The cobalt-60 gamma radiolysis of cysteine in deaerated aqueous solutions at pH values between 5 and 6

Blue-light-driven photoactivity of L-cysteine-modified graphene quantum dots and their antibacterial effects

The widespread abuse of traditional antibiotics has led to a global rise in antibiotic-resistant bacteria, which give in return unprecedented health risks. Therefore, there is a large and urgent need for the development of new, smart antibacterial agents able to efficiently kill or inhibit bacterial growth. In this study, we investigated the antibacterial activity of S, N-doped Graphene Quantum Dots (GQDs) as a light-triggered antibacterial agent. Gamma irradiation was employed as a tool to achieve one-step modification of GQDs in the presence of L-cysteine amino acid as a source of heteroatoms. X-ray Photoelectron Spectroscopy (XPS), nuclear magnetic resonance (NMR), and zeta potential measurements provided the necessary data to clarify the structure of modified dots and verify the introduction of both S- and N-atoms in GQDs structure, but also severe changes in the aromatic, sp2 domains. Namely, γ-irradiation caused a bonding of S atoms in 1.14 at.% mainly as thiol groups, and N in 1.81 at.% as amino groups, but sp2 contribution in GQD structure was lowered from 63.00 to 4.86 at.%, as measured in dots irradiated at a dose of 200 kGy. Fluorescence quenching measurements showed that L-cysteine-modified dots are able to bind to human serum albumin. The antibacterial activity of GQDs combined with 1 and 6 h of blue light (470 nm) irradiation was tested against 8 bacterial strains. GQD-cys-25 sample provided the best results, with minimum inhibitory concentration (MIC) as low as 125 μg/mL against S. aureus, E. faecalis, and E. coli after only 1 h of blue light exposure.

Comprehensive model for X-ray-induced damage in protein crystallography

Acquisition of X-ray crystallographic data is always accompanied by structural degradation owing to the absorption of energy. The application of high-fluency X-ray sources to large biomolecules has increased the importance of finding ways to curtail the onset of X-ray-induced damage. A significant effort has been under way with the aim of identifying strategies for protecting protein structure. A comprehensive model is presented that has the potential to explain, both qualitatively and quantitatively, the structural changes induced in crystalline protein at ∼100 K. The first step is to consider the qualitative question: what are the radiation-induced intermediates and expected end products? The aim of this paper is to assist in optimizing these strategies through a fundamental understanding of radiation physics and chemistry, with additional insight provided by theoretical calculations performed on the many schemes presented.

Synchrotron X-radiolysis of l-cysteine at the sulfur K-edge: Sulfurous products, experimental surprises, and dioxygen as an oxidoreductant

In situ inventory of sulfurous products from the sulfur K-edge synchrotron X-radiolysis of l-cysteine in solid-phase and anaerobic (pH 5) and air-saturated (pH 5, 7, and 9) solutions without and with 40% glycerol is reported. Sequential K-edge X-ray Absorption Spectroscopic (XAS) spectra were acquired. l-cysteine degraded systematically in the X-ray beam. Radiolytic products were inventoried by fits using the XAS spectra of sulfur model compounds. Solid l-cysteine declined to 92% fraction after a single K-edge XAS scan. After six scans, 60% remained, accompanied by 14% cystine, 16% thioether, 5.4% elemental sulfur, and smaller fractions of more highly oxidized products. In air-saturated pH 5 solution, 73% of l-cysteine remained after ten scans, with 2% cystine and 19% elemental sulfur. Oxidation increased with 40% glycerol, yielding 67%, 5%, and 23% fractions, respectively, after ten scans. Higher pH solutions exhibited less radiolytic chemistry. All the reactivity followed first-order kinetics. The anaerobic experiment displayed two reaction phases, with sharp changes in kinetics and radiolytic chemistry. Unexpectedly, the radiolytic oxidation of l-cysteine was increased in anaerobic solution. After ten scans, only 60% of the l-cysteine remained, along with 17% cystine, 22% elemental sulfur, and traces of more highly oxidized products. A new aerobic reaction cycle is hypothesized, wherein dissolved dioxygen captures radiolytic H• or eaq -, enters HO2 •/O2 •-, reductively quenches cysteine thiyl radicals, and cycles back to O2. This cycle is suggested to suppress the radiolytic production of cystine in aerobic solution.

