Irradiation of proteins with intense X-ray radiation produced by third-generation synchrotron sources generates specific structural and chemical alterations, including breakage of disulfide bonds and decarboxylation. In this paper, disulfide bond lengths in irradiated crystals of the enzyme Torpedo californica acetylcholinesterase are examined based on quantum simulations and on experimental data published previously. The experimental data suggest that one disulfide bond elongates by approximately 0.7 A upon X-ray irradiation as seen in a series of nine data sets collected on a single crystal. Simulation of the same bond suggests elongation by a similar value if a disulfide-radical anion is formed by trapping an electron. The absorption spectrum of a crystal irradiated under similar conditions shows a peak at approximately 400 nm, which in aqueous solution has been attributed to disulfide radicals. The results suggest that the formation of disulfide radicals in protein crystals owing to X-ray irradiation can be observed experimentally, both by structural means and by absorption spectroscopy.

An alternative to classical? In square‐planar d8 complexes the metal ion can interact with axial H2O molecules either as a Lewis acid or as a Lewis base. Ab initio calculations predicted that uncharged PtII complexes form a hydrogen‐bond‐like interaction with H2O, in which PtII acts as a Lewis base. Such a nonclassical OH⋅⋅⋅Pt hydrogen bond has now been identified in crystals of trans‐[PtCl2(NH3)(N‐glycine)]⋅H2O by neutron diffraction.

Although a two-center three-electron (2c-3e) bond between homonuclear atoms is well characterized, this is not the case for the S∴O bond, especially in neutral radicals resulting from the addition of a hydroxyl group on various sulfur substrates. A structural, energetical, and topological study is presented for prototypical radicals, ionic and neutral, RSOH -, RR′SOH 2 + , and RR′SOH, with R, R′ ) H, CH 3 . Three calculation methods have been applied, BH&HLYP, MP2, and CCSD(T), with different basis sets to determine the domains of accuracy of the more approximate ones to use them for larger systems. Qualitative and quantitative criteria, defined from the topological analysis of the electron localization function, are proposed to characterize such a 2c-3e bond. They specify the number and type of basins and their hierarchy of bifurcation, the global charge transfer between the fragments, the localization of the integrated spin density, and the electron delocalization between the lone pairs of the interacting atoms. Surprisingly, the neutral radicals show an intermediate behavior between the pure 2c-3e S∴O bond in anions and the electrostatic interaction in cations, despite the low energy of bond formation. As in the radical anions, the substitution favors the formation of a 2c-3e bond.

By means of Rydberg electron-transfer spectroscopy (RETS), negative ion photoelectron spectroscopy (NIPES), and quantum chemistry calculations, we have studied electron attachment properties of a series of saturated disulfides: dimethyl disulfide, diethyl disulfide, and dipropyl disulfide. Both RETS and NIPES experiments show that the valence anions of these disulfides are stable. RETS further shows that these negative ions result from attachment of nonzero energy electrons (0.2 eV), in contrast to dimers and larger complexes. NIPES experiments provide vertical detachment energies for the three disulfide monomer anions along with their Franck-Condon profiles. Fitting these spectra, using model potentials for the S-S stretch coordinate, finds that the adiabatic electron affinities of these disulfides are positive but rather small, about 0.1 eV. These experimental data compare well with the results of ab initio calculations, performed at the MP2 level with large basis sets.

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ABSTRACT:The topological analysis of the electron localization function (ELF) provides a convenient mathematical framework enabling an unambiguous characterization of bonds and more particularly in terms of bond types. In this contribution, we present an overview of the applications of this approach to biological and biomimetic systems.

One electron oxidation of methionine in peptides is highly dependent on the local structure. The sulfur-centered radical cation can complex with oxygen, nitrogen, or other sulfur atoms from a neighboring residue or from the peptidic bond, forming an intramolecular S therefore X two-center three-electron bond (X = S, N, O). This stabilization was investigated computationally in the radical cations of three peptides, methionine glycine (Met Gly) and its reverse sequence Gly Met, and Met Met. Geometry optimizations were done at the BH&HLYP/6-31G(d) level of theory and the effect of solvation was taken into account using a continuum model (CPCM). Up to seven stable conformations were considered for each peptide, with formation of 5-10 member cycles involving nitrogen from the peptidic bond or from the amine, oxygen from the peptidic bond or from the carboxylate group, or sulfur from the other residue for Met Met. The absorption wavelengths corresponding to the sigma --> sigma* transition calculated for each complex at the TD-BH&HLYP/6-311+G(d,p)//BH&HLYP/6-31G(d) level of theory vary from the near-UV for the S therefore O bonds to the green visible for the S therefore S bonds. For X = N, they increase with the SN distance as expected for a 2c-3e bond, whereas for X = O they slightly decrease. Characterization of these 2c-3e bonds as a function of the sequence, using the ELF and the AIM topological analyses, shows the different natures of the S therefore X bonds, which is purely 2c-3e for X = S, mainly 2c-3e with a part of electrostatic interaction for X = N and mainly electrostatic for X = O.

The protein residue methionine (Met) is one of the main targets of oxidizing free radicals produced in oxidative stress. Despite its biological importance, the mechanism of the oxidation of this residue is still partly unknown. In particular the one-electron redox potentials of the couple Met(•+)/Met have not been measured. In this work, two approaches, experimental as well as theoretical, were applied for three dipeptides L-Met L-Gly, L-Gly L-Met and L-Met L-Met. Measurements by electrochemistry indicated differences in the ease of oxidation. Two DFT methods (BH&HLYP and PBE0) with two basis sets (6-31G(d) and 6-311+G(2d,2p)) were used to determine the redox potentials of Met in these peptides present in different conformations. In agreement with experimental results, we show that they vary with the sequence and the spatial structure of the peptide, most of the values being higher than 1 V (up to 2 V) vs NHE.