Magnetic Resonance Characterization of One-Electron Oxidized Cyclic Dipeptides with Thioether Groups

Magnetic-Field Dependence of LC-Photo-CIDNP in the Presence of Target Molecules Carrying a Quasi-Isolated Spin Pair

NMR spectroscopy is well known for its superb resolution, especially at high applied magnetic field. However, the sensitivity of this technique is very low. Liquid-state low-concentration photo-chemically-induced dynamic nuclear polarization (LC-photo-CIDNP) is a promising emerging methodology capable of enhancing NMR sensitivity in solution. LC-photo-CIDNP works well on solvent-exposed Trp and Tyr residues, either in isolation or within proteins. This study explores the magnetic-field dependence of the LC-photo-CIDNP experienced by two tryptophan isotopologs in solution upon in situ LED-mediated optical irradiation. Out of the two uniformly 13C,15N-labeled Trp (Trp-U-13C,15N) and Trp-α-13C-β,β,2,4,5,6,7-d7 species employed here, only the latter bears a quasi-isolated 1Hα-13Cα spin pair. Computer simulations of the predicted polarization due to geminate recombination of both species display a roughly bell-shaped field dependence. However, while Trp-U-13C,15N is predicted to show a maximum at ca. 500 MHz (11.7 T) and a fairly weak field dependence, Trp-α-13C-β,β,2,4,5,6,7-d7 is expected to display a much sharper field dependence accompanied by a dramatic polarization increase at lower field (ca. 200 MHz, 4.7 T). Experimental LC-photo-CIDNP studies on both Trp isotopologs at 1μM concentration, performed at selected fields, are consistent with the theoretical predictions. In summary, this study highlights the prominent field-dependence of LC-photo-CIDNP enhancements (ε ) experienced by Trp isotopologs bearing a quasi-isolated spin pair.

Photo- and Radiation-Induced One-Electron Oxidation of Methionine in Various Structural Environments Studied by Time-Resolved Techniques

Oxidation of methionine (Met) is an important reaction that plays a key role in protein modifications during oxidative stress and aging. The first steps of Met oxidation involve the creation of very reactive and short-lived transients. Application of complementary time-resolved radiation and photochemical techniques (pulse radiolysis and laser flash photolysis together with time-resolved CIDNP and ESR techniques) allowed comparing in detail the one-electron oxidation mechanisms initiated either by ^●OH radicals and other one-electron oxidants or the excited triplet state of the sensitizers e.g., 4-,3-carboxybenzophenones. The main purpose of this review is to present various factors that influence the character of the forming intermediates. They are divided into two parts: those inextricably related to the structures of molecules containing Met and those related to external factors. The former include (i) the protection of terminal amine and carboxyl groups, (ii) the location of Met in the peptide molecule, (iii) the character of neighboring amino acid other than Met, (iv) the character of the peptide chain (open vs cyclic), (v) the number of Met residues in peptide and protein, and (vi) the optical isomerism of Met residues. External factors include the type of the oxidant, pH, and concentration of Met-containing compounds in the reaction environment. Particular attention is given to the neighboring group participation, which is an essential parameter controlling one-electron oxidation of Met. Mechanistic aspects of oxidation processes by various one-electron oxidants in various structural and pH environments are summarized and discussed. The importance of these studies for understanding oxidation of Met in real biological systems is also addressed.

Radical rearrangement and transfer reactions in proteins

Radical rearrangement and transfer reactions play an important role in the chemical modifications of proteins in vivo and in vitro. These reactions depend on protein sequence, as well as structure and dynamics. Frequently, these reactions have well-defined precedents in the organic chemistry literature, but their occurrence in proteins provides a stage for a number of novel and, perhaps, unexpected reaction products. This essay will provide an overview over a few representative examples of radical rearrangement and transfer reactions.

Radiolytic Oxidation of Two Inverse Dipeptides, Methionine-Valine and Valine-Methionine: A Joint Experimental and Computational Study

The two inverse peptides methionine-valine (Met-Val) and valine-methionine (Val-Met) are investigated in an oxidative radiolysis process in water. The OH radical yields products with very different absorption spectra and concentration effects: Met-Val yields one main product with a band at about 400 nm and other products at higher energies; there is no concentration effect. Val-Met yields at least three products, with a striking concentration effect. Molecular simulations are performed with a combination of the Monte Carlo, density functional theory, and reaction field methods. The simulation of the possible transients enables an interpretation of the radiolysis: (1) Met-Val undergoes an H atom uptake leaving mainly a neutral radical with a 2-center-3-electron (2c-3e) SN bond, which cannot dimerize. Other radicals are present at higher energies. (2) Val-Met undergoes mainly an electron uptake leaving a cation monomer with a (2c-3e) SO bond and a cation dimer with a (2c-3e) SS bond. At higher energies, neutral radicals are possible. This cation monomer can transfer a proton toward a neutral peptide, leaving a neutral radical.