Triphenylsulfonium topophotochemistry.

The products from the 193 nm irradiation of triphenylsulfonium nonaflate (TPS) embedded in a poly(methyl methacrylate) (PMMA) film have been characterized. The analysis of the photoproduct formation was performed using chromatographic techniques including HPLC, GPC and GC-MS as well as UV-vis and NMR spectroscopic methods. Two previously unreported TPS photoproducts, triphenylene and dibenzothiophene, were detected; additionally, GPC and DOSY-NMR spectroscopic analyses after irradiation suggested that TPS fragments had been incorporated into the polymer film. The irradiation of acetonitrile solutions containing 10% w/v PMMA and 1% w/v TPS in a 1 cm-path-length cuvette showed only a trace amount of triphenylene or dibenzothiophene, indicating that topochemical factors were important for the formation of these molecules. The accumulated evidence indicates that both products were formed by in-cage, secondary photochemical reactions: 2-(phenylthio)biphenyl to triphenylene, and diphenylsulfide to dibenzothiophene.

Chromium corroles in four oxidation States.

We have isolated and characterized chromium complexes of 5,10,15-tris(pentafluorophenyl)corrole [(tpfc)H(3)] (1) in four oxidation states: [(tpfc(*))CrO][SbCl(6)] (6); [(tpfc)CrO] (2); [(tpfc)CrO][Cp(2)Co] (4); and [(tpfc)Cr(py)(2)] (3). Complex 6 was prepared both by electrochemical and chemical oxidation of 2; its formulation as a Cr(V)O ligand-radical species is based on UV-visible absorption as well as EPR measurements. Cobaltocene reduction of 2 gave 4; it was identified as a diamagnetic d(2) Cr(IV)O complex from its sharp (1)H NMR spectrum. Reaction of 2 with triphenylphosphine yielded a chromium(III) corrole, [(tpfc)Cr(OPPh(3))(2)] (5). Owing to its air sensitivity, 5 could not be isolated in the absence of excess OPPh(3). The structure of the Cr(III) bis-pyridine complex (3) was determined by X-ray crystallography (Cr-N distances: 1.926-1.952 A, pyrrole; 2.109, 2.129 A, pyridine).

Electron tunneling in single crystals of Pseudomonas aeruginosa azurins.

Rates of reduction of Os(III), Ru(III), and Re(I) by Cu(I) in His83-modified Pseudomonas aeruginosa azurins (M-Cu distance approximately 17 A) have been measured in single crystals, where protein conformation and surface solvation are precisely defined by high-resolution X-ray structure determinations: 1.7(8) x 10(6) s(-1) (298 K), 1.8(8) x 10(6) s(-1) (140 K), [Ru(bpy)2(im)(3+)-]; 3.0(15) x 10(6) s(-1) (298 K), [Ru(tpy)(bpy)(3+)-]; 3.0(15) x 10(6) s(-1) (298 K), [Ru(tpy)(phen)(3+)-]; 9.0(50) x 10(2) s(-1) (298 K), [Os(bpy)2(im)(3+)-]; 4.4(20) x 10(6) s(-1) (298 K), [Re(CO)3(phen)(+)] (bpy = 2,2'-bipyridine; im = imidazole; tpy = 2,2':6',2' '-terpyridine; phen = 1,10-phenanthroline). The time constants for electron tunneling in crystals are roughly the same as those measured in solution, indicating very similar protein structures in the two states. High-resolution structures of the oxidized (1.5 A) and reduced (1.4 A) states of Ru(II)(tpy)(phen)(His83)Az establish that very small changes in copper coordination accompany reduction but reveal a shorter axial interaction between copper and the Gly45 peptide carbonyl oxygen [2.6 A for Cu(II)] than had been recognized previously. Although Ru(bpy)2(im)(His83)Az is less solvated in the crystal, the reorganization energy for Cu(I) --> Ru(III) electron transfer falls in the range (0.6-0.8 eV) determined experimentally for the reaction in solution. Our work suggests that outer-sphere protein reorganization is the dominant activation component required for electron tunneling.

