Isotropic Proton Hyperfine Coupling Constants Research Articles

Is Deprotonation of the Oxygen-Evolving Complex of Photosystem II during the S1 → S2 Transition Suppressed by Proton Quantum Delocalization?

We address the protonation state of the water-derived ligands in the oxygen-evolving complex (OEC) of photosystem II (PSII), prepared in the S2 state of the Kok cycle. We perform quantum mechanics/molecular mechanics calculations of isotropic proton hyperfine coupling constants, with direct comparisons to experimental data from two-dimensional hyperfine sublevel correlation (HYSCORE) spectroscopy and extended X-ray absorption fine structure (EXAFS). We find a low-barrier hydrogen bond with significant delocalization of the proton shared by the water-derived ligand, W1, and the aspartic acid residue D1-D61 of the D1 polypeptide. The lowering of the zero-point energy of a shared proton due to quantum delocalization precludes its release to the lumen during the S1→ S2 transition. Retention of the proton facilitates the shuttling of a proton during the isomerization of the tetranuclear manganese-calcium-oxo (Mn4Ca-oxo) cluster, from the "open" to "closed" conformation, a step suggested to be necessary for oxygen evolution from previous studies. Our findings suggest that quantum-delocalized protons, stabilized by low-barrier hydrogen bonds in model catalytic systems, can facilitate the accumulation of multiple oxidizing equivalents at low overpotentials.

Magnetic Resonance Studies and Molecular Orbital Calculations on the Doublet and Triplet States of Bacteriopurpurin: a Potential Second-Generation Photosensitizer for Photodynamic Therapy

Bacteriopurpurin is a macrocyclic compound at the saturation level of bacteriochlorins that was specifically synthesized as a potential photosensitizer in photodynamic therapy. It exhibits a long-wavelength Qy absorption band at 843 nm, which is a necessary prerequisite for a deep tissue penetration. Magnetic resonance spectroscopic properties of bacteriopurpurin have been examined by fluorescence-detected magnetic resonance (FDMR) and electron paramagnetic resonance (EPR) in conjunction with molecular orbital calculations. The electronic structure of the radical cation state obtained by oxidation of bacteriopurpurin with iodine is mapped by the determination of isotropic proton hyperfine coupling constants from electron−nuclear double-resonance spectroscopy at 298 K. A tentative assignment of the couplings to the various proton positions in the molecule is achieved by comparison of the experimental values with simulated ones based on semiempirical intermediate neglect of differential overlap calculations...

Substrate Binding to DNA Photolyase Studied by Electron Paramagnetic Resonance Spectroscopy

Structural changes in Escherichia coli DNA photolyase induced by binding of a ( cis, syn)-cyclobutane pyrimidine dimer (CPD) are studied by continuous-wave electron paramagnetic resonance and electron-nuclear double resonance spectroscopies, using the flavin adenine dinucleotide (FAD) cofactor in its neutral radical form as a naturally occurring electron spin probe. The electron paramagnetic resonance/electron-nuclear double resonance spectral changes are consistent with a large distance (≥0.6 nm) between the CPD lesion and the 7,8-dimethyl isoalloxazine ring of FAD, as was predicted by recent model calculations on photolyase enzyme-substrate complexes. Small shifts of the isotropic proton hyperfine coupling constants within the FAD’s isoalloxazine moiety can be understood in terms of the cofactor binding site becoming more nonpolar because of the displacement of water molecules upon CPD docking to the enzyme. Molecular orbital calculations of hyperfine couplings using density functional theory, in conjunction with an isodensity polarized continuum model, are presented to rationalize these shifts in terms of the changed polarity of the medium surrounding the FAD cofactor.

Hyperfine structure of some hydrocarbon radical cations: a B3LYP and MP2 study

Hyperfine structures of n-alkene cations (C 2H 4 +, C 3H 6 +), c-alkane cations (C 3H 6 +, C 5H 10 +, C 6H 12 +), and benzene cation (C 6H 6 +), have been studied by using B3LYP and MP2 methods based on the B3LYP optimized grometries, and the isotropic proton hyperfine coupling constants ( a(H)) are predicted. A 6-311G(d,p) basis was used. For C 2H 4 +, the B3LYP torsion angle value of 28° and MP2//B3LYP a(H) value of −3.2 G are the best ones reported to date. For n-C 3H 6 +, the B3LYP calculations predict a planar geometry (no twisting around the CC bond) and the calculated a(H) values are in agreement with experiment. For c-C 3H 6 +, and C 6H 6 +, the a(H) values predicted by the B3LYP//B3LYP and MP2//B3LYP calculations are in good agreement with the ESR experiments. For c-C 5H 10 +, the B3LYP calculations predict a twisted C 2 structure and a bent C s structure (the previous MP2 calculations failed to predict the twisted structure), and the hyperfine structure calculations imply that assignment of the ESR spectra to the 2A state of C 2 conformation is favored. For c-C 6H 12 +, the B3LYP calculations predict that the ground state is 2A g of the C 2 h chair conformation, to which the ESR spectrum at 140 K is assigned, and suggest that the previously reported C s structure does not exit.

