to calculate the intensity of each Raman-active vibration. The RR intensities derive mainly from expansion of the C R C m and C C bonds in the excited state, with the relative intensities strongly influenced by the phasing between C R C m and C C stretching coordinates. Analysis of the ν 8 overtone shows the INDO predicted geometry changes to be about 25% too large. Results are compared at successive levels of approximation, demonstrating that inclusion of displacements along bending coordinates in the excited state are essential, as are frequency-dependent scaling factors which are determined from the absorption spectrum by the transform approach to RR scattering. Finally, the activation of non-totally symmetric modes by an A-term mechanism is modeled by distortion of the excited state along a b 1g coordinate. Enhancement of the experimentally observed non-totally symmetric modes is correctly predicted, although quantitative modeling of their intensity requires the inclusion of nonCondon coupling.

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Resonance Raman (RR) and FT-IR spectra are reported for nickel(II) 1,5-dihydroxy-1,5-dimethyloctaethylbacteriochlorin [Ni(HOEBC)] and its meso-d4 isotopomer. All the in-plane skeletal RR-active modes and most IR-active modes are assigned with the aid of a normal mode analysis by using a force field developed for nickel(II) octaethylporphyrin and by scaling the bond stretch force constants to bond lengths revealed in the crystal structure of nickel(H) octaethylbacteriochlorin. The calculated eigenvectors provide insight into the essential vibrational characteristics of metallobacteriochlorins. The RR spectra of Ni(HOEBC) were acquired with a variety of excitation wavelengths, near resonance with the Bx, Qx, and transitions. The enhancement pattern of the observed RR intensities reveals that the Bx-and near-Q^-resonant spectra are dominated by Franck-Condon-active modes while the Qx-resonant spectrum is dominated by vibronically active modes. The Bx-resonant spectrum also shows significant vibronic scattering, via coupling between the Bx-and B^-excited states. Frequencies correlate well among Ni(II) complexes of octaethylporphine (OEP) and hydroporphyrins for modes containing similar local mode contributions, when allowance is made for C^-C^b ond order reduction and the effects of symmetry lowering. Assignments are proposed for the existing RR data on bacteriochlorophyll a.