Axial ligand modulation of the electronic structures of binuclear copper sites: analysis of paramagnetic 1H NMR spectra of Met160Gln Cu(A).

Cu(A) is an electron-transfer copper center present in heme-copper oxidases and N2O reductases. The center is a binuclear unit, with two cysteine ligands bridging the metal ions and two terminal histidine residues. A Met residue and a peptide carbonyl group are located on opposite sides of the Cu2S2 plane; these weaker ligands are fully conserved in all known Cu(A) sites. The Met160Gln mutant of the soluble subunit II of Thermus thermophilus ba3 oxidase has been studied by NMR spectroscopy. In its oxidized form, the binuclear copper is a fully delocalized mixed-valence pair, as are all natural Cu(A) centers. The faster nuclear relaxation in this mutant suggests that a low-lying excited state has shifted to higher energies compared to that of the wild-type protein. The introduction of the Gln residue alters the coordination mode of His114 but does not affect His157, thereby confirming the proposal that the axial ligand-to-copper distances influence the copper-His interactions (Robinson, H.; Ang, M. C.; Gao, Y. G.; Hay, M. T.; Lu, Y.; Wang, A. H. Biochemistry 1999, 38, 5677). Changes in the hyperfine coupling constants of the Cys beta-CH2 groups are attributed to minor geometrical changes that affect the Cu-S-C(beta)-H(beta) dihedral angles. These changes, in addition, shift the thermally accessible excited states, thus influencing the spectral position of the Cys beta-CH2 resonances. The Cu-Cys bonds are not substantially altered by the Cu-Gln160 interaction, in contrast to the situation found in the evolutionarily related blue copper proteins. It is possible that regulatory subunits in the mitochondrial oxidases fix the relative positions of thermally accessible Cu(A) excited states by tuning axial ligand interactions.

Probing the open state of cytochrome P450cam with ruthenium-linker substrates.

Cytochromes P450 play key roles in drug metabolism and disease by oxidizing a wide variety of natural and xenobiotic compounds. High-resolution crystal structures of P450cam bound to ruthenium sensitizer-linked substrates reveal an open conformation of the enzyme that allows substrates to access the active center via a 22-A deep channel. Interactions of alkyl and fluorinated biphenyl linkers with the channel demonstrate the importance of exploiting protein dynamics for specific inhibitor design. Large changes in peripheral enzyme structure (F and G helices) couple to conformational changes in active center residues (I helix) implicated in proton pumping and dioxygen activation. Common conformational states among P450cam and homologous enzymes indicate that static and dynamic variability in the F/G helix region allows the 54 human P450s to oxidize thousands of substrates.

Cytochrome c' folding triggered by electron transfer: fast and slow formation of four-helix bundles.

Reduced (Fe(II)) Rhodopseudomonas palustris cytochrome c' (Cyt c') is more stable toward unfolding ([GuHCl](1/2) = 2.9(1) M) than the oxidized (Fe(III)) protein ([GuHCl](1/2) = 1.9(1) M). The difference in folding free energies (Delta Delta G(f) degrees = 70 meV) is less than half of the difference in reduction potentials of the folded protein (100 mV vs. NHE) and a free heme in aqueous solution ( approximately -150 mV). The spectroscopic features of unfolded Fe(II)-Cyt c' indicate a low-spin heme that is axially coordinated to methionine sulfur (Met-15 or Met-25). Time-resolved absorption measurements after CO photodissociation from unfolded Fe(II)(CO)-Cyt c' confirm that methionine can bind to the ferroheme on the microsecond time scale [k(obs) = 5(2) x 10(4) s(-1)]. Protein folding was initiated by photoreduction (two-photon laser excitation of NADH) of unfolded Fe(III)-Cyt c' ([GuHCl] = 2.02--2.54 M). Folding kinetics monitored by heme absorption span a wide time range and are highly heterogeneous; there are fast-folding ( approximately 10(3) s(-1)), intermediate-folding (10(2)-10(1) s(-1)), and slow-folding (10(-1) s(-1)) populations, with the last two likely containing methionine-ligated (Met-15 or Met-25) ferrohemes. Kinetics after photoreduction of unfolded Fe(III)-Cyt c' in the presence of CO are attributable to CO binding [1.4(6) x 10(3) s(-1)] and Fe(II)(CO)-Cyt c' folding [2.8(9) s(-1)] processes; stopped-flow triggered folding of Fe(III)-Cyt c' (which does not contain a protein-derived sixth ligand) is adequately described by a single kinetics phase with an estimated folding time constant of approximately 4 ms [Delta G(f) degrees = -33(3) kJ mol(-1)] at zero denaturant.

