Zinc thiolate reactivity toward nitrogen oxides: insights into the interaction of Zn2+ with S-nitrosothiols and implications for nitric oxide synthase

Zinc thiolate complexes containing N(2)S tridentate ligands were prepared to investigate their reactivity toward reactive nitrogen species, chemistry proposed to occur at the zinc tetracysteine thiolate site of nitric oxide synthase (NOS). The complexes are unreactive toward nitric oxide (NO) in the absence of dioxygen, strongly indicating that NO cannot be the species directly responsible for S-nitrosothiol formation and loss of Zn(2+) at the NOS dimer interface in vivo. S-Nitrosothiol formation does occur upon exposure of zinc thiolate solutions to NO in the presence of air, however, or to NO(2) or NOBF(4), indicating that these reactive nitrogen/oxygen species are capable of liberating zinc from the enzyme, possibly through generation of the S-nitrosothiol. Interaction between simple Zn(2+) salts and preformed S-nitrosothiols leads to decomposition of the -SNO moiety, resulting in release of gaseous NO and N(2)O. The potential biological relevance of this chemistry is discussed.     

Electron exchange involving a sulfur-stabilized ruthenium radical cation

Half-sandwich Ru(II) amine, thiol, and thiolate complexes were prepared and characterized by X-ray crystallography. The thiol and amine complexes react slowly with acetonitrile to give free thiol or amine and the acetonitrile complex. With the thiol complex, the reaction is dissociative. The thiolate complex has been oxidized to its Ru(III) radical cation and the solution EPR spectrum of that radical cation recorded. Cobaltocene reduces the thiol complex to the thiolate complex. The 1H and 31P NMR signals of the thiolate complex in acetonitrile become very broad whenever the thiolate and thiol complexes are present simultaneously. The line broadening is primarily due to electron exchange between the thiolate complex and its radical cation; the latter is generated by an unfavorable redox equilibrium between the thiol and thiolate complexes. Pyramidal inversion of sulfur in the thiol complex is fast at room temperature but slow at lower temperatures; major and minor conformers of the thiol complex were observed by 31P NMR at -98 degrees C in CD2Cl2.     

Tris(thioimidazolyl)borate-zinc-thiolate complexes for the modeling of biological thiolate alkylations

The S3Zn-SR coordination of thiolate-alkylating enzymes such as the Ada DNA repair protein was reproduced in tris(thioimidazolyl)borate-zinc-thiolate complexes Tti(R)Zn-SR'. Four different Tti(R) ligands and nine different thiolates were employed, yielding a total of 12 new complexes. In addition, one Tti(R)Zn-SH complex and two thiolate-bridged [Tti(R)-SEt-Tti(R)]+ complexes were obtained. A selection of six thiolate complexes was converted with methyl iodide to the corresponding methyl thioethers and Tti(R)Zn-I. According to a kinetic analysis these reactions are second-order processes, which implies that the alkylations are likely to occur at the zinc-bound thiolates. They are much faster than the alkylations of zinc thiolates with N3 or N2S tripod ligands. The most reactive thiolate, Tti(Xyl)Zn-SEt, reacts slowly with trimethyl phosphate in a nonpolar medium at room temperature, yielding methyl-ethyl-thioether and Tti(Xyl)Zn-OPO(OMe)2 which can be converted back to the thiolate complex with NaSEt. This is the closest reproduction of the Ada repair process so far.     

Universal Anticancer Cu(DTC)2 Discriminates between Thiols and Zinc(II) Thiolates Oxidatively

Aerobic organisms must rely on abundant intracellular thiols to reductively protect various vital functional units, especially ubiquitous zinc(II) thiolate sites of proteins, from deleterious oxidations resulting from oxidizing environments. Disclosed here is the first well‐defined model study for reactions between zinc(II) thiolate complexes and copper(II) complexes. Among all the studied ligands of copper(II), diethyldithiocarbamate (DTC) displays a unique redox‐tuning ability that enables copper(II) to resist the reduction by thiols while retaining its ability to oxidize zinc(II) thiolates to form disulfides. This work proves for the first time that it is possible to develop oxidants to discriminate between thiols and zinc(II) thiolates, alluding to a new chemical principle for how oxidants, especially universal anticancer Cu(DTC)₂, might circumvent the intracellular reductive defense around certain zinc(II) thiolate sites of proteins to kill malignant cells.     

