Photogeneration and Quenching of Tryptophan Radical in Azurin

Proximal Methionine Amino Acid Residue Affects the Properties of Redox-Active Tryptophan in an Artificial Model Protein

Redox-active amino acid residues are at the heart of biological electron-transfer reactions. They play important roles in natural protein functions and are implicated in disease states (e.g., oxidative-stress-associated disorders). Tryptophan (Trp) is one such redox-active amino acid residue, and it has long been known to serve a functional role in proteins. Broadly speaking, there is still much to learn about the local features that make some Trp redox active and others inactive. Herein, we describe a new protein model system where we investigate how a methionine (Met) residue proximal to a redox-active Trp affects its reactivity and spectroscopy. We use an artificial variant of azurin from Pseudomonas aeruginosa to produce these models. We employ a series of UV-visible spectroscopy, electrochemistry, electron paramagnetic resonance, and density functional theory experiments to demonstrate the effect that placing Met near Trp radicals has in the context of redox proteins. The introduction of Met proximal to Trp lowers its reduction potential by ca. 30 mV and causes clear shifts in the optical spectra of the corresponding radicals. While the effect may be small, it is significant enough to be a way for natural systems to tune Trp reactivity.

Electrochemical and Structural Study of the Buried Tryptophan in Azurin: Effects of Hydration and Polarity on the Redox Potential of W48

Tryptophan serves as an important redox-active amino acid in mediating electron transfer and mitigating oxidative damage in proteins. We previously showed a difference in electrochemical potentials for two tryptophan residues in azurin with distinct hydrogen-bonding environments. Here, we test whether reducing the side chain bulk at position Phe110 to Leu, Ser, or Ala impacts the electrochemical potentials (E°) for tryptophan at position 48. X-ray diffraction confirmed the influx of crystallographically resolved water molecules for both the F110A and F110L tyrosine free azurin mutants. The local environments of W48 in all azurin mutants were further evaluated by UV resonance Raman (UVRR) spectroscopy to probe the impact of mutations on hydrogen bonding and polarity. A correlation between the frequency of the ω17 mode─considered a vibrational marker for hydrogen bonding─and E° is proposed. However, the trend is opposite to the expectation from a previous study on small molecules. Density functional theory calculations suggest that the ω17 mode reflects hydrogen bonding as well as local polarity. Further, the UVRR data reveal different intensity/frequency shifts of the ω9/ω10 vibrational modes that characterize the local H-bonding environments of tryptophan. The cumulative data support that the presence of water increases E° and reveal properties of the protein microenvironment surrounding tryptophan.

Theory of Protein Charge Transfer: Electron Transfer between Tryptophan Residue and Active Site of Azurin

One reaction step in the conductivity relay of azurin, electron transfer between the Cu-based active site and the tryptophan residue, is studied theoretically and by classical molecular dynamics simulations. Oxidation of tryptophan results in electrowetting of this residue. This structural change makes the free energy surfaces of electron transfer nonparabolic as described by the Q-model of electron transfer. We analyze the medium dynamical effect on protein electron transfer produced by coupled Stokes-shift dynamics and the dynamics of the donor-acceptor distance modulating electron tunneling. The equilibrium donor-acceptor distance falls in the plateau region of the rate constant, where it is determined by the protein-water dynamics, and the probability of electron tunneling does not affect the rate. The crossover distance found here puts most intraprotein electron-transfer reactions under the umbrella of dynamical control. The crossover between the medium-controlled and tunneling-controlled kinetics is combined with the effect of the protein-water medium on the activation barrier to formulate principles of tunability of protein-based charge-transfer chains. The main principle in optimizing the activation barrier is the departure from the Gaussian-Gibbsian statistics of fluctuations promoting activated transitions. This is achieved either by incomplete (nonergodic) sampling, breaking the link between the Stokes-shift and variance reorganization energies, or through wetting-induced structural changes of the enzyme's active site.

