Fluctuating hydrogen-bond networks govern anomalous electron transfer kinetics in a blue copper protein

Abstract

SignificanceProtein fluctuations and hydrogen-bond networks play an important—although incompletely understood—role in facilitating efficient biological electron transfer (ET). Experimental mutagenesis results provide evidence for the role of protein motions in Ru-modified azurin ET, a quintessential example of biological ET. A recently developed nonadiabatic molecular dynamics method allows for exploration of the nature of protein fluctuations, providing insight into the conformational motions that accompany ET in Ru-modified azurin. In particular, a fluctuating hydrogen-bond network is identified that transiently ruptures to allow for donor–acceptor compression during ET.

Charge‐Transfer versus Charge‐Separated Triplet Excited States of [ReI(dmp)(CO)3(His124)(Trp122)]+ in Water and in Modified Pseudomonas aeruginosa Azurin Protein

“…Numerous experimental studies performed on mutants of the azurin redox protein of Pseudomonas aeruginosa , sensitized with UV/Vis‐light‐absorbing Ru or Re chromophores, have established the important role of the low‐lying triplet metal‐to‐ligand‐charge‐transfer (MLCT) and charge‐separated (CS) states in photoinduced charge transfer to the native copper center over more than 30 Å. This process is supposed to be initiated by the population of MLCT states localized on the metal complex, followed by charge separation, with the formation of a CS state between the metal complex and a coupled, proximal tryptophan (Trp), the evolution of which will ultimately lead to charge transfer to the copper center.…”

“…[7][8][9][10][11] As ap aradigmatic example, one can cite the study of the emission properties anda bsorption spectra of organometallic Ru II DNA intercalators [7,9,12] or of aR e I complex probe for DNA-mediated charge transport. [10] Numerous experimental studies performed on mutantso f the azurin redox protein of Pseudomonas aeruginosa, [13][14][15][16][17][18][19] sensitized with UV/Vis-light-absorbing Ru or Re chromophores, have established the important role of the low-lying triplet metal-to-ligand-charge-transfer( MLCT) and charge-separated (CS) states in photoinduced charget ransfer to the native copperc enter over more than 30 .T his processi ss upposed to be initiated by the population of MLCT states localized on the metal complex, followed by charge separation, with the formation of aCSs tate between the metal complex and ac oupled, proximal tryptophan (Trp), the evolution of which will ultimately lead to charge transfer to the copper center.M oreover,t ime-resolved experiments have provided evidenceo ft he formation of a 3 CS state in the protein through two different channels, namely,d irect ultrafast decay (< 1ps) from the metal complex chromophore, [Re I (dmp)(CO) 3 (His124)(Trp122)] + (dmp = 4,7-dimethylphenanthroline), or through as lower process (500 ps)a cross a 3 MLCT/ 3 CS equilibrium.…”

What Are We Missing by Not Measuring the Net Charge of Proteins?

“…[1,4] Deoxyribonuclease and ovalbumin, for example, have identical isoelectricp oints of pI = 5.1, but the formal and measured net charge of both proteins differ by approximately 7u nits at pH 8.4. [4] The systematic absence of experimentally determinedv alues of Z has likely impeded ar igorous understanding of most chemicalp rocesses in which proteinsa re involved including aggregation and self-assembly, [20][21][22][23][24][25][26] ligand binding, [27][28][29][30][31][32][33][34] catalysis, [35][36][37][38][39] electron transfer, [3,6,[40][41][42][43][44][45][46][47] protein crystallization, [14,48] analytical separation, [49,50] and protein engineering. [51][52][53][54][55][56] It is tempting to assume that the formal net chargeo faprotein predicted from generalized residue pK a values (Z seq )issosimilar to the actual net charge that any difference is irrelevant, and the isoelectric point tells us all we need to know about ap rotein's net charge.…”

Ab initio electronic structure calculations of entire blue copper azurins

“…39,40 Moreover, the possibility of tuning the transport properties via single amino acid mutations makes this proposal even more attractive. 41,42 However, a proper description of transport mechanisms is extremely complicated, not only because of the complex nature of the proteins but also because of a number of various factors which, to date, remains unclear: the interaction with the electrodes, 38 the orientation of the protein relative to them, 43 the oxidation state of the metallic center and its environment, 34,44 and the conformational state of the protein, 43 that might be relevant even in small globular proteins like azurins. There is no general agreement about the quantitative influence of these factors and there are still many unanswered questions but it is well known that protein-electrode coupling plays a major role in the electron transport, 38 similarly to what it is observed in small organic molecules.…”