C.A. Steren

ABACUS, a Direct Method for Protein NMR Structure Computation via Assembly of Fragments

The ABACUS algorithm obtains the protein NMR structure from unassigned NOESY distance restraints. ABACUS works as an integrated approach that uses the complete set of available NMR experimental information in parallel and yields spin system typing, NOE spin pair identities, sequence specific resonance assignments, and protein structure, all at once. The protocol starts from unassigned molecular fragments (including single amino acid spin systems) derived from triple‐resonance 1H/13C/15N NMR experiments. Identifications of connected spin systems and NOEs precede the full sequence specific resonance assignments. The latter are obtained iteratively via Monte Carlo‐Metropolis and/or probabilistic sequence selections, molecular dynamics structure computation and BACUS filtering ( A. Grishaev and M. Llinás, J Biomol NMR 2004;28:1–10 ). ABACUS starts from scratch, without the requirement of an initial approximate structure, and improves iteratively the NOE identities in a self‐consistent fashion. The procedure was run as a blind test on data recorded on mth1743, a 70‐amino acid genomic protein from M. thermoautotrophicum. It converges to a structure in ca. 15 cycles of computation on a 3‐GHz processor PC. The calculated structures are very similar to the ones obtained via conventional methods (1.22 Å backbone RMSD). The success of ABACUS on mth1743 further validates BACUS as a NOESY identification protocol. Proteins 2005.

FT-EPR study of triplet state C60. Spin dynamics and electron transfer quenching

Abstract A FT-EPR study was made of paramagnetic species formed by pulsed-laser excitation of C 60 in fluid solution. Earlier findings that photoexcitation of C 60 in fluid solution gives rise to an EPR signal with narrow linewidth were confirmed. The lifetime of the signal corresponds to that of the C 60 triplet as measured by flash photolysis. In the presence of donors, the rate of signal decay is increased and matches the growing in of EPR signals from oxidized donors. The time dependence of the FT-EPR spectra gives values for electron transfer rate constants which agree with those derived from flash photolysis measurements on the same systems. Based on these findings, we assign the narrow line signal to the triplet of C 60 . The time evolution of the FT-EPR signal establishes that the triplets are born with less spin polarization that the thermal equilibrium value. As a result, signal growth is controlled by spin-lattice relaxation.

EPR of layered magnetic metal-amino acid salts. II. Cu(L-Met)2

Abstract Single crystals of bis(L-methioninato)copper(II), Cu(L-Met) 2 , were studied by EPR at 9.7 and 33.6 GHz, at 300 K. The position and the peak-to-peak linewidth of the single observed EPR line were measured in three perpendicular planes of the samples. This single resonance is due to the collapse of the resonances of the two magnetically inequivalent copper ions in the lattice caused by the exchange interaction. The components of the molecular g tensor for isolated copper ions were obtained using a model which assumes axial symmetry. The results indicate that the unpaired electron occupies the d( x 2 − y 2 ) orbital, and the orientation of the molecule obtained from the EPR data agrees with the crystallographic result. The linewidth data support a model which assumes exchange narrowing of the magnetic dipolar interaction in a two-dimensional magnetic lattice, an incomplete collapse of the hyperfine structure, and a frequency-dependent contribution in the planes where the g factors for the two sites of copper are different. An analysis of this latter contribution, allows to evaluate an exchange coupling constant | J ′| = 0.10 K between inequivalent copper neighbors. Besides, the analysis of the hyperfine contribution to the linewidth gives | J | = 0.18 K for the average value of the exchange interaction of one copper ion with its six nearest neighbors within the layer. Concerning the possible superexchange paths between inequivalent copper ions, we suggest they can be of two types: the CuNH…OCu one, involving hydrogen bonds between equatorial nitrogens and equatorial oxygens, and the other consisting of CuOCOCu carboxylate bridges involving apical and equatorial oxygens. They are discussed in view of the experimental results.