Photoinduced Electron Transfer from the Tryptophan Triplet State in Zn-Azurin

Tryptophan is one of few residues that participates in biological electron transfer reactions. Upon substitution of the native Cu²⁺ center with Zn²⁺ in the blue-copper protein azurin, a long-lived tryptophan neutral radical can be photogenerated. We report the following quantum yield values for Zn-substituted azurin in the presence of the electron acceptor Cu(II)-azurin: formation of the tryptophan neutral radical (Φ_rad), electron transfer (Φ_ET), fluorescence (Φ_fluo), and phosphorescence (Φ_phos), as well as the efficiency of proton transfer of the cation radical (Φ_PT). Increasing the concentration of the electron acceptor increased Φ_rad and Φ_ET values and decreased Φ_phos without affecting Φ_fluo. At all concentrations of the acceptor, the value of Φ_PT was nearly unity. These observations indicate that the phosphorescent triplet state is the parent state of electron transfer and that nearly all electron transfer events lead to proton loss. Similar results regarding the parent state were obtained with a different electron acceptor, [Co(NH₃)₅Cl]²⁺; however, Stern-Volmer graphs revealed that [Co(NH₃)₅Cl]²⁺ was a more effective phosphorescence quencher (K _SV = 230 000 M^-1) compared to Cu(II)-azurin (K _SV = 88 000 M^-1). Competition experiments in the presence of both [Co(NH₃)₅Cl]²⁺ and Cu(II)-azurin suggested that [Co(NH₃)₅Cl]²⁺ is the preferred electron acceptor. Implications of these results in terms of quenching mechanisms are discussed.

Role of the Triplet State and Protein Dynamics in the Formation and Stability of the Tryptophan Radical in an Apoazurin Mutant

The protein, azurin, has enabled the study of the tryptophan radical. Upon UV excitation of tyrosine-deficient apoazurin and in the presence of a Co(III) electron acceptor, the neutral radical (W48•) is formed. The lifetime of W48• in apoazurin is 41 s, which is shorter than the lifetime of several hours in Zn-substituted azurin. Molecular dynamics simulations revealed enhanced fluctuations of apoazurin which likely destabilize W48•. The photophysics of W48 was investigated to probe the precursor state for ET. The phosphorescence intensity was eliminated in the presence of an electron acceptor while the fluorescence was unchanged; this quenching of the phosphorescence is attributed to ET. The kinetics associated with W48• were examined with a model that incorporates intersystem crossing, ET, deprotonation, and decay of the cation radical. The estimated rate constants for ET (6 × 10⁶ s^–1) and deprotonation (3 × 10⁵ s^–1) are in agreement with a photoinduced mechanism where W48• is derived from the triplet state. The triplet as the precursor state for ET was supported by photolysis of apoazurin with 280 nm in the absence and presence of triplet-absorbing 405 nm light. Absorption bands from the neutral radical were observed only in the presence of blue light.

Long-Range Charge Delocalization Mediates the Ultrafast Ligand-to-Metal Charge Transfer Dynamics at the Cu2+-Active Site in Azurin

The blue color metalloprotein in azurin has traditionally been attributed to the intense cysteine-to-Cu²⁺ ligand-to-metal charge transfer transition centered at 628 nm. Although resonance Raman measurements of the Cu²⁺ active site have implied that the LMCT transition electronically couples to the protein scaffold well beyond its primary metal-ligand coordination shell, the structural extent of this electronic coupling and visualization of the protein-mediated charge transfer dynamics have remained elusive. Here, using femtosecond broadband transient absorption and impulsive Raman spectroscopy, we provide direct evidence for a rapid relaxation between two distinct charge transfer states, having different spatial delocalization, within ∼300 fs followed by recombination of charges in subpicosecond time scales. We invoke the formation of a protein-centered radical cation, possibly Trp48 or a Phe residue, within 100 fs substantiating the long-range electronic coupling for the first time beyond the traditional copper active site. The Raman spectra of the excited CT state show the presence of protein-centric vibrations along with the vibrational modes assigned to the copper active site. Our results demonstrate a large delocalization length scale of the initially populated CT state, thereby highlighting the possibility of exploiting azurin photochemistry for energy conversion techniques.