Substituent effects on Ni-S bond dissociation energies and kinetic stability of nickel arylthiolate complexes supported by a bis(phosphinite)-based pincer ligand

Pincer complexes of the type [2,6-(R(2)PO)(2)C(6)H(3)]NiSC(6)H(4)Z (R = Ph and i-Pr; Z = p-OCH(3), p-CH(3), H, p-Cl, and p-CF(3)) have been synthesized from [2,6-(R(2)PO)(2)C(6)H(3)]NiCl and sodium arylthiolate. X-ray structure determinations of these thiolate complexes have shown a somewhat constant Ni-S bond length (approx. 2.20 ?) but an almost unpredictable orientation of the thiolate ligand. Equilibrium constants for various thiolate exchange (between a nickel thiolate complex and a free thiol, or between two different nickel thiolate complexes) reactions have been measured. Evidently, the thiolate ligand with an electron-withdrawing substituent prefers to bond with "[2,6-(Ph(2)PO)(2)C(6)H(3)]Ni" rather than "[2,6-(i-Pr(2)PO)(2)C(6)H(3)]Ni", and bonds least favourably with hydrogen. The reactions of the thiolate complexes with halogenated compounds such as PhCH(2)Br, CH(3)I, CCl(4), and Ph(3)CCl have been examined and several mechanistic pathways have been explored.     

Models of hypoxia activated prodrugs: Co(III) complexes of hydroxamic acids

Co(III) complexes of simple hydroxamic acids have been evaluated as models of hypoxia activated prodrugs containing MMP inhibitors. The complexes are based upon a proposed carrier system comprising the tripodal tetradentate ligand tris(2-methylpyridyl)amine (tpa) with the hydroxamate functionality occupying the remaining coordination sites of the Co centre. Acetohydroxamato (aha), propionhydroxamato (pha), and benzohydroxamato (bha) complexes were synthesised and characterised by single crystal X-ray diffraction. For aha and pha both the hydroxamato and hydroximato (deprotonated) forms were obtained and were readily interconverted by pH manipulation; for bha only the hydroximato complex was obtained as a stable species. Electrochemical analysis was used to probe the redox chemistry of the complexes and assess their ease of reduction. All of the complexes displayed irreversible reduction and had low cathodic peak potentials. This suggests that the Co-tpa carrier system would provide a suitably inert framework to deliver the drugs to target sites intact yet would release the ligands upon reduction to the more labile Co(II) oxidation state.     

Synthesis and structures of bis(dithiolene)tungsten(IV,VI) thiolate and selenolate complexes: approaches to the active sites of molybdenum and tungsten formate dehydrogenases

Formate dehydrogenases are molybdenum- or tungsten-containing enzymes that catalyze the oxidation of formate to carbon dioxide. Among the significant characteristics of the mononuclear active sites are coordination of two pyranopterindithiolene ligands and selenocysteinate to the metal in oxidation states IV-VI. The first detailed investigation of the synthesis and structures of bis(dithiolene)tungsten selenolate and analogous thiolate complexes of relevance to formate dehydrogenases has been undertaken. Some 17 complexes of the types [WIV(QR)(S2C2Me2)2]-, [WVIO(QR)(S2C2Me2)2]-, and [WVIS(QR)(S2C2Me2)2]- (Q = S, Se; R = tert-butyl, 1-adamantyl) and the desoxo species [WVI(SR)(OSiR'3)(S2C2Me2)2] (R' = Me, Ph) were prepared. Ten structures of representative members of these types were determined; WIV complexes are square-pyramidal and WVI complexes are six-coordinate, with geometries intermediate between octahedral and trigonal-prismatic. Selenolate complexes are less stable than similar thiolate species; decomposition products were identified as [WV2(mu2-Q)2(S2C2Me2)2]2- and [WIV,V2(mu2-Se)(S2C2Me2)4]-. The several [MoIV(QR)(S2C2Me2)2]- complexes prepared earlier and the tungsten compounds synthesized in this work form a family of molecules whose overall stereochemistry and metric features are those expected in the absence of protein structural constraints.     

Electronic structure of ferric heme nitrosyl complexes with thiolate coordination

The effect of trans thiolate ligation on the coordinated nitric oxide in ferric heme nitrosyl complexes as a function of the thiolate donor strength, induced by variation of NH-S(thiolate) hydrogen bonds, is explored. Density functional theory (DFT) calculations (BP86/TZVP) are used to define the electronic structures of corresponding six-coordinate ferric [Fe(P)(SR)(NO)] complexes. In contrast to N-donor-coordinated ferric heme nitrosyls, an additional Fe-N(O) sigma interaction that is mediated by the dz2/dxz orbital of Fe and a sigma*-type orbital of NO is observed in the corresponding complexes with S-donor ligands. Experimentally, this is reflected by lower nu(N-O) and nu(Fe-N) stretching frequencies and a bent Fe-N-O moiety in the thiolate-bound case.     

