Computational modeling of metalloporphyrin structure and vibrational spectra: Porphyrin ruffling in NiTPP

This study extends DFT-SQM (density functional theory-scaled quantum mechanical) analysis to infrared and resonance Raman spectra of nickel(II) tetraphenylporphyrin (NiTPP), the largest molecular system so far analyzed with this methodology. NiTPP is of interest because of extensive empirical studies: its tendency to undergo porphyrin ruffling provides a way to model out-of-plane distortions in heme proteins. This ruffling tendency is captured by DFT, which predicts imaginary frequencies for D-4h NiTPP, along coordinates which lead to porphyrin ruffling and to phenyl rotation. Relaxation of symmetry constraints from D-4h lower the calculated energy by 0.61 kcal/mol for a D-2d structure [phenyl rotation] and an additional 1.08 kcal/mol for a S-4 structure [ruffling]. The S-4 structure is supported unequivocally by the observed activation of two out-of-plane modes, gamma(12) and gamma(13), in the Soret-excited RR spectrum. Raman intensity calculations, employing an INDO-level evaluation of excited-state gradients, give the correct gamma(12) and gamma(13) magnitudes for the S-4 conformation. Deconvolution of the Ni-N stretching RR band supports the population ratio [0.28] for planar [D-4h + D-2d] and nonplanar [S-4] conformations which is expected on the basis of the DFT energies. The computed frequencies and intensities permit assignment of all the RR bands, including several reassignments From previous studies, and of the IR spectrum. The previous NiTPP empirical force field has also been refined. Our analysis illustrates the utility of DFT-SQM in making detailed connections between the metalloporphyrin structure and its vibrational spectra. This capability promises to yield precise determinations of heme protein structural variations using resonance Raman spectroscopy.