Electron tunneling in protein crystals.

The current understanding of electron tunneling through proteins has come from work on systems where donors and acceptors are held at fixed distances and orientations. The factors that control electron flow between proteins are less well understood, owing to uncertainties in the relative orientations and structures of the reactants during the very short time that tunneling occurs. As we report here, the way around such structural ambiguity is to examine oxidation-reduction reactions in protein crystals. Accordingly, we have measured and analyzed the kinetics of electron transfer between native and Zn-substituted tuna cytochrome c (cyt c) molecules in crystals of known structure. Electron transfer rates [(320 s(-1) for *Zn-cyt c --> Fe(III)-cyt c; 2000 s(-1) for Fe(II)-cyt c --> Zn-cyt c(+))] over a Zn-Fe distance of 24.1 A closely match those for intraprotein electron tunneling over similar donor-acceptor separations. Our results indicate that van der Waals interactions and water-mediated hydrogen bonds are effective coupling elements for tunneling across a protein-protein interface.

Copper coordination in blue proteins.

The spectroscopic and electrochemical properties of blue copper proteins are strikingly different from those of inorganic copper complexes in aqueous solution. Over three decades ago this unusual behavior was ascribed to constrained coordination in the folded protein; consistent with this view, crystal structure determinations of blue proteins have demonstrated that the ligand positions are essentially unchanged on reduction as well as in the apoprotein. Blue copper reduction potentials are tuned to match the particular function of a given protein by exclusion of water from the metal site and strict control of the positions of axial ligands in the folded structure. Extensive experimental work has established that the reorganization energy of a prototypal protein, Pseudomonas aeruginosa azurin, is approximately 0.7 eV, a value that is much lower than those of inorganic copper complexes in aqueous solution. The lowered reorganization energy in the protein, which is attributable to constrained coordination, is critically important for function, since the driving forces for electron transfer often are low (approximately 0.1 eV) between blue copper centers and distant (>10 A) donors and acceptors.

Enantiomeric discrimination of Ru-substrates by cytochrome P450cam.

Molecules with photosensitizers attached to substrates (Wilker et al., Angew. Chem. Int. Ed. 38 (1999) 90-92) or cofactors (Hamachi et al., J. Am. Chem. Soc. 121 (1999) 5500-5506) can rapidly deliver redox equivalents to buried active sites. The structure of cytochrome P450cam (P450) co-crystallized with a prototypal sensitizer-substrate, [Ru-C9-Ad]Cl2, has been determined (Dmochowski et al., Proc. Natl. Acad. Sci. USA 96 (1999) 12987-12990); and, in separate UV-vis absorption and time-resolved luminescence experiments, the binding of the lambda and delta enantiomers of Ru-C9-Ad to P450 has been measured. The results, KD(delta/lambda) approximately 2, indicate that the bipyridyl ligands of the lambda isomer interact more favorably with hydrophobic residues at the entrance to the substrate channel. We conclude that enantiospecific interactions may be exploited in the design of enzyme-metallosubstrate conjugates.

Stabilization of high-valent metals by corroles: oxo[tris(pentafluorophenyl)corrolato]chromium(V).