Concerted and Stepwise Proton-Coupled Electron Transfer for Tryptophan-Derivative Oxidation with Water as the Primary Proton Acceptor: Clarifying a Controversy

Concerted electron-proton transfer (CEPT) reactions avoid charged intermediates and may be energetically favorable for redox and radical-transfer reactions in natural and synthetic systems. Tryptophan (W) often partakes in radical-transfer chains in nature but has been proposed to only undergo sequential electron transfer followed by proton transfer when water is the primary proton acceptor. Nevertheless, our group has shown that oxidation of freely solvated tyrosine and W often exhibit weakly pH-dependent proton-coupled electron transfer (PCET) rate constants with moderate kinetic isotope effects (KIE ≈ 2-5), which could be associated with a CEPT mechanism. These results and conclusions have been questioned. Here, we present PCET rate constants for W derivatives with oxidized Ru- and Zn-porphyrin photosensitizers, extracted from laser flash-quench studies. Alternative quenching/photo-oxidation methods were used to avoid complications of previous studies, and both the amine and carboxylic acid groups of W were protected to make the indole the only deprotonable group. With a suitably tuned oxidant strength, we found an ET-limited reaction at pH < 4 and weakly pH-dependent rates at pH > ∼5 that are intrinsic to the PCET of the indole group with water (H₂O) as the proton acceptor. The observed rate constants are up to more than 100 times higher than those measured for initial electron transfer, excluding the electron-first mechanism. Instead, the reaction can be attributed to CEPT. These conclusions are important for our view of CEPT in water and of PCET-mediated radical reactions with solvent-exposed tryptophan in natural systems.

Spectroscopic observation and ultrafast coherent vibrational dynamics of the aqueous phenylalanine radical

The phenylalanine radical (Phe˙) has been proposed to mediate biological electron transport (ET) and exhibit long-lived electronic coherences following attosecond photoionization. However, the coupling of ultrafast structural reorganization to the oxidation/ionization of biomolecules such as phenylalanine remains unexplored. Moreover, studies of ET involving Phe˙ are hindered by its hitherto unobserved electronic spectrum. Here, we report the spectroscopic observation and coherent vibrational dynamics of aqueous Phe˙, prepared by sub-6 fs photodetachment of phenylalaninate anions. Sub-picosecond transient absorption spectroscopy reveals the ultraviolet absorption signature of Phe˙. Ultrafast structural reorganization drives coherent vibrational motion involving nine fundamental frequencies and one overtone. DFT calculations rationalize the absence of the decarboxylation reaction, a photodegradation pathway previously identified for Phe˙. Our findings guide the interpretation of future attosecond experiments aimed at elucidating coherent electron motion in photoionized aqueous biomolecules and pave way for the spectroscopic identification of Phe˙ in studies of biological ET.

Observation of intra- and intermolecular vibrational coherences of the aqueous tryptophan radical induced by photodetachment

The study of the photodetachment of amino acids in aqueous solution is pertinent to the understanding of elementary processes that follow the interaction of ionizing radiation with biological matter. In the case of tryptophan, the tryptophan radical that is produced by electron ejection also plays an important role in numerous redox reactions in biology, although studies of its ultrafast molecular dynamics are limited. Here, we employ femtosecond optical pump-probe spectroscopy to elucidate the ultrafast structural rearrangement dynamics that accompany the photodetachment of the aqueous tryptophan anion by intense, ∼5-fs laser pulses. The observed vibrational wave packet dynamics, in conjunction with density functional theory calculations, identify the vibrational modes of the tryptophan radical, which participate in structural rearrangement upon photodetachment. Aside from intramolecular vibrational modes, our results also point to the involvement of intermolecular modes that drive solvent reorganization about the N-H moiety of the indole sidechain. Our study offers new insight into the ultrafast molecular dynamics of ionized biomolecules and suggests that the present experimental approach can be extended to investigate the photoionization- or photodetachment-induced structural dynamics of larger biomolecules.

Copper-induced spectroscopic and structural changes in short peptides derived from azurin

The active sites of metalloproteins may be mimicked by designing peptides that bind to their respective metal ions. Studying the binding of protein ligands to metal ions along with the associated structural changes is important in understanding metal uptake, transport and electron transfer functions of proteins. Copper-binding metalloprotein azurin is a 128-residue electron transfer protein with a redox-active copper cofactor. Here, we report the copper-binding associated spectroscopic and structural properties of peptide loops (11 and 13 residues) from the copper-binding site of azurin. These peptides develop a β-turn upon copper-binding with a 1:1 Cu²⁺:peptide stoichiometry as seen in circular dichroism and exhibit electronic transitions centered at 340 nm and 540 nm. Further addition of copper develops a helical feature along with a shift in the absorption maxima to ~360 nm and ~580 nm at 2:1 Cu²⁺:peptide stoichiometry, indicating stoichiometric dependence of copper-binding geometry. Mass spectrometry indicates the copper-binding to cysteine, histidine and methionine in the peptide with 1:1 stoichiometry, and interestingly, dimerization through a disulfide linkage at 2:1 stoichiometry, as observed previously for denatured azurin. Fluorescence quenching studies on peptides with tryptophan further confirm the copper-binding induced changes in the two peptides are bi-phasic.