Reactions of [aryloxy(phenyl)carbene]pentacarbonylchromium(0) complexes with thiolate ions. Decreasing reactivity with increasing basicity of the nucleophile

A kinetic study of the reactions of thiolate ions with three Fischer-type [aryloxy(phenyl)carbene]pentacarbonyl chromium(0) complexes in 50% MeCN-50% water (v/v) is reported. Br?nsted plots of the second-order rate constants are biphasic with an initial steep rise for weakly basic thiolate ions (beta(nuc) approximately equal to 1.0) followed by a slightly descending leg with a negative slope (beta(nuc) approximately equal to -0.2) for strongly basic thiolate ions. This indicates a change from rate-limiting leaving group departure at low pK(RSH)(a) to rate-limiting nucleophilic attachment at high pK(RSH)(a). The negative beta(nuc) values result from a combination of minimal progress of C-S bond formation at the transition state and the requirement for partial desolvation of the nucleophile before it enters the transition state. Possible factors that may affect the degree of bond formation in reactions of Fischer carbene complexes as well as reactions of other unsaturated electrophiles with thiolate ions are discussed.     

Experimental and computational investigation of thiolate alkylation in Ni(II) and Zn(II) complexes: role of the metal on the sulfur nucleophilicity

The biologically relevant S-alkylation reactions of thiolate ligands bound to a transition metal ion were investigated with particular attention paid to the role of the metal identity: Zn(II) versus Ni(II). The reactivity of two mononuclear diamine dithiolate Zn and Ni complexes with CH(3)I was studied. With the [ZnL] complex (1) (LH(2) = 2,2'-(2,2'-bipyridine-6,6'-diyl)bis(1,1-diphenylethanethiolate)), a double S-methylation occurs leading to [ZnL(Me2)I(2)] (1(Me2)), while with [NiL] (2), only the mono-S-methylated product [NiL(Me)]I (2(Me)) is formed. Complexes 1 and 1(Me2) have been characterized by X-ray crystallography, while the structures of 2 and 2(Me) have been previously described. The kinetics of the first S-methylation reaction, investigated by (1)H NMR, is found to follow a second-order rate law, and the activation parameters, ΔH(?) and ΔS(?), are similar for both 1 and 2. S K-edge X-ray absorption spectroscopy measurements have been carried out on 1, 2, and 2(Me), and a TD-DFT approach was employed to interpret the data. The electronic structures of 1 and 2 calculated by DFT reveal that the thiolate-metal bond is predominantly ionic in 1 and covalent in 2. However, evaluation of the molecular electrostatic potential minima around the lone pairs of the thiolate sulfur atoms gives similar values for 1 and 2, suggesting a comparable nucleophilicity. The DFT-optimized structures of the mono-S-methylation products have been calculated for the Zn and Ni complexes. Molecular electrostatic potential analysis of these products shows that (i) the nucleophilicity of the remaining thiolate sulfur atom is partly quenched for the Ni complex while it is conserved in the Zn complex and, more importantly, (ii) that the accessibility for the methyl transfer agent to the remaining thiolate is favored for the mono-S-methylated Zn complex compared to the Ni one. This explains the absence of a double S-methylation process in the case of the Ni complex at room temperature.     

Oxidation of zinc-thiolate complexes of biological interest by hydrogen peroxide: a theoretical study

