Infrared spectroscopy of the nitrogenase MoFe protein under electrochemical control: potential-triggered CO binding

Abstract

Electrochemical control over nitrogenase allows us to examine electrocatalytic proton reduction and potential-triggered CO inhibition using infrared spectroscopy.

Nitrogen Reduction to Ammonia on a Biomimetic Mononuclear Iron Centre: Insights into the Nitrogenase Enzyme

“…The lowest lying transition state for protonation of 3 dd is on the quartet spin state surface with ab arrier( 4 TS3 d )o fa bout 15.0 kcal mol À1 .T herefore, the energetic barrierf or the third consecutive protonation step is considerably larger than the first two protont ransfer barriers.F urthermore, the reaction from 4 3 dd leadingt o 4 4 ddd is endothermic by 4.0 kcal mol À1 . Structurally,i ti sa kin to 4 3 dd with Fe-N and N-N distances of 1.819 and 1.325 .S imilar to the 2 TS1 d and 2, 4 TS2 d structures discussed above, the donating proton is at ar elativelyl arge distance from N d of well over 2 and has as malli maginary frequency of i148 cm À1 .A fter passingt he proton transfer transition states 2,4,6 TS3 d ,t he geometry scans fall to complex 4 ddd , which is an iron-nitrido with aw eakly bound ammonia molecule. This complex dissociates ammonia to form the final products.…”

“…Subsequently,w el ocated several transition states ( 2,4,6 TS3' d ) for the distal protonation of 2,4,6 3 dp as wella st hose for proximal protonation from 2,4,6 3 dd (see Figure 2). Thus, startingf rom 4 3 dp ,aproton-transfer transition state 4 TS3' d was located for distal protonation to give [(TPB)FeN(H)NH 2 ] 2 + ( 4 4 dpd )p roducts. Structure 3 dp ,i na nalogyt o3 dd ,h as aq uartet spin ground state that is separated from the sexteta nd doublet spin states by 10.8 and 23.1 kcal mol À1 ,r espectively.T he proton-transfer barrier( via 4 TS3' d )i sa bout 4.5 kcal mol À1 in energy (DE + ZPE) above the value of 4 3 dp andi ts structure has long Fe-N andN -Nd istances of 1.941 and 1.331 ,r espectively.S imilart ot he transition states reporteda bove, the imaginary frequency is small (i285 cm À1 ).…”

“…[3] The exact position of the nitrogenb ind-ing to the FeMoco cluster is unknown and its catalytic mechanism remains am ystery;t herefore, extensive spectroscopic, kinetics ande lectrochemical studies have been performed to obtain insights into the intricated etailso ft he mechanism. [4] Detailed computational studies tested possible mechanismsa s well as the coordinatione nvironment and the function of the centralc arbon of the FeMoco cluster. [5,6] However, as the reaction process on the FeMoco clusteri sf ast, there remain many controversies relatedt ot he mechanism and particularly to the order of proton and electron transfer.A saresult, model complexesh ave been developed to gain insight into these chemical processes.…”

Activation of CO2 by Vanadium Nitrogenase

“…Several approaches have been pioneered to directly inject electrons into the Mo‐nitrogenase P‐cluster, thereby circumventing the need for Fe protein and ATP. Notable efforts include covalently tethering a photoexcitable receptor complex proximal to the P‐cluster, generating nanocrystal‐protein hybrids for light‐driven catalysis, and using low‐potential reductants such as bisdicyclopentadienylcobalt(II) and europium(II)L (Eu II L) complexes with immobilized proteins . To date, only the method using the strong reductant europium(II)diethylenetriaminepentaacetate (Eu II DTPA, E 1/2 =−1.14 V vs. SHE at pH 8) has been applied to V‐nitrogenase (System II, Figure b, c) .…”

“…Notablee fforts include covalently tethering ap hotoexcitable receptor complex proximal to the P-cluster, [65] generating nanocrystal-protein hybrids for light-driven catalysis, [66] and using low-potential reductantss uch as bisdicyclopentadienylcobalt(II) and europium(II)L (Eu II L) complexes with immobilized proteins. [67,68] To date, only the methodusing the strongreductanteuropium(II)diethylenetriaminepentaacetate( Eu II DTPA, E 1/2 = À1.14 Vv s. SHE at pH 8) [69] has been applied to V-nitrogenase (System II, Figure 1b,c ). [70] Generation of Eu II Lc omplexes is as traightforward approacht hat involves the simple mixingo fa queous Eu II ,aweakr eductant (E 1/2 = À0.35 Vv s. SHE), with the appropriate deprotonated ligand.…”