Reaction rates of glutathione and ascorbate with alkyl radicals are too slow for protection against protein peroxidation in vivo

Reaction kinetics of amino acid and peptide alkyl radicals with GSH and ascorbate, the two most abundant endogenous antioxidants, were investigated by pulse radiolysis. Rate constants in the order of 10⁶ M^-1s^-1 were found. Alkyl radicals react at almost diffusion controlled rates and irreversibly with oxygen to form peroxyl radicals, and competition with this reaction is the benchmark for efficient repair in vivo. We consider repair of protein radicals and assume comparable rate constants for the reactions of GSH/ascorbate with peptide alkyl radicals and with alkyl radicals on a protein surface. Given physiological concentrations of oxygen, GSH and ascorbate, protein peroxyl radicals will always be a major product of protein alkyl radicals in vivo. Therefore, if they are formed by oxidative stress, protein alkyl radicals are a probable cause for biological damage.

Electrons initiate efficient formation of hydroperoxides from cysteine

Amino acid and protein hydroperoxides can constitute a significant hazard if formed in vivo. It has been suggested that cysteine can form hydroperoxides after intramolecular hydrogen transfer to the commonly produced cysteine sulfur-centered radical. The resultant cysteine-derived carbon-centered radicals can react with oxygen at almost diffusion-controlled rate, forming peroxyl radicals which can oxidize other molecules and be reduced to hydroperoxides in the process. No cysteine hydroperoxides have been found so far. In this study, dilute air-saturated cysteine solutions were exposed to radicals generated by ionizing radiation and the hydroperoxides measured by an iodide assay. Of the three primary radicals present, the hydroxyl, hydrogen atoms and hydrated electrons, the first two were ineffective. However, electrons did initiate the generation of hydroperoxides by removing the -SH group and forming cysteine-derived carbon radicals. Under optimal conditions, 100% of the electrons reacting with cysteine produced the hydroperoxides with a 1:1 stoichiometry. Maximum hydroperoxide yields were at pH 5.5, with fairly rapid decline under more acid or alkaline conditions. The hydroperoxides were stable between pH 3 and 7.5, and decomposed in alkaline solutions. The results suggest that formation of cysteine hydroperoxides initiated by electrons is an unlikely event under physiological conditions.

Kinetics and Mechanisms of Chlorine Dioxide and Chlorite Oxidations of Cysteine and Glutathione

Chlorine dioxide oxidation of cysteine (CSH) is investigated under pseudo-first-order conditions (with excess CSH) in buffered aqueous solutions, p[H+] 2.7-9.5 at 25.0 degrees C. The rates of chlorine dioxide decay are first order in both ClO2 and CSH concentrations and increase rapidly as the pH increases. The proposed mechanism is an electron transfer from CS- to ClO2 (1.03 x 10(8) M(-1) s(-1)) with a subsequent rapid reaction of the CS* radical and a second ClO2 to form a cysteinyl-ClO2 adduct (CSOClO). This highly reactive adduct decays via two pathways. In acidic solutions, it hydrolyzes to give CSO(2)H (sulfinic acid) and HOCl, which in turn rapidly react to form CSO3H (cysteic acid) and Cl-. As the pH increases, the (CSOClO) adduct reacts with CS- by a second pathway to form cystine (CSSC) and chlorite ion (ClO2-). The reaction stoichiometry changes from 6 ClO2:5 CSH at low pH to 2 ClO2:10 CSH at high pH. The ClO2 oxidation of glutathione anion (GS-) is also rapid with a second-order rate constant of 1.40 x 10(8) M(-1) s(-1). The reaction of ClO2 with CSSC is 7 orders of magnitude slower than the corresponding reaction with cysteinyl anion (CS-) at pH 6.7. Chlorite ion reacts with CSH; however, at p[H+] 6.7, the observed rate of this reaction is slower than the ClO2/CSH reaction by 6 orders of magnitude. Chlorite ion oxidizes CSH while being reduced to HOCl, which in turn reacts rapidly with CSH to form Cl-. The reaction products are CSSC and CSO3H with a pH-dependent distribution similar to the ClO2/CSH system.