Impact of Local Electrostatics on the Redox Properties of Tryptophan Radicals in Azurin: Implications for Redox-Active Tryptophans in Proton-Coupled Electron Transfer

Tyrosine and tryptophan play critical roles in facilitating proton-coupled electron transfer (PCET) processes essential to life. The local protein environment is anticipated to modulate the thermodynamics of amino acid radicals to achieve controlled, unidirectional PCET. Herein, square-wave voltammetry was employed to investigate the electrostatic effects on the redox properties of tryptophan in two variants of the protein azurin. Each variant contains a single redox-active tryptophan, W48 or W108, in a unique and buried protein environment. These tryptophan residues exhibit reversible square-wave voltammograms. A Pourbaix plot, representing the reduction potentials versus pH, is presented for the non-H-bonded W48, which has potentials comparable to those of tryptophan in solution. The reduction potentials of W108 are seen to be increased by more than 100 mV across the same pH range. Molecular dynamics shows that, despite its buried indole ring, the N-H of W108 hydrogen bonds with a water cluster, while W48 is completely excluded from interactions with water or polar groups. These redox properties provide insight into the role of the protein in tuning the reactivity of tryptophan radicals, a requirement for controlled biological PCET.

Raman and Quantum Yield Studies of Trp48-d5 in Azurin: Closed-Shell and Neutral Radical Species

Isotopologues are valuable vibrational probes that shift features in a vibrational spectrum while preserving the electronic structure of the molecule. We report the vibrational and electronic spectra of perdeuterated tryptophan in solution (l-Trp-d₅), as Trp48-d₅ in azurin, and as the photogenerated neutral tryptophan radical, Trp48-d₅^•, in azurin. The UV resonance Raman bands of the perdeuterated closed-shell tryptophan in solution and in azurin are lower in frequency relative to the protiated counterpart. The observed decrease in frequencies of l-Trp-d₅ bands relative to l-Trp-h₅ enables the analysis of vibrational markers of other amino acids, e.g., phenylalanine, that overlap with some modes of l-Trp-h₅. The Raman intensities vary between l-Trp-d₅ and l-Trp-h₅; these differences likely reflect modifications in normal mode composition upon perdeuteration. Analysis of the W3, W6, and W17 modes suggests that the W3 mode retains its utility as a conformational marker; however, the H-bond markers W6 and W17 appear to be less sensitive upon perdeuteration. The neutral tryptophan radical, Trp48-d₅^•, was generated in azurin with a slightly lower radical quantum yield than for Trp48-h₅^•. The visible resonance Raman spectrum of Trp48-d₅^• is different from that of Trp48-h₅^•, especially in terms of relative intensities, and all assignable peaks decreased in frequency upon perdeuteration. The absorption and emission spectra of the perdeuterated closed-shell and radical species exhibited hypsochromic shifts of less than 1 nm relative to the protiated species. The data presented here indicate that l-Trp-d₅ is a valuable probe of vibrational structure, with minimal modification of photoreactivity and photophysics compared to l-Trp-h₅.

Structure and Function of Tryptophan-Tyrosine Dyads in Biomimetic β Hairpins

Tyrosine–tryptophan (YW) dyads are ubiquitous structural motifs in enzymes and play roles in proton-coupled electron transfer (PCET) and, possibly, protection from oxidative stress. Here, we describe the function of YW dyads in de novo designed 18-mer, β hairpins. In Peptide M, a YW dyad is formed between W14 and Y5. A UV hypochromic effect and an excitonic Cotton signal are observed, in addition to singlet, excited state (W*) and fluorescence emission spectral shifts. In a second Peptide, Peptide MW, a Y5–W13 dyad is formed diagonally across the strand and distorts the backbone. On a picosecond timescale, the W* excited-state decay kinetics are similar in all peptides but are accelerated relative to amino acids in solution. In Peptide MW, the W* spectrum is consistent with increased conformational flexibility. In Peptide M and MW, the electron paramagnetic resonance spectra obtained after UV photolysis are characteristic of tyrosine and tryptophan radicals at 160 K. Notably, at pH 9, the radical photolysis yield is decreased in Peptide M and MW, compared to that in a tyrosine and tryptophan mixture. This protective effect is not observed at pH 11 and is not observed in peptides containing a tryptophan–histidine dyad or tryptophan alone. The YW dyad protective effect is attributed to an increase in the radical recombination rate. This increase in rate can be facilitated by hydrogen-bonding interactions, which lower the barrier for the PCET reaction at pH 9. These results suggest that the YW dyad structural motif promotes radical quenching under conditions of reactive oxygen stress.

