Alexandra V. Soldatova

New light on NO bonding in Fe(III) heme proteins from resonance Raman spectroscopy and DFT modeling.

Visible and ultraviolet resonance Raman (RR) spectra are reported for Fe(III)(NO) adducts of myoglobin variants with altered polarity in the distal heme pockets. The stretching frequencies of the Fe(III)-NO and N-O bonds, nu(FeN) and nu(NO), are negatively correlated, consistent with backbonding. However, the correlation shifts to lower nu(NO) for variants lacking a distal histidine. DFT modeling reproduces the shifted correlations and shows the shift to be associated with the loss of a lone-pair donor interaction from the distal histidine that selectively strengthens the N-O bond. However, when the model contains strongly electron-withdrawing substituents at the heme beta-positions, nu(FeN) and nu(NO) become positively correlated. This effect results from Fe(III)-N-O bending, which is induced by lone-pair donation to the N(NO) atom. Other mechanisms for bending are discussed, which likewise lead to a positive nu(FeN)/nu(NO) correlation, including thiolate ligation in heme proteins and electron-donating meso-substituents in heme models. The nu(FeN)/nu(NO) data for the Fe(III) complexes are reporters of heme pocket polarity and the accessibility of lone pair, Lewis base donors. Implications for biologically important processes, including NO binding, reductive nitrosylation, and NO reduction, are discussed.

Protein Dynamics from Time-Resolved UV Raman Spectroscopy

Raman spectroscopy can provide unique information on the evolution of structure in proteins over a wide range of time scales; the picosecond to millisecond range can be accessed with pump-probe techniques. Specific parts of the molecule are interrogated by tuning the probe laser to a resonant electronic transition, including the UV transitions of aromatic residues and of the peptide bond. Advances in laser technology have enabled the characterization of transient species at an unprecedented level of structural detail. Applications to protein unfolding and allostery are reviewed.

Mn(II,III) oxidation and MnO2 mineralization by an expressed bacterial multicopper oxidase

Reactive Mn(IV) oxide minerals are ubiquitous in the environment and control the bioavailability and distribution of many toxic and essential elements and organic compounds. Their formation is thought to be dependent on microbial enzymes, because spontaneous Mn(II) to Mn(IV) oxidation is slow. Several species of marine Bacillus spores oxidize Mn(II) on their exosporium, the outermost layer of the spore, encrusting them with Mn(IV) oxides. Molecular studies have identified the mnx (Mn oxidation) genes, including mnxG, encoding a putative multicopper oxidase (MCO), as responsible for this two-electron oxidation, a surprising finding because MCOs only catalyze single-electron transfer reactions. Characterization of the enzymatic mechanism has been hindered by the lack of purified protein. By purifying active protein from the mnxDEFG expression construct, we found that the resulting enzyme is a blue (absorption maximum 590 nm) complex containing MnxE, MnxF, and MnxG proteins. Further, by analyzing the Mn(II)- and (III)-oxidizing activity in the presence of a Mn(III) chelator, pyrophosphate, we found that the complex facilitates both electron transfers from Mn(II) to Mn(III) and from Mn(III) to Mn(IV). X-ray absorption spectroscopy of the Mn mineral product confirmed its similarity to Mn(IV) oxides generated by whole spores. Our results demonstrate that Mn oxidation from soluble Mn(II) to Mn(IV) oxides is a two-step reaction catalyzed by an MCO-containing complex. With the purification of active Mn oxidase, we will be able to uncover its mechanism, broadening our understanding of Mn mineral formation and the bioinorganic capabilities of MCOs.

The molecular biogeochemistry of manganese(II) oxidation

Micro-organisms capable of oxidizing the redox-active transition metal manganese play an important role in the biogeochemical cycle of manganese. In the present mini-review, we focus specifically on Mn(II)-oxidizing bacteria. The mechanisms by which bacteria oxidize Mn(II) include a two-electron oxidation reaction catalysed by a novel multicopper oxidase that produces Mn(IV) oxides as the primary product. Bacteria also produce organic ligands, such as siderophores, that bind to and stabilize Mn(III). The realization that this stabilized Mn(III) is present in many environments and can affect the redox cycles of other elements such as sulfur has made it clear that manganese and the bacteria that oxidize it profoundly affect the Earths biogeochemistry.

Near-infrared-emitting phthalocyanines. A combined experimental and density functional theory study of the structural, optical, and photophysical properties of Pd(II) and Pt(II) α-butoxyphthalocyanines.