Zinc-thiolate complexes play a major structural and functional role in the living cell. Their stability is directly related to the thiolate reactivity toward reactive oxygen species naturally present in the cell. Oxidation of some zinc-thiolate complexes has a functional role, as is the case of zinc finger redox switches. Herein, we report a theoretical investigation on the oxidation of thiolate by hydrogen peroxide in zinc finger cores of CCCC, CCHC, and CCHH kinds containing either cysteine or histidine residues. In the case of the CCCC core, the calculated energy barrier for the oxidation to sulfenate of the complexed thiolate was found to be 16.0 kcal mol(-1), which is 2 kcal mol(-1) higher than that for the free thiolate. The energy barrier increases to 19.3 and 22.2 kcal mol(-1) for the monoprotonated and diprotonated CCCC cores, respectively. Substitution of cysteine by histidine also induces an increase in the magnitude of the reaction energy barrier: It becomes 20.0 and 20.9 kcal mol(-1) for the CCCH and CCHH cores, respectively. It is concluded that the energy barrier for the oxidation of zinc fingers is strictly dependent on the type of ligands coordinated to zinc and on the protonation state of the complex. These changes in the thiolate reactivity can be explained by the lowering of the nucleophilicity of complexed sulfur and by the internal reorganization of the complex (changes in the metal-ligand distances) upon oxidation. The next reaction steps subsequent to sulfenate formation are also considered. The oxidized thiolate (sulfenate) is predicted to dissociate very fast: For all complexes, the calculated dissociation energy barrier is lower than 3 kcal mol(-1). It is also shown that the dissociated sulfenic acid can interact with a free thiolate to form a sulfur-sulfur (SS) bridge in a reaction that is predicted to be quasi-diffusion limited. The interesting biological consequences of the modulation of thiolate reactivity by the chemical composition of the zinc finger cores are discussed.     

PtII diimine chromophores with perfluorinated thiolate ligands: nature and dynamics of the charge-transfer-to-diimine lowest excited state

The synthesis of new Pt(II) diimine complexes bearing perfluorinated thiolate ligands, Pt(II)(NN)(4-X-C(6)F(4)-S)(2), where NN = 2,2'-bipyridine or 1,10-phenanthroline and X = F or CN, is reported, together with an investigation of the nature and dynamics of their lowest excited states. A combined UV-vis, (spectro)electrochemical, resonance Raman, and time-resolved infrared (TRIR) study has suggested that the HOMO is mainly composed of thiolate(pi)/S(p)/Pt(d) orbitals and that the LUMO is largely localized on the pi*(diimine) orbital, thus revealing the [charge-transfer-to-diimine] nature of the lowest excited state. An enhancement of the thiolate ring vibrations, C-F vibrations, and the vibration of the CN-substituent on the thiolate moiety was observed in the resonance Raman spectra, whereas no such enhancement was seen for the nonfluorinated analogues. Thus, the introduction of fluorine substituents on the thiolate moiety probably leads to a more pronounced contribution of the intrathiolate modes to the HOMO compared to the analogous complexes with nonfluorinated thiolates. Furthermore, the introduction of the p-CN group into the thiolate moiety has allowed the dynamics of the lowest excited state of Pt(bpy)(4-CN-C(6)F(4)-S)(2) to be monitored by picosecond TRIR spectroscopy. The dynamics of the lowest [charge-transfer-to-diimine] excited state are governed by ca. 2-ps vibrational cooling and 35-ps back electron transfer.     

Azahelicenes from the Oxidative Photocyclization of Boron Hydroxamate Complexes

Aromatic hydroxamic acids (Ar–CO–NOH–Ar′) were used as bidentate chelating ligands to generate the corresponding boron hydroxamate complexes, which were subsequently transformed into nitrogen‐containing helicenes (azahelicenes) using an oxidative photocyclization method that is frequently used for stilbene‐type (Ar–CH=CH–Ar′) precursors of carbohelicenes. The nitrogen atom of the hydroxamate linker was thus directly embedded into the helicene core without using nitrogen‐containing aromatic rings in the stilbene‐type precursors. In a batch photoreaction, aza[4]helicenes were readily and efficiently prepared, but aza[6]helicenes underwent severe decomposition upon irradiation. Alternatively, a continuous flow photoreactor was employed to furnish an amide‐type aza[6]helicene.     

Complexes of Fe(III) and Ga(III) Derived from the Cyclic 6- and 7-Membered Hydroxamic Acids Found in Mixed Siderophores

Six- and seven-membered cyclic hydroxamic acids are found as terminal binding units in different families of siderophores, including exochelins and mycobactins. The simplest models of these preorganized chelating ligands were known, but their coordination chemistry with Fe³⁺, the target metal ion of siderophores, had never been reported. Four complexes were synthesized and studied: two Fe³⁺ complexes, one with the six-membered ring hydroxamate PIPO⁻ and one with the seven-membered ring hydroxamate AZEPO⁻, and the two corresponding Ga³⁺ complexes. X-ray diffraction studies showed that the interligand repulsion energies were better minimized in the case of the AZEPO⁻ complexes whatever the metal cation considered, and that the Fe−O bond distances were shorter in [Fe(AZEPO)₃] by comparison with [Fe(PIPO)₃].     