A novel role for PsbO1 in photosynthetic electron transport as suggested by its light-triggered selective nitration in Arabidopsis thaliana

Exposure of Arabidopsis leaves to nitrogen dioxide (NO2) results in the selective nitration of specific proteins, such as PsbO1. The 9th tyrosine residue (9Tyr) of PsbO1 has been identified as the nitration site. This nitration is triggered by light and inhibited by photosynthetic electron transport inhibitors. During protein nitration, tyrosyl and NO2 radicals are formed concurrently and combine rapidly to form 3-nitrotyrosine. A selective oxidation mechanism for 9Tyr of PsbO1 is required. We postulated that, similar to 161Tyr of D1, 9Tyr of PsbO1 is selectively photo-oxidized by photosynthetic electron transport in response to illumination to a tyrosyl radical. In corroboration, after reappraising our oxygen evolution analysis, the nitration of PsbO1 proved responsible for decreased oxygen evolution from the thylakoid membranes. NO2 is reportedly taken into cells as nitrous acid, which dissociates to form NO2-. NO2- may be oxidized into NO2 by the oxygen-evolving complex. Light may synchronize this reaction with tyrosyl radical formation. These findings suggest a novel role for PsbO1 in photosynthetic electron transport.

Unusual Reduction Mechanism of Copper in Cysteine-Rich Environment

Copper-cysteine interactions play an important role in Biology and herein we used the copper-substituted rubredoxin (Cu-Rd) from Desulfovibrio gigas to gain further insights into the copper-cysteine redox chemistry. EPR spectroscopy results are consistent with Cu-Rd harboring a Cu^II center in a sulfur-rich coordination, in a distorted tetrahedral structure ( g_∥,⊥ = 2.183 and 2.032 and A_∥,⊥ = 76.4 × 10^-4 and 12 × 10^-4 cm^-1). In Cu-Rd, two oxidation states at Cu-center (Cu^II and Cu^I) are associated with Cys oxidation-reduction, alternating in the redox cycle, as pointed by electrochemical studies that suggest internal geometry rearrangements associated with the electron transfer processes. The midpoint potential of [Cu^I(S-Cys)₂(Cys-S-S-Cys)]/[Cu^II(S-Cys)₄] redox couple was found to be -0.15 V vs NHE showing a large separation of cathodic and anodic peaks potential (Δ E_p = 0.575 V). Interestingly, sulfur-rich Cu^II-Rd is highly stable under argon in dark conditions, which is thermodynamically unfavorable to Cu-thiol autoreduction. The reduction of copper and concomitant oxidation of Cys can both undergo two possible pathways: oxidative as well as photochemical. Under O₂, Cu^II plays the role of the electron carrier from one Cys to O₂ followed by internal geometry rearrangement at the Cu site, which facilitates reduction at Cu-center to yield Cu^I(S-Cys)₂(Cys-S-S-Cys). Photoinduced (irradiated at λ_ex = 280 nm) reduction of the Cu^II center is observed by UV-visible photolysis (above 300 nm all bands disappeared) and tryptophan fluorescence (∼335 nm peak enhanced) experiments. In both pathways, geometry reorganization plays an important role in copper reduction yielding an energetically compatible donor-acceptor system. This model system provides unusual stability and redox chemistry rather than the universal Cu-thiol auto redox chemistry in cysteine-rich copper complexes.