The structural, optical, and photophysical properties of 1,4,8,11,15,18,22,25-octabutoxyphthalocyaninato-palladium(II), PdPc(OBu)(8), and the newly synthesized platinum analogue PtPc(OBu)(8) are investigated combining X-ray crystallography, static and transient absorption spectroscopy, and relativistic zeroth-order regular approximation (ZORA) Density Functional Theory (DFT)/Time Dependent DFT (TDDFT) calculations where spin-orbit coupling (SOC) effects are explicitly considered. The results are compared to those previously reported for NiPc(OBu)(8) (J. Phys. Chem. A 2005, 109, 2078) in an effort to highlight the effect of the central metal on the structural and photophysical properties of the group 10 transition metal octabutoxyphthalocyanines. Different from the nickel analogue, PdPc(OBu)(8) and PtPc(OBu)(8) show a modest and irregular saddling distortion of the macrocycle, but share with the first member of the group similar UV-vis spectra, with the deep red and intense Q-band absorption experiencing a blue shift down the group, as observed in virtually all tetrapyrrolic complexes of this triad. The blue shift of the Q-band along the MPc(OBu)(8) (M = Ni, Pd, Pt) series is interpreted on the basis of the metal-induced electronic structure changes. Besides the intense deep red absorption, the title complexes exhibit a distinct near-infrared (NIR) absorption due to a transition to the double-group 1E (π,π*) state, which is dominated by the lowest single-group (3)E (π,π*) state. Unlike NiPc(OBu)(8), which is nonluminescent, PdPc(OBu)(8) and PtPc(OBu)(8) show both deep red fluorescence emission and NIR phosphorescence emission. Transient absorption experiments and relativistic spin-orbit TDDFT calculations consistently indicate that fluorescence and phosphorescence emissions occur from the S(1)(π,π*) and T(1)(π,π*) states, respectively, the latter being directly populated from the former, and the triplet state decays directly to the S(0) surface (the triplet lifetime in deaerated benzene solution was 3.04 μs for Pd and 0.55 μs for Pt). Owing to their triplet properties, PdPc(OBu)(8) and PtPc(OBu)(8) have potential for use in photodynamic therapy (PDT) and are potential candidates for NIR light emitting diodes or NIR emitting probes.

Multicopper oxidase involvement in both Mn(II) and Mn(III) oxidation during bacterial formation of MnO2

Global cycling of environmental manganese requires catalysis by bacteria and fungi for MnO2 formation, since abiotic Mn(II) oxidation is slow under ambient conditions. Genetic evidence from several bacteria indicates that multicopper oxidases (MCOs) are required for MnO2 formation. However, MCOs catalyze one-electron oxidations, whereas the conversion of Mn(II) to MnO2 is a two-electron process. Trapping experiments with pyrophosphate (PP), a Mn(III) chelator, have demonstrated that Mn(III) is an intermediate in Mn(II) oxidation when mediated by exosporium from the Mn-oxidizing bacterium Bacillus SG-1. The reaction of Mn(II) depends on O2 and is inhibited by azide, consistent with MCO catalysis. We show that the subsequent conversion of Mn(III) to MnO2 also depends on O2 and is inhibited by azide. Thus, both oxidation steps appear to be MCO-mediated, likely by the same enzyme, which is indicated by genetic evidence to be the MnxG gene product. We propose a model of how the manganese oxidase active site may be organized to couple successive electron transfers to the formation of polynuclear Mn(IV) complexes as precursors to MnO2 formation.

Soluble guanylate cyclase is activated differently by excess NO and by YC-1: Resonance Raman spectroscopic evidence

Modulation of soluble guanylate cyclase (sGC) activity by nitric oxide (NO) involves two distinct steps. Low-level activation of sGC is achieved by the stoichiometric binding of NO (1-NO) to the heme cofactor, while much higher activation is achieved by the binding of additional NO (xsNO) at a non-heme site. Addition of the allosteric activator YC-1 to the 1-NO form leads to activity comparable to that of the xsNO state. In this study, the mechanisms of sGC activation were investigated using electronic absorption and resonance Raman (RR) spectroscopic methods. RR spectroscopy confirmed that the 1-NO form contains five-coordinate NO-heme and showed that the addition of NO to the 1-NO form has no significant effect on the spectrum. In contrast, addition of YC-1 to either the 1-NO or xsNO forms alters the RR spectrum significantly, indicating a protein-induced change in the heme geometry. This change in the heme geometry was also observed when BAY 41-2272 was added to the xsNO form. Bands assigned to bending and stretching motions of the vinyl and propionate substituents undergo changes in intensity in a pattern suggesting altered tilting of the pyrrole rings to which they are attached. In addition, the N-O stretching frequency increases, with no change in the Fe-NO stretching frequency, an effect modeled via DFT calculations as resulting from a small opening of the Fe-N-O angle. These spectral differences demonstrate different mechanisms of activation by synthetic activators, such as YC-1 and BAY 41-2272, and excess NO.

