Spectroscopic, Computational, and Kinetic Studies of the 4-Sulfide-Bridged Tetranuclear CuZ Cluster in N2
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- 作者：Somdatta Ghosh ; Serge I. Gorelsky ; Serena DeBeer George ; Jeannine M. Chan ; Inê ; s Cabrito ; David M. Dooley ; José ; J. G. Moura ; Isabel Moura ; Edward I. Solomon
- 刊名：Journal of the American Chemical Society
- 出版年：2007
- 出版时间：April 4, 2007
- 年：2007
- 卷：129
- 期：13
- 页码：3955 - 3965
- 全文大小：480K
- 年卷期：v.129,no.13(April 4, 2007)
- ISSN：1520-5126
文摘
A combination of spectroscopy and density functional theory (DFT) calculations has been usedto evaluate the pH effect at the CuZ site in Pseudomonas nautica (Pn) nitrous oxide reductase (N2OR) andAchromobacter cycloclastes (Ac) N2OR and its relevance to catalysis. Absorption, magnetic circulardichroism, and electron paramagnetic resonance with sulfur K-edge X-ray absorption spectra of the enzymesat high and low pH show minor changes. However, resonance Raman (rR) spectroscopy of PnN2OR athigh pH shows that the 415 cm-1 Cu-S vibration (observed at low pH) shifts to higher frequency, losesintensity, and obtains a 9 cm-1 18O shift, implying significant Cu-O character, demonstrating the presenceof a OH- ligand at the CuICuIV edge. From DFT calculations, protonation of either the OH- to H2O or the
4-S2- to 4-SH- would produce large spectral changes which are not observed. Alternatively, DFTcalculations including a lysine residue at an H-bonding distance from the CuICuIV edge ligand show thatthe position of the OH- ligand depends on the protonation state of the lysine. This would change the couplingof the Cu-(OH) stretch with the Cu-S stretch, as observed in the rR spectrum. Thus, the observed pHeffect (pKa ~9.2) likely reflects protonation equilibrium of the lysine residue, which would both raise E 
and provide a proton for lowering the barrier for the N-O cleavage and for reduction of the[Cu4S(im)7OH]2+ to the fully reduced 4CuI active form for turnover.
4-S2- to 4-SH- would produce large spectral changes which are not observed. Alternatively, DFTcalculations including a lysine residue at an H-bonding distance from the CuICuIV edge ligand show thatthe position of the OH- ligand depends on the protonation state of the lysine. This would change the couplingof the Cu-(OH) stretch with the Cu-S stretch, as observed in the rR spectrum. Thus, the observed pHeffect (pKa ~9.2) likely reflects protonation equilibrium of the lysine residue, which would both raise E and provide a proton for lowering the barrier for the N-O cleavage and for reduction of the[Cu4S(im)7OH]2+ to the fully reduced 4CuI active form for turnover.