Nuclear resonance vibrational spectroscopy - NRVS

The recent, synchrotron-based vibrational technique nuclear resonance vibrational spectroscopy (NRVS) is introduced. The method can be used for a number of Mössbauer active isotopes including 57Fe, which has yielded most of the results to date. The NRVS experiment can be thought of as Mössbauer spectroscopy with vibrational sidebands. Importantly, the NRVS experiment provides the complete set of bands corresponding to modes that involve motion of the iron atom. The method has a selectivity reminiscent of that of resonance Raman spectroscopy, but with the significant advantage that NRVS is not subject to the optical selection rules of Raman or infrared spectroscopy. Indeed, NRVS provides the ultimate limit in selectivity because only the vibrational dynamics of the probe nucleus contribute to the observed signal. All iron-ligand modes will be observed, including many that had not been previously observed. For hemes, these include in-plane iron vibrations that have not yet been reported by resonance Raman studies and the iron-imidazole stretch that has not been identified in six-coordinate porphyrins. Other modes that can be investigated include that of heme doming that is expected to be a low-frequency mode. The experimental setup at a beam line and sample requirements for iron-based derivatives are presented. Both powder and polarized single-crystal measurements can be made. The general features of data extraction and analysis are given. Data for heme and heme proteins are given. Examples of assignment of spectra for nitrosyl and carbonyl derivatives are given. These data demonstrate the importance of peripheral substituents on the vibrational spectrum of heme derivatives. Delocalization of modes appears to be common. Although this technique has only been available for a relatively short time, this early progress report indicates that NRVS has significant potential for probing the dynamics of Fe-containing molecules of biological interest.