Photophysical Behavior of Open-Shell, First-Row Transition Metal meso-Tetraphenyltetrabenzoporphyrins: Insights from Experimental and Theoretical Studies†

The ground and excited state properties of two metallo-tetraphenyltetrabenzoporphyrins (MTPTBP) have been investigated by a combination of DFT/TDDFT and transient absorption spectrometry to draw a complete picture of the excited state deactivation. The Cu(II) and Co(II) complexes were chosen to investigate the impact of the half-filled d orbitals on the photophysical properties of the tetrapyrrole macrocycle. The first observed transient in CuTPTBP was assigned to the triplet state that equilibrated with a ligand-to-metal charge transfer (LMCT) state. Ground state repopulation, completed within 53 ps, occurred via a lower-lying LMCT state. The dependence of the observed lifetime on solvent polarity confirmed the participation of the LMCT state in the overall deactivation process. For CoTPTBP, the first observed transient, a π-localized triplet state, converted to a hot d,d state, wherein intramolecular cooling occurred and completed within 3 ps. The cooled d,d state decayed to the ground state in an exponential manner with a 17 ps lifetime.

Photophysical behavior of open-shell first-row transition-metal octabutoxynaphthalocyanines: CoNc(OBu)8 and CuNc(OBu)8 as case studies.

Ultrafast photodynamics and density functional theory/time-dependent density functional theory (DFT/TDDFT) results for complexes of Co and Cu with 5,9,14,18,23,27,32,36-octabutoxynaphthalocyanine [CoNc(OBu)8 and CuNc(OBu)8] are reported. As a basis for this work, details concerning the syntheses of these complexes and the corresponding Zn complex (used as a reference) are given. Transient absorption spectrometry with femtosecond time resolution combined with a detailed DFT/TDDFT analysis has been employed to construct a complete picture of the excited-state dynamics after Q-band excitation of the Co and Cu complexes and to gain an understanding of the relationship between the nature of the metal center and the excited-state lifetime. The Co complex was shown to return to its ground state via two competing channels: a (2)T1(pi, pi*) state that decayed with a lifetime of 1 ps and a low-lying (2)(d, d) state that repopulated the ground-state surface with a lifetime of 15 ps. CuNc(OBu)8 showed ground-state repopulation from the (2)T1(pi, pi*) state via a lower-lying ligand-to-metal charge-transfer (LMCT) state that was completed within a few nanoseconds. The photophysical behavior of the cobalt and copper complexes was compared to that previously reported for the nickel analog in an effort to highlight the effect of the central metal on the nature and rates of the deactivation pathways. The results described in this work provide basic knowledge that is relevant to the use of these compounds as photothermal sensitizers in cancer therapy.

Ultrafast Charge Transfer in Nickel Phthalocyanine Probed by Femtosecond Raman-Induced Kerr Effect Spectroscopy

The recently developed technique of femtosecond stimulated Raman spectroscopy, and its variant, femtosecond Raman-induced Kerr effect spectroscopy (FRIKES), offer access to ultrafast excited-state dynamics via structurally specific vibrational spectra. We have used FRIKES to study the photoexcitation dynamics of nickel(II) phthalocyanine with eight butoxy substituents, NiPc(OBu)8. NiPc(OBu)8 is reported to have a relatively long-lived ligand-to-metal charge-transfer (LMCT) state, an essential characteristic for efficient electron transfer in photocatalysis. Following photoexcitation, vibrational transitions in the FRIKES spectra, assignable to phthalocyanine ring modes, evolve on the femtosecond to picosecond time scales. Correlation of ring core size with the frequency of the ν10 (asymmetric C–N stretching) mode confirms the identity of the LMCT state, which has a ∼500 ps lifetime, as well as that of a precursor d-d excited state. An even earlier (∼0.2 ps) transient is observed and tentatively assigned to a higher-lying Jahn–Teller-active LMCT state. This study illustrates the power of FRIKES spectroscopy in elucidating ultrafast molecular dynamics.