The asymmetric structure of the n-pentane radical cation: a theoretical study

An asymmetrically distorted structure of the n-pentane radical cation ( trans– trans) has been located in B3LYP/6-311G(d,p) calculations, and it has a C s symmetric geometry with one of the two central C–C bonds elongated. The B3LYP and CI calculations indicate that the C s structure is lower in energy than the C 2ν ( 2 B 2 ) structure. The calculated isotropic proton hyperfine coupling constants for the C s and C 2ν ( 2 B 2 ) structures are in fairly good agreement with the experimental couplings observed in an SF 6 matrix and in an C 6F 14 matrix, respectively.

Density functional and ab initio study of the ketene and methylketene radical cations

The present study for the ketene and methylketene radical cations involves geometry, conformation, and hyperfine structure. The geometry and conformation (for the methylketene cation) investigations were based on the B3LYP/6-311G(d,p) and MP2/6-311G(d,p) calculations, and the isotropic proton hyperfine coupling constants (a(H)) were calculated using various density functional theory (DFT) methods and ab initio UHF, UMP2, and multireference single and double excitation configuration interaction (MRSDCI) methods with large basis sets. The ketene cation is predicted to have a C2v geometry. The barriers to methyl group rotation in the methylketene cation is predicted to be very low (<1 kcal/mol) and the cis (staggered) conformation is the most stable. On the rotation, the sum of the B3LYP/6-311G(d,p) a(H) values on the three β protons in the methylketene cation is almost a constant approximately equal to three times the experimental β coupling value. All the three ab initio methods predict too small a(H) values for the β protons. The B3LYP (also B3P86 and B3PW91) calculations predict a(H) values for the α and β protons in the two radicals in excellent agreement with experiment and the DFT methods do not underestimate the β couplings. The UMP2(FULL)/6-311G(d,p) hyperfine structure calculations predict a(H) values for the α protons in excellent agreement with experiment and the UMP2 hfcc results are overall better than the MRSDCI results. As an ab initio method for hyperfine structure calculations, the UMP2 method is suggested.

The electronic structure of the primary donor cation radical in Rhodobacter sphaeroides R-26: ENDOR and TRIPLE resonance studies in single crystals of reaction centers

The electron spin density distribution of the cation radical of the primary donor, D +, a bacteriochlorophyll a dimer was determined by ENDOR and TRIPLE resonance experiments performed on single crystals of reaction centers (RCs) of Rhodobacter sphaeroides R-26. Nine isotropic proton hyperfine coupling constants (hfc's) were obtained and from the angular dependence of the hfc's in three crystallographic planes, five complete hyperfine (hf) tensors were determined. Theoretical hf tensors were calculated by the all-valence-electron SCF molecular orbital method RHF-INDO/SP using the X-ray structure data of the dimer D and its amino acid environment. A comparison of the directions of the principal axes of the experimental and calculated hf tensors enabled us to identify the hfc's with specific protons on the two bacteriochlorophyll halves D L and D M of the dimer. The result shows that the unpaired valence electron is unequally distributed over the dimer halves, favoring D L by approx. 2:1. This ratio has been obtained from the proton hfc's of rotating methyl groups, which directly reflect the π-spin densities at the corresponding positions in the two macrocycles, D L and D M. It was further confirmed by recent 15N-ENDOR experiments on RC single crystals (Lendzian, F., Bönigk, B., Plato, M., Möbius, K. and Lubitz, W. (1992) in The Photosynthetic Bacterial Reaction Center II (Breton, J. and Verméglio, A., eds.), pp. 89–97, Plenum Press, New York). The observed asymmetry of D + is attributed to the difference in energies of the highest filled molecular π-orbitals of the monomeric halves, D L and D M, which is caused by differences in the structure of the two bacteriochlorophylls and/or their environment. Possible implications of this asymmetry for the electron transfer in the RC are discussed.