Pourbaix Diagram, Proton-Coupled Electron Transfer, and Decay Kinetics of a Protein Tryptophan Radical: Comparing the Redox Properties of W32• and Y32• Generated Inside the Structurally Characterized α3W and α3Y Proteins

Protein-based "hole" hopping typically involves spatially arranged redox-active tryptophan or tyrosine residues. Thermodynamic information is scarce for this type of process. The well-structured α₃W model protein was studied by protein film square wave voltammetry and transient absorption spectroscopy to obtain a comprehensive thermodynamic and kinetic description of a buried tryptophan residue. A Pourbaix diagram, correlating thermodynamic potentials (E°') with pH, is reported for W₃₂ in α₃W and compared to equivalent data recently presented for Y₃₂ in α₃Y ( Ravichandran , K. R. ; Zong , A. B. ; Taguchi , A. T. ; Nocera , D. G. ; Stubbe , J. ; Tommos , C. J. Am. Chem. Soc. 2017 , 139 , 2994 - 3004 ). The α₃W Pourbaix diagram displays a pK_OX of 3.4, a E°'(W₃₂(N^•+/NH)) of 1293 mV, and a E°'(W₃₂(N^•/NH); pH 7.0) of 1095 ± 4 mV versus the normal hydrogen electrode. W₃₂(N^•/NH) is 109 ± 4 mV more oxidizing than Y₃₂(O^•/OH) at pH 5.4-10. In the voltammetry measurements, W₃₂ oxidation-reduction occurs on a time scale of about 4 ms and is coupled to the release and subsequent uptake of one full proton to and from bulk. Kinetic analysis further shows that W₃₂ oxidation likely involves pre-equilibrium electron transfer followed by proton transfer to a water or small water cluster as the primary acceptor. A well-resolved absorption spectrum of W₃₂^• is presented, and analysis of decay kinetics show that W₃₂^• persists ∼10⁴ times longer than aqueous W^• due to significant stabilization by the protein. The redox characteristics of W₃₂ and Y₃₂ are discussed relative to global and local protein properties.

Multielectron Chemistry within a Model Nickel Metalloprotein: Mechanistic Implications for Acetyl-CoA Synthase

The acetyl coenzyme A synthase (ACS) enzyme plays a central role in the metabolism of anaerobic bacteria and archaea, catalyzing the reversible synthesis of acetyl-CoA from CO and a methyl group through a series of nickel-based organometallic intermediates. Owing to the extreme complexity of the native enzyme systems, the mechanism by which this catalysis occurs remains poorly understood. In this work, we have developed a protein-based model for the Ni_P center of acetyl coenzyme A synthase using a nickel-substituted azurin protein (NiAz). NiAz is the first model nickel protein system capable of accessing three (Ni^I/Ni^II/Ni^III) distinct oxidation states within a physiological potential range in aqueous solution, a critical feature for achieving organometallic ACS activity, and binds CO and -CH₃ groups with biologically relevant affinity. Characterization of the Ni^I-CO species through spectroscopic and computational techniques reveals fundamentally similar features between the model NiAz system and the native ACS enzyme, highlighting the potential for related reactivity in this model protein. This work provides insight into the enzymatic process, with implications toward engineering biological catalysts for organometallic processes.

A computational study of radical initiated protein backbone homolytic dissociation on all natural amino acids

Hydroxyl radical (˙OH) is known to be one of the most reactive species. The attack of this radical onto the backbone of all natural amino acids is investigated.

Electrochemical, Spectroscopic, and Density Functional Theory Characterization of Redox Activity in Nickel-Substituted Azurin: A Model for Acetyl-CoA Synthase

Nickel-containing enzymes are key players in global hydrogen, carbon dioxide, and methane cycles. Many of these enzymes rely on Ni(I) oxidation states in critical catalytic intermediates. However, due to the highly reactive nature of these species, their isolation within metalloenzymes has often proved elusive. In this report, we describe and characterize a model biological Ni(I) species that has been generated within the electron transfer protein, azurin. Replacement of the native copper cofactor with nickel is shown to preserve the redox activity of the protein. The Ni(II/I) couple is observed at -590 mV versus NHE, with an interfacial electron transfer rate of 70 s(-1). Chemical reduction of Ni(II)Az generates a stable species with strong absorption features at 350 nm and a highly anisotropic, axial EPR signal with principal g-values of 2.56 and 2.10. Density functional theory calculations provide insight into the electronic and geometric structure of the Ni(I) species, suggesting a trigonal planar coordination environment. The predicted spectroscopic features of this low-coordinate nickel site are in good agreement with the experimental data. Molecular orbital analysis suggests potential for both metal-centered and ligand-centered reactivity, highlighting the covalency of the metal-thiolate bond. Characterization of a stable Ni(I) species within a model protein has implications for understanding the mechanisms of complex enzymes, including acetyl coenzyme A synthase, and developing scaffolds for unique reactivity.