Characterization of [4Fe-4S] Cluster Vibrations and Structure in Nitrogenase Fe Protein at Three Oxidation Levels via Combined NRVS, EXAFS, and DFT Analyses

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• 作者：Devrani Mitra ; Simon J. George ; Yisong Guo ; Saeed Kamali ; Stephen Keable ; John W. Peters ; Vladimir Pelmenschikov ; David A. Case ; Stephen P. Cramer
• 刊名：The Journal of the American Chemical Society
• 出版年：2013
• 出版时间：February 20, 2013
• 年：2013
• 卷：135
• 期：7
• 页码：2530-2543
• 全文大小：682K
• 年卷期：v.135,no.7(February 20, 2013)
• ISSN：1520-5126

文摘

Azotobacter vinelandii nitrogenase Fe protein (Av2) provides a rare opportunity to investigate a [4Fe-4S] cluster at three oxidation levels in the same protein environment. Here, we report the structural and vibrational changes of this cluster upon reduction using a combination of NRVS and EXAFS spectroscopies and DFT calculations. Key to this work is the synergy between these three techniques as each generates highly complementary information and their analytical methodologies are interdependent. Importantly, the spectroscopic samples contained no glassing agents. NRVS and DFT reveal a systematic 10鈥?0 cm^鈥? decrease in Fe鈥揝 stretching frequencies with each added electron. The 鈥渙xidized鈥?[4Fe-4S]²⁺ state spectrum is consistent with and extends previous resonance Raman spectra. For the 鈥渞educed鈥?[4Fe-4S]¹⁺ state in Fe protein, and for any 鈥渁ll-ferrous鈥?[4Fe-4S]⁰ cluster, these NRVS spectra are the first available vibrational data. NRVS simulations also allow estimation of the vibrational disorder for Fe鈥揝 and Fe鈥揊e distances, constraining the EXAFS analysis and allowing structural disorder to be estimated. For oxidized Av2, EXAFS and DFT indicate nearly equal Fe鈥揊e distances, while addition of one electron decreases the cluster symmetry. However, addition of the second electron to form the all-ferrous state induces significant structural change. EXAFS data recorded to k = 21 脜^鈥? indicates a 1:1 ratio of Fe鈥揊e interactions at 2.56 脜 and 2.75 脜, a result consistent with DFT. Broken symmetry (BS) DFT rationalizes the interplay between redox state and the Fe鈥揝 and Fe鈥揊e distances as predominantly spin-dependent behavior inherent to the [4Fe-4S] cluster and perturbed by the Av2 protein environment.