The aerobic reaction of Cr(CO)6 with tris(pentafluorophenyl)corrole (H3(TpFPC)) in toluene gives the dark red oxochromium(V) compound (TpFPC)Cr(O), which has been characterized by X-ray crystallography, electrochemistry, and EPR spectroscopy. Short Cr-N (1.927-1.943 A) bonds as well as relatively large 14N and small 53Cr coupling constants suggest that sigma (N-->Cr) donation is responsible for the unusual stability of chromium(V) in this complex. The CrV/IV reduction potential (0.11 V vs Ag/AgCl) is 0.65 V below that of oxo(tetramesitylporphinato)chromium(V).

Lowest electronic excited states of platinum(II) diimine complexes.

Absorption and emission spectra of Pt(diimine)L2 complexes (diimine = 2,2'-bipyridine (bpy) or 4,4'-dimethyl-2,2'-bipyridine (dmbpy); L = pyrazolate (pz-), 3,5-dimethylpyrazolate (dmpz-), or 3,4,5-trimethylpyrazolate (tmpz-)) have been measured. Solvent-sensitive absorption bands (370-440 nm) are attributed to spin-allowed metal-to-ligand charge-transfer (1MLCT) transitions. As solids and in 77 K glassy solution, Pt(bpy)(pz)2 and Pt(dmbpy)(pz)2 exhibit highly structured emission systems (lambda max approximately 494 nm) similar to those of the diprotonated forms of these complexes. The highly structured bands (spacings 1000-1400 cm-1) indicate that the transition originates in a diimine-centered 3(pi-->pi*) (3LL) excited state. The intense solid-state and 77 K glassy solution emissions from 3MLCT[d(Pt)-->pi*(bpy)] excited states of complexes with dmpz- and tmpz- ligands occur at longer wavelengths (lambda max = 500-610 nm), with much broader vibronic structure. These findings are consistent with increasing electron donation of the pyrazolate ligands, leading to a distinct crossover from a lowest 3LL to a 3MLCT excited state.

Optical detection of cytochrome P450 by sensitizer-linked substrates.

The ability to detect, characterize, and manipulate specific biomolecules in complex media is critical for understanding metabolic processes. Particularly important targets are oxygenases (cytochromes P450) involved in drug metabolism and many disease states, including liver and kidney dysfunction, neurological disorders, and cancer. We have found that Ru photosensitizers linked to P450 substrates specifically recognize submicromolar cytochrome P450(cam) in the presence of other heme proteins. In the P450:Ru-substrate conjugates, energy transfer to the heme dramatically accelerates the Ru-luminescence decay. The crystal structure of a P450(cam):Ru-adamantyl complex reveals access to the active center via a channel whose depth (Ru-Fe distance is 21 A) is virtually the same as that extracted from an analysis of the energy-transfer kinetics. Suitably constructed libraries of sensitizer-linked substrates could be employed to probe the steric and electronic properties of buried active sites.

The red form of [Re(phen)(CO)3(H2O)]-CF3SO3.H2O.

The coordination geometry of the cations in the red form of aquatricarbonyl(1,10-phenanthroline-N,N')rhenium(I) trifluoromethanesulfonate hydrate, [Re(C12H8N2)(CO)3-(H2O)]CF3SO3.H2O, is approximately octahedral, with a facial arrangement of the linearly coordinated carbonyl ligands. The phenanthroline (phen) ligands interleave to form a columnar pi-stacked structure.

Inhibition of thermolysin and human alpha-thrombin by cobalt(III) Schiff base complexes.

Cobalt(III) Schiff base complexes have been shown to inhibit the replication of the ocular herpes virus. It is well known that these complexes have a high affinity for nitrogenous donors such as histidine residues, and it is possible that they bind to (and inhibit) an enzyme that is crucial to viral replication. In model studies, we have found that [Co(acacen)(NH3)2]+ is an effective irreversible inhibitor of thermolysin at millimolar concentrations; it also inhibits human alpha-thrombin. Axial ligand exchange with an active-site histidine is the proposed mechanism of inhibition. The activity of thermolysin and thrombin can be protected by binding a reversible inhibitor to the active site before addition of the cobalt(III) complex.