An ENDOR study of the canthaxanthin cation radical in solution

The cation radical of canthaxanthin was generated electrochemically in dichloromethane and its solution ENDOR spectrum is reported for the first time. The isotropic proton hyperfine coupling constants are in reasonable agreement with those calculated by RHF-INDO/SP molecular orbital methods for a planar all-trans-polyene conformation. Proton couplings less than 1.1 MHz were not detected due in part to incomplete hyperfine separation and chemical exchange processes.

Spin density distribution of the photoexcited triplet state of anthracene via electron spin echo envelope modulation spectroscopy

The values of spin density of the anthracene triplet have been determined from the analyses of electron spin echo envelope modulations and a program of non-linear least-squares fitting as follows: p 1 (α) = 0.116, p 2 (β) = 0.05, p 9(meso) = 0.285 and p 11 = −0.058. The values of anisotropic and isotropic proton hyperfine coupling constants per unit spin density on the adjacent carbon have also been determined. Comparisons with previously reported values are presented.

ChemInform Abstract: ISOTROPIC PROTON HYPERFINE COUPLING CONSTANTS OF TWO CATIONIC NITROXIDES

The isotropic proton hyperfine coupling constants of 2,2,6,6-tetramethylpiperidineoxide–trimethylammonium bromide and 2,2,6,6-tetramethylpiperidineoxide–dodecyldimethylammonium bromide have been determined using a nuclear magnetic resonance technique. The measured coupling constants have been used to generate computer simulated electron paramagnetic resonance (e.p.r.) spectra, which show moderate agreement with experimental e.p.r. spectra. Evidence is presented which indicates that intermolecular exchange effects in the e.p.r. spectra may account for the discrepancies.

Isotropic proton hyperfine coupling constants of two cationic nitroxides

The isotropic proton hyperfine coupling constants of 2,2,6,6-tetramethylpiperidineoxide–trimethylammonium bromide and 2,2,6,6-tetramethylpiperidineoxide–dodecyldimethylammonium bromide have been determined using a nuclear magnetic resonance technique. The measured coupling constants have been used to generate computer simulated electron paramagnetic resonance (e.p.r.) spectra, which show moderate agreement with experimental e.p.r. spectra. Evidence is presented which indicates that intermolecular exchange effects in the e.p.r. spectra may account for the discrepancies.

Hyperfine Coupling Constants and Covalency in Sandwich Complexes

An analysis of the isotropic proton hyperfine coupling constants of V(C5H5)2, Ni(C5H5)2, and Cr(C6H6)2+ is given. The unpaired electrons in the e1g* molecular orbitals in Ni(C5H5)2 are found to be rather strongly delocalized into the carbon 2pz–π framework of the ligands. In agreement with bonding theories this indicates that the interaction between the metal 3dxz and 3dyz orbitals and the ligand e1g–π molecular orbitals is very important for the chemical bonding in sandwich complexes. In Cr(C6H6)2+ and V(C5H5)2 the proton coupling constants are determined mainly by σ delocalization of the unpaired electron in the a1g(3dz2) orbital into the hydrogen 1s orbitals. In V(C5H5)2, also, delocalization into the ligand π framework is present and this is thought to be due to the e2g orbitals. It is concluded that the interaction of the metal 3dxy and 3dx2−y2 orbitals with the ligand e2g–π molecular orbitals is appreciable.

Electron Spin Resonance of Ultraviolet-Irradiated Compounds. II. Peroxide-Initiated Radicals in Straight Chain Alkenes

Selected low molecular weight hydrocarbon glasses at approximately 77°K were irradiated with ultraviolet light, producing trapped radicals which were studied by ESR. Straight chain alkenes exhibited spectra which were demonstrated, through tests with 1-hexene, to be the result of free radical reactions initiated by the photolysis of peroxide impurities. Other experiments with 1-hexene are reported. Symmetric alkene spectra were interpreted in terms of the following model: (A) Primary (peroxide) radicals, formed by uv photolysis, react with molecular end groups by abstraction or terminal addition to produce the observed spectra. (B) In the π-electron radicals assumed to be formed, the isotropic proton hyperfine coupling constants (which determine the observed approximate component separations) are taken to have the fixed values: α protons, 22 G; methylene protons, 22 G (β1) and 44 G (β2); methyl protons, 22 G. (The α-proton anisotropic splitting is manifested only as a broadening of the components.) In terms of this model, interpretations of the spectra obtained from 2-pentene, 1- and 2-hexene, 1-, 2-, and 3-heptene, 1- and 2-octene, and 1-decene are presented.