FT-EPR study of the CIDEP of 2-propanolyl radical produced by the reaction of acetone with triethylamine

Abstract The results of an FT-EPR study of the CIDEP of 2-propanolyl radical produced by the photochemical reaction of acetone with triethylamine are reported. The time profiles of the signals are explained satisfactorily on the basis of a hydrogen abstraction reaction involving the acetone singlet and/or triplet excited states depending upon the amine concentration and CIDEP mechanisms involving singlet and/or triplet geminate pairs and triplet F pairs together with spin-lattice relaxation.

FT-EPR and HPLC study of the mechanism of 4-chlorophenol photolysis

The mechanism of 4-chlorophenol (4CP) photolysis was investigated with the aid of Fourier Transform Electron Paramagnetic Resonance (FT-EPR) and pulsed-laser photolysis combined with High Performance Liquid Chromatography (HPLC) detection-of stable (diamagnetic) products. With FT-EPR transient free radicals produced by pulsed-laser excitation of solutions of 4CP in alcohols could be identified. Time profiles of the FT-EPR spectra provided information on reaction kinetics and Chemically Induced Dynamic, Electron Polarization (CIDEP) effects. It was found that 4CP photolysis in alcohols leads to the simultaneous formation of the phenoxyl radical and radicals produced by hydrogen abstraction from the solvent. CIDEP patterns establish that these radicals are formed in a reaction sequence involving a triplet state precursor and radical pair intermediate. Results of earlier transient optical absorption measurements indicate that the triplet precursor must be the carbene 4-oxocyclohexa-2,5-dienylidene. This assignment is supported by the finding that photolysis of quinone diazide in a hydrogen-donating solvent gives the same free radical products as those obtained from 4CP. The formation of the phenoxyl radical intermediate accounts for the finding that photolysis of deoxygenated solutions of 4CP in alcohols gives phenol as stable diamagnetic product. By contrast, photolysis of aerated and deoxygenated aqueous solutions of 4CP produces benzoquinone and hydroquinone as primary products, respectively.

Photoinduced spin polarization of paramagnetic centers in solid C60

Abstract Results of time-resolved cw-EPR and spin-echo measurements of photoexcited C 60 are presented. It is found that visible light induces a reversal in sign of the spin polarization of paramagnetic impurities present in solid C 60 . The photoinduced spin polarization is explained in terms of spin-selective triplet exciton quenching by the spin 1 2 paramagnetic centers.

Reactions of Group 4 Amide Guanidinates with Dioxygen or Water. Studies of the Formation of Oxo Products

Reactions of the zirconium amide guanidinates (R2N)2M[(i)PrNC(NR2)N(i)Pr]2 (R = Me, M = Zr, 1; M = Hf, 2; R = Et, M = Zr, 3) with O2 or H2O give products that are consistent with the oxo dimers {M(μ-O)[(i)PrNC(NR2)N(i)Pr]2}2 (R = Me, M = Zr, 4; M = Hf, 5; R = Et, M = Zr, 6) and polymers {M(μ-O)[(i)PrNC(NR2)N(i)Pr]2}n (R = Me, M = Zr, 7; M = Hf, 8; R = Et, M = Zr, 9). Mass spectrometric (MS) analyses of the reactions of water in air with 1 and 2 show formation of the Zr monomer Zr(═O)[(i)PrNC(NMe2)N(i)Pr]2 (10), oxo dimers 4 and 5, and dihydroxyl complexes M(OH)2[(i)PrNC(NMe2)N(i)Pr]2 (M = Zr, 11; Hf, 12). Similar MS analyses of the reaction of diethylamide guanidinate 3 with water in air show the formation of Zr(═O)[(i)PrNC(NEt2)N(i)Pr]2 (13), Zr(OH)2[(i)PrNC(NEt2)N(i)Pr]2 (14), 6, and {(Et2N)Zr[(i)PrNC(NEt2)N(i)Pr]2}(+) (15). Kinetic studies of the reaction between 1 and a continuous flow of 1.0 atm of O2 at 80-105 °C indicate that it follows pseudo-first-order kinetics with ΔH(‡) = 8.7(1.1) kcal/mol, ΔS(‡) = -54(3) eu, ΔG(‡)(358 K) = 28(2) kcal/mol, and a half-life of 213(1) min at 85 °C.