Synthesis and characterization of {Ni(NO)}10 and {Co(NO)2}10 complexes supported by thiolate ligands

Nitric oxide is an important molecule in biology and modulates a variety of physiological and pathophysiological processes. Some of its regulatory functions are exerted through interactions with redox-active elements, including iron, nickel, cobalt, and sulfur. Metalloenzymes containing [ nFe- nS] ( n = 2 or 4) clusters can be activated or inactivated by reaction with NO, affording dinitrosyl iron complexes. Studies of the NO chemistry of small-molecule iron thiolate complexes have provided insight into these biological processes and suggested probable intermediates. To explore this chemistry from a different perspective, we prepared nickel and cobalt thiolate complexes and investigated their reactions with NO and related compounds. We report here the first examples of anionic complexes containing {Ni(NO)} (10) and {Co(NO) 2} (10) units, the reactivity of which suggests possible intermediates in the interconversion of iron thiolate nitrosyl compounds. Our results demonstrate new chemistry involving NO and simple complexes of nickel and cobalt supported by thiolates, which have been known for more than 30 years. The use of mass balance methodology was key to their discovery. Among the novel complexes reported are (Et 4N) 2[Ni(NO)(SPh) 3] ( 2), from (Et 4N) 2[Ni(SPh) 4] ( 1) and NO, (Et 4N) 2[Ni 2(NO) 2(mu-SPh) 2(SPh) 2] ( 3), from 1 and NO (+) or 2 and Me 3O (+), (Et 4N)[Co(NO) 2(SPh) 2] ( 5), from (Et 4N) 2[Co(SPh) 4] ( 4) and NO, and [Co 3(NO) 6(mu-SPh) 3] ( 6), from 5 and Me 3O (+). In the syntheses of 2 and 5, NO could be replaced by the convenient solid Ph 3CSNO.     

Sulfur K-edge XAS and DFT studies on NiII complexes with oxidized thiolate ligands: implications for the roles of oxidized thiolates in the active sites of Fe and Co nitrile hydratase

S K-edge X-ray absorption spectroscopy data on a series of NiII complexes with thiolate (RS-) and oxidized thiolate (RSO2-) ligands are used to quantify Ni-S bond covalency and its change upon ligand oxidation. Analyses of these results using geometry-optimized density functional theory (DFT) calculations suggest that the Ni-S sigma bonds do not weaken on ligand oxidation. Molecular orbital analysis indicates that these oxidized thiolate ligands use filled high-lying S-O pi* orbitals for strong sigma donation. However, the RSO2- ligands are poor pi donors, as the orbital required for pi interaction is used in the S-O sigma-bond formation. The oxidation of the thiolate reduces the repulsion between electrons in the filled Ni t2 orbital and the thiolate out-of-plane pi-donor orbital leading to shorter Ni-S bond length relative to that of the thiolate donor. The insights obtained from these results are relevant to the active sites of Fe- and Co-type nitrile hydratases (Nhase) that also have oxidized thiolate ligands. DFT calculations on models of the active site indicate that whereas the oxidation of these thiolates has a major effect in the axial ligand-binding affinity of the Fe-type Nhase (where there is both sigma and pi donation from the S ligands), it has only a limited effect on the sixth-ligand-binding affinity of the Co-type Nhases (where there is only sigma donation). These oxidized residues may also play a role in substrate binding and proton shuttling at the active site.     

Multifunctional hydrolytic catalyses. IX. Hydrolysis of p-nitrophenyl acetate by bifunctional polymer catalysts containing zwitterionic (hydroxamate) nucleophiles

The hydrolysis of p-nitrophenyl acetate by bifunctional polymer catalysts was studied in 28.9% EtOH–H₂0 at 30°C. Partial (ca. 10 mole%) quaternization of poly(4-vinylpyridine) and poly(1-vinyl-2-ethylimidazole) with benzyl N-benzyl chloroacetohydroxamate and the subsequent removal of the benzyl group produced water-soluble polymers containing zwitterionic hydroxamate groups and free pyridine (or imidazole) groups. The zwitterionic hydroxamate was a nucleophile more than 10 times as effective as simple hydroxamate anions, and the acetyl hydroxamate intermediate was efficiently hydrolyzed by the intrapolymeric pyridine (or imidazole) group. Thus, the catalytic efficiency of these bifunctional polymers was much better than monofunctional polymers which contain either hydroxamate or imidazole, and amounted to 10–20% of that of α-chymotrypsin under a comparable condition.     