The role of sulphur peptide functions in free radical transfer: a pulse radiolysis study

Cascading transfers of free radical centres, involving sulphur and aromatic protein functions, have been studied in further detail. The disulphide radical anion appears to be an important terminus of both oxidative and reductive radical transfer. In deaerated solutions of cysteine (20 mmol dm-3) the yield of Cys2/SS.- closely resembles the yield of all primary free radicals generated by water radiolysis (.OH, H. and eaq-). The alanyl Ala/C beta., formed by electron addition to cysteine and subsequent SH- elimination, oxidizes cysteine with a rate constant of k8 = 5.0 x 10(6)dm3mol-1s-1 at pH 6 to 7 and 3.6 x 10(6)dm3mol-1s-1 at pH 9 to 10. In the case of glutathione (GSH) the eaq--induced carbon-centred radical oxidizes the parent thiol with rate constants k(G. + GSH) of 7.0 x 10(6) and 1.3 x 10(6)dm3mol-1s-1 at pH 8 and pH 10, respectively; and with dithiothreitol (D(SH)2) the corresponding reaction rate is k(.DSH + D(SH)2) = 5.5 x 10(6)dm3mol-1s-1 at pH 7.0. The decarboxylated methionyl Met/C. alpha, formed by reaction of .OH with methionine, is capable of electron transfer to cystine, indicating a reduction potential for decarboxylated methione more negative than -1.6 V. The ring-closed methionyl radical cation Met/SN.+, formed by reaction of .OH with Met-Gly, oxidizes azide via equilibration, Met/SN.+ + H+ + N3- in equilibrium Met + N3., which enables an estimate to be given for the one-electron reduction potential: E degrees (Met/SN.+ + H+; Met) = +1.42 +/- 0.3 V (pH 6.8). Some further reactions of oxidizing dimeric Met2/SS.+ species in neutral solution have been demonstrated. The direction and nature of the transfers can be expressed by the scheme: (formula; see text).

On the effect of oxygen or copper(II) in radiation-induced degradation of DNA in the presence of thiols

Degradation of DNA when gamma-irradiated in aqueous solutions containing cysteine can be efficiently enhanced not only with oxygen, but to the same extent also with Cu2+ ions under hypoxic conditions. The result can be explained by 'self-repair' in this system due to recombination of DNA. with RSS.-R intermediates, and repair inhibition by oxygen or copper involving RSS.-R scavenging. It is emphasized that oxygen enhancement in DNA-thiol systems may occur not only by peroxidation, via defect fixation (DNA-O2.) or thiol activation (RS-O2.), but also by the well-established inactivation of RSS.-R by oxygen. There is evidence also from literature data for a correlation between oxygen enhancement and RSS.-R stability, which varies with thiol concentration, pH and thiol structure.

Unpaired electron migration between aromatic and sulfur peptide units

Cysteine thiyl radicals (Cys/S.) were found capable of one-electron oxidation of tyrosine. Equilibration occurred, using Cys and Gly-Tyr, with an equilibrium constant of K5 = 20 +/- 4 at pH 9.15: Cys/S. + Tyr in equilibrium Cys + Tyr/O. (5) Hence the reduction potentials of these couples differ at pH 9.15 by E(Cys/S., Cys) - E(Tyr/O., Tyr) = 80 mV. Oxidation of Trp-Gly by Cys/S. was not detectable from pH 7 to 12. The methionyl radical cation (Met/S.N), formed via .OH-attack on Met-Gly, reacts with Trp-Gly to generate the indolyl radical (Trp/N.). New results on intramolecular Trp/N.----Tyr/O. transitions indicate that the reaction requires direct contact between the two redox centers. Various possible pathways for migration of unpaired electrons between peptide units are compiled in a scheme.

Free radical initiation in proteins and amino acids by ionizing and ultraviolet radiations and lipid oxidation--Part I: ionizing radiation

Parallels and similarities in chemical and functional damage to proteins by ionizing and UV radiations and oxidizing lipids have been recognized for some time. However, only recently have oxidizing lipids been shown directly by electron spin resonance to be radiomimetic also in their capacity for protein free radical production. Free radicals play a key role in the transformation of energy to molecular and cellular damage. It is thus of critical importance to elucidate the general mechanisms of free radical formation and reactions in proteins in order to understand protein involvement in various pathological conditions and in food deterioration. Accordingly, this review is a detailed comparison of gamma radiation, UV radiation, and lipid oxidation for what is presently known concerning (1) the specific modes of energy deposition and free radical formation, (2) the free radicals formed in proteins and amino acids, and (3) the typical damage correlating with these radicals.