Magnetic and structural properties of trans-bis (D,L-isoleucine) copper(II)

A magnetic and structural characterization of single crystals of the copper derivative of the amino acid D,L-isoleucine, Cu(NH{sub 2}(CH){sub 2}CH{sub 2}(CH{sub 3}){sub 2}CO{sub 2}){sub 2}, was performed by EPR and X-ray diffraction techniques. The complex crystallizes in the orthorhombic space group Aba2, with a = 11.165(3) {angstrom}, b = 11.111(3) {angstrom}, c = 25.985(6) {angstrom}, and Z = 8. The copper ions occupy sites of point symmetry C{sub 2}. The position and peak-to-peak linewidth of the single EPR line observed were measured at 9.7 GHz and 293 K in three perpendicular planes of the sample. The gyromagnetic tensor {lvec {rvec g}} has near axial symmetry around {cflx c}, with a small anisotropy in the perpendicular plane, in agreement with the orthorhombic symmetry indicated by the crystallographic results. The principal values of {lvec {rvec g}} are g{sub 1} = 2.0607(5), g{sub 2} = 2.0616(5), and g{sub 3} = 2.2619(3), with principal directions parallel to the crystal axes. The observed angular variation of the linewidth suggests a layered arrangement of the copper ions.

Time-resolved EPR study of 3C60 in solid matrices

Using time-resolved cw EPR and pulsed EPR techniques, a study was made of the characteristics of the photoexcited triplet state of C60 adsorbed in silica gel pores and embedded in a polymethylmethacrylate matrix. It was found that the time-resolved spectra from 3C60 in these matrices at room temperature retain the absorption/emission features of spectra from 3C60 in frozen solution (≤120 K). Apparently, the combination of molecular rotation and pseudo rotation (resulting from interconversion between Jahn-Teller states) is not fast enough, to lead to complete averaging of the dipole-dipole interaction between the unpaired electrons. The study explored the effect of introduction of solvent molecules in the silica gel pores on the 3C60 spectrum and quenching of the triplet by electron donors.

Molecular structure and single crystal EPR spectra of bis(L-Valinato)copper(II) monohydrate, Cu[H2NCH(CH3)2CHCO2]2·H2O

Abstract A magnetic-structural study of the title compound, Cu(L-Val)2·H2O, employing EPR spectroscopy and X-ray diffraction methods is presented. The complex crystallizes in the space group C2 with a = 21.314(5) A , b = 9.586(2) A , c = 7.417(2) A , β = 108.89(2)°, and Z=4. The Cu(II) ions, which are in a five-fold pyramidal coordination, with two valine molecules and a water oxygen (Ow), are arranged in layers parallel to (100). Neighboring Cu(L-Val)2·H2O molecules in a layer are linked to each other by a net of Ow-H…O and N-H…O hydrogen bonds. The single EPR line observed arises from the collapse of the resonances due to the two magnetically inequivalent Cu(II) ions in the lattice caused by the exchange interaction. The obtained g⌈ = 2.254(1) and g⊥ = 2.061(1) principal values of the molecular gyromagnetic tensor point to a d(x2−y2) orbital for the unpaired electron. The orientation of this orbital was also obtained from our EPR data. The angular variation of the resonance line width has contributions arising from the dipolar interaction modulated by exchange effects in a two-dimensional magnetic lattice, and from the incomplete collapse of the hyperfine structure. The EPR results are compared to those obtained in other copper amino acid complexes.