Collision-induced dissociation tandem mass spectrometry of desferrioxamine siderophore complexes from electrospray ionization of UO2(2+), Fe3+ and Ca2+ solutions

Desferrioxamine (DEF) is a trihydroxamate siderophore typical of those produced by bacteria and fungi for the purpose of scavenging Fe(3+) from environments where the element is in short supply. Since this class of molecules has excellent chelating properties, reaction with metal contaminants such as actinide species can also occur. The complexes that are formed can be mobile in the environment. Because the natural environment is extremely diverse, strategies are needed for the identification of metal complexes in aqueous matrices having a high degree of chemical heterogeneity, and electrospray ionization mass spectrometry (ESI-MS) has been highly effective for the characterization of siderophore-metal complexes. In this study, ESI-MS of solutions containing DEF and either UO(2)(2+), Fe(3+) or Ca(2+) resulted in generation of abundant singly charged ions corresponding to [UO(2)(DEF - H)](+), [Fe(DEF - 2H)](+) and [Ca(DEF - H)](+). In addition, less abundant doubly charged ions were produced. Mass spectrometry/mass spectrometry (MS/MS) studies of collision-induced dissociation (CID) reactions of protonated DEF and metal-DEF complexes were contrasted and rationalized in terms of ligand structure. In all cases, the most abundant fragmentation reactions involved cleavage of the hydroxamate moieties, consistent with the idea that they are most actively involved with metal complexation. Singly charged complexes tended to be dominated by cleavage of a single hydroxamate, while competitive fragmentation between two hydroxamate moieties increased when the doubly charged complexes were considered. Rupture of amide bonds was also observed, but these were in general less significant than the hydroxamate fragmentations. Several lower abundance fragmentations were unique to the metal examined: abundant loss of H(2)O occurred only for the singly charged UO(2)(2+) complex. Further, NH(3) was eliminated only from the singly charged Fe(3+) complex; this and fragmentation of C-C and C-N bonds derived from neither the hydroxamate nor the amide groups suggested that Fe(3+) insertion reactions were competing with ligand complexation. In no experiments were coordinating solvent molecules observed, attached either to the intact complexes or to the fragment ions, which indicated that both intact DEF and its fragments were occupying all of the coordination sites around the metal centers. This conclusion was based on previous experiments that showed that undercoordinated UO(2)(2+) and Fe(3+) readily added H(2)O and methanol in the ESI quadrupole ion trap mass spectrometer that was used in this study.     

Rational tuning of the thiolate donor in model complexes of superoxide reductase: direct evidence for a trans influence in Fe(III)-OOR complexes

Iron peroxide species have been identified as important intermediates in a number of nonheme iron as well as heme-containing enzymes, yet there are only a few examples of such species either synthetic or biological that have been well characterized. We describe the synthesis and structural characterization of a new series of five-coordinate (N4S(thiolate))Fe(II) complexes that react with tert-butyl hydroperoxide ((t)BuOOH) or cumenyl hydroperoxide (CmOOH) to give metastable alkylperoxo-iron(III) species (N4S(thiolate)Fe(III)-OOR) at low temperature. These complexes were designed specifically to mimic the nonheme iron active site of superoxide reductase, which contains a five-coordinate iron(II) center bound by one Cys and four His residues in the active form of the protein. The structures of the Fe(II) complexes are analyzed by X-ray crystallography, and their electrochemical properties are assessed by cyclic voltammetry. For the Fe(III)-OOR species, low-temperature UV-vis spectra reveal intense peaks between 500-550 nm that are typical of peroxide to iron(III) ligand-to-metal charge-transfer (LMCT) transitions, and EPR spectroscopy shows that these alkylperoxo species are all low-spin iron(III) complexes. Identification of the vibrational modes of the Fe(III)-OOR unit comes from resonance Raman (RR) spectroscopy, which shows nu(Fe-O) modes between 600-635 cm(-1) and nu(O-O) bands near 800 cm(-1). These Fe-O stretching frequencies are significantly lower than those found in other low-spin Fe(III)-OOR complexes. Trends in the data conclusively show that this weakening of the Fe-O bond arises from a trans influence of the thiolate donor, and density functional theory (DFT) calculations support these findings. These results suggest a role for the cysteine ligand in SOR, and are discussed in light of the recent assessments of the function of the cysteine ligand in this enzyme.