Low frequency dynamics of the nitrogenase MoFe protein via femtosecond pump probe spectroscopy - Observation of a candidate promoting vibration.
We have used femtosecond pump-probe spectroscopy (FPPS) to study the FeMo-cofactor within the nitrogenase (N2ase) MoFe protein from Azotobacter vinelandii. A sub-20-fs visible laser pulse was used to pump the sample to an excited electronic state, and a second sub-10-fs pulse was used to probe changes in transmission as a function of probe wavelength and delay time. The excited protein relaxes to the ground state with a ~1.2ps time constant. With the short laser pulse we coherently excited the vibrational modes associated with the FeMo-cofactor active site, which are then observed in the time domain. Superimposed on the relaxation dynamics, we distinguished a variety of oscillation frequencies with the strongest band peaks at ~84, 116, 189, and 226cm(-1). Comparison with data from nuclear resonance vibrational spectroscopy (NRVS) shows that the latter pair of signals comes predominantly from the FeMo-cofactor. The frequencies obtained from the FPPS experiment were interpreted with normal mode calculations using both an empirical force field (EFF) and density functional theory (DFT). The FPPS data were also compared with the first reported resonance Raman (RR) spectrum of the N2ase MoFe protein. This approach allows us to outline and assign vibrational modes having relevance to the catalytic activity of N2ase. In particular, the 226cm(-1) band is assigned as a potential 'promoting vibration' in the H-atom transfer (or proton-coupled electron transfer) processes that are an essential feature of N2ase catalysis. The results demonstrate that high-quality room-temperature solution data can be obtained on the MoFe protein by the FPPS technique and that these data provide added insight to the motions and possible operation of this protein and its catalytic prosthetic group.
IFN-CNR, Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo da Vinci 32, I-20133 Milano, Italy.
Margherita
M
Maiuri
Dipartimento di Scienze Ecologiche e Biologiche, Università della Tuscia, Largo dell'Università, I-01100 Viterbo, Italy.
Ines
I
Delfino
IFN-CNR, Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo da Vinci 32, I-20133 Milano, Italy.
Giulio
G
Cerullo
IFN-CNR, Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo da Vinci 32, I-20133 Milano, Italy.
Cristian
C
Manzoni
Institut für Chemie, Technische Universität Berlin, Strasse des 17 Juni 135, 10623 Berlin, Germany.
Vladimir
V
Pelmenschikov
Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA 15213, United States.
Yisong
Y
Guo
Department of Chemistry, University of California, Davis, CA 95616, United States.
Hongxin
H
Wang
Department of Chemistry, University of California, Davis, CA 95616, United States.
Leland B
LB
Gee
Department of Biochemistry, Virginia Polytechnic Institute & State University, Blacksburg, VA 24061, United States.
Christie H
CH
Dapper
Department of Biochemistry, Virginia Polytechnic Institute & State University, Blacksburg, VA 24061, United States.
William E
WE
Newton
Department of Chemistry, University of California, Davis, CA 95616, United States; Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, United States. Electronic address: spjcramer@ucdavis.edu.
Stephen P
SP
Cramer
D016948:Azotobacter vinelandii / Q000378:metabolism*; D055162:Biocatalysis; D008956:Models, Chemical; D008983:Molybdoferredoxin / Q000737:chemistry*; D013057:Spectrum Analysis; D013696:Temperature; D014732:Vibration
D008983:Molybdoferredoxin
Femtosecond; NRVS; Nitrogenase; Proton transfer; Pump-probe