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Article

The Diagnostic Vibrational Signature of Pentacoordination in Heme Carbonyls

Journal of the american chemical society. 2014. Vol. 136. No. 28. P. 9818-9821.
Linder D. P., Silvernail N. J., Barabanschikov A., Zhao J., Alp E. E., Sturhahn W., Sage J. T., Scheidt W. R., Rodgers K. R.

Heme-carbonyl complexes are widely exploited for the insight they provide into the structural basis of function in heme-based proteins and enzymes, by revealing the nature of their bonded and nonbonded interactions with the protein. This report presents two novel results which, taken together, clearly establish a FeCO vibrational signature for crystallographically verified penta-coordination. First, anysotropy in the NRVS density of states for \nu_{Fe-C} and \delta_{FeCO} in oriented single crystals of [Fe(OEP)(CO)] clearly reveals that the Fe-C stretch occurs at higher frequency than the FeCO bend and considerably higher than any previously reported heme carbonyl. Second, DFT calculations on a series of heme carbonyls reveal that the frequency crossover occurs near the weak trans O-atom donor, furan. As \nu_{Fe-C} occurs at lower frequencies than \delta_{FeCO} in all heme protein carbonyls reported to date, the results reported herein suggest that they are all hexacoordinate.