Cytochrome b562 folding triggered by electron transfer: approaching the speed limit for formation of a four-helix-bundle protein.

Ferrocytochrome b562 [Fe(II)cyt b562] folding can be triggered by photoinduced electron transfer to unfolded Fe(III)cyt b562 in 2-3 M guanidine hydrochloride solutions. The folding rates increase with decreasing guanidine hydrochloride; the extrapolated time constant for this folding process in the absence of denaturant (5 micros) is near the predicted value for intrachain diffusion. The relatively smooth energy landscape indicated for Fe(II)cyt b562 folding accords with the helical, highly symmetrical structure of the protein.

Structures of ruthenium-modified Pseudomonas aeruginosa azurin and [Ru(2,2'-bipyridine)2(imidazole)2]SO4 x 10H2O.

The crystal structure of Ru(2, 2'-bipyridine)2(imidazole)(His83)azurin (RuAz) has been determined to 2.3 A -resolution by X-ray crystallography. The spectroscopic and thermodynamic properties of both the native protein and [Ru(2, 2'-bipyridine)2(imidazole)2]2+ are maintained in the modified protein. Dark-green RuAz crystals grown from PEG 4000, LiNO3, CuCl2 and Tris buffer are monoclinic, belong to the space group C2 and have cell parameters a = 100.6, b = 35.4, c = 74.7 A and beta = 106. 5 degrees. In addition, [Ru(2,2'-bipyridine)2(imidazole)2]SO4 x 10H2O was synthesized, crystallized and structurally characterized by X-ray crystallography. Red-brown crystals of this complex are monoclinic, space group P21/n, unit-cell parameters a = 13.230 (2), b = 18.197 (4), c = 16.126 (4) A, beta = 108.65 (2) degrees. Stereochemical parameters for the refinement of Ru(2, 2'-bipyridine)2(imidazole)(His83) were taken from the atomic coordinates of [Ru(2,2'-bipyridine)2(imidazole)2]2+. The structure of RuAz confirms that His83 is the only site of chemical modification and that the native azurin structure is not perturbed significantly by the ruthenium label.

Role of ligand substitution in ferrocytochrome c folding.

The ligand substitutions that occur during the folding of ferrocytochrome c [Fe(II)cyt c] have been monitored by transient absorption spectroscopy. The folding reaction was triggered by photoinduced electron transfer to unfolded Fe(III)cyt c in guanidine hydrochloride (GuHCl) solutions. Assignments of ligation states were made by reference to the spectra of the imidazole and methionine adducts of N-acetylated microperoxidase 8. At pH 7, the heme in unfolded Fe(II)cyt c is ligated by native His18 and HisX (X = 26, 33) residues. The native Met80 ligand displaces HisX only in the last stages of folding. The ferroheme is predominantly five-coordinate in acidic solution; it remains five-coordinate until the native methionine binds the heme to give the folded protein (the rate of the methionine binding step is 16 +/- 5 s-1 at pH 5, 3.2 M GuHCl). The evidence suggests that the substitution of histidine by methionine is strongly coupled to backbone folding.

High-potential states of blue and purple copper proteins.

Electrochemical measurements show that there are high-potential states of two copper proteins, Pseudomonas aeruginosa azurin and Thermus thermophilus CuA domain; these perturbed states are formed in guanidine hydrochloride (GuHCl) solution in which the proteins are still blue (azurin) and purple (CuA). In each case, the high-potential state forms reversibly. Absorption (azurin, CuA), visible circular dichroism (azurin, CuA), resonance-Raman (CuA), and EPR (CuA) spectra indicate that the structure of the oxidized copper site of each high-potential form is very similar to that of the native protein. It is proposed that GuHCl perturbs one or more H-bonds in the blue or purple copper active site, thereby allowing Cu(I) to adopt a more favorable coordination structure than that in the rigid cavity of the native protein.