Gold Nanoparticle as a Lewis Catalyst for Water Elimination of Tyrosine-•OH Adducts: A Radiation and Quantum Chemical Study

Influence of Ionizing Radiation on Spontaneously Formed Aggregates in Proteins or Enzymes Solutions

We have shown that many proteins and enzymes (ovalbumin, β-lactoglobulin, lysozyme, insulin, histone, papain) undergo concentration-dependent reversible aggregation as a result of the interaction of the studied biomolecules. Moreover, irradiation of those protein or enzyme solutions under oxidative stress conditions results in the formation of stable soluble protein aggregates. We assume that protein dimers are mainly formed. A pulse radiolysis study has been made to investigate the early stages of protein oxidation by N3• or ^•OH radicals. Reactions of the N3• radical with the studied proteins lead to the generation of aggregates stabilized by covalent bonds between tyrosine residues. The high reactivity of the ^•OH with amino acids contained within proteins is responsible for the formation of various covalent bonds (including C-C or C-O-C) between adjacent protein molecules. In the analysis of the formation of protein aggregates, intramolecular electron transfer from the tyrosine moiety to Trp^• radical should be taken into account. Steady-state spectroscopic measurements with a detection of emission and absorbance, together with measurements of the dynamic scattering of laser light, made it possible to characterize the obtained aggregates. The identification of protein nanostructures generated by ionizing radiation using spectroscopic methods is difficult due to the spontaneous formation of protein aggregates before irradiation. The commonly used fluorescence detection of dityrosyl cross-linking (DT) as a marker of protein modification under the influence of ionizing radiation requires modification in the case of the tested objects. A precise photochemical lifetime measurement of the excited states of radiation-generated aggregates is useful in characterizing their structure. Resonance light scattering (RLS) has proven to be an extremely sensitive and useful technique to detect protein aggregates.

Enhanced Single-Strand Breaks of a Nucleic Acid by Gold Nanoparticles under X-ray Irradiation

The hydroxyl radical concentration-dependent yield of single-strand breaks (SSBs), obtained through correction of scavenging and hindrance effects caused by gold nanoparticles (AuNPs), for fluorophore- and quencher-labeled DNA on AuNPs was 10 times that of free DNA based on fluorescence measurements of X-ray-irradiated DNA on AuNPs. By comparing the fluorescence data that revealed the number of SSBs with the results of mass spectrometry measurements that detected the total damage to DNA, we found that SSBs dominated DNA damage for DNA on AuNPs whereas non-SSB damage dominated for free DNA. The yield of RNA SSBs under X-ray irradiation was similar to that of DNA in the presence of AuNPs, whereas free RNA was more resistive to radiation than DNA. These results indicated the enhanced SSBs were likely catalyzed through the conversion from nucleobase damage to SSBs by AuNPs. The outcome of this work impacts materials and environmental science, sensing, nanotechnology, biology, and medicine.

Spontaneous and Ionizing Radiation-Induced Aggregation of Human Serum Albumin: Dityrosine as a Fluorescent Probe

The use of spectroscopic techniques has shown that human serum albumin (HSA) undergoes reversible self-aggregation through protein−protein interactions. It ensures the subsequent overlapping of electron clouds along with the stiffening of the conformation of the interpenetrating network of amino acids of adjacent HSA molecules. The HSA oxidation process related to the transfer of one electron was investigated by pulse radiolysis and photochemical methods. It has been shown that the irradiation of HSA solutions under oxidative stress conditions results in the formation of stable protein aggregates. The HSA aggregates induced by ionizing radiation are characterized by specific fluorescence compared to the emission of non-irradiated solutions. We assume that HSA dimers are mainly responsible for the new emission. Dityrosine produced by the intermolecular recombination of protein tyrosine radicals as a result of radiolysis of an aqueous solution of the protein is the main cause of HSA aggregation by cross-linking. Analysis of the oxidation process of HSA confirmed that the reaction of mild oxidants (Br2•−, N3•, SO4•−) with albumin leads to the formation of covalent bonds between tyrosine residues. In the case of •OH radicals and partly, Cl2•−, species other than DT are formed. The light emission of this species is similar to the emission of self-associated HSA.

The mechanism of organic radical oxidation catalysed by gold nanoparticles

Metal nanoparticles can catalyze reactions involving organic free radicals. From the first studies focused on the catalytic reduction of water by free radicals until today, the catalytic oxidation of organic radicals has not received attention. In this work, we present the results on the catalytic activity of gold nanoparticles in the oxidation of 2-propanol to acetone and acetanilide hydroxylation during water radiolysis. A detailed reaction mechanism of α-hydroxyisopropyl radical oxidation is discussed, explaining the increase in acetone formation by ca. 340% in the presence of gold nanoparticles. In the case of acetanilide hydroxylation in the presence of nanoparticles, a strong effect of oxygen in the reaction mechanism was observed: the increase in the oxygen concentration from 0 to 1.22 mM leads to a 40-fold decrease in hydroxylation product formation. This observation is unexpected since, in the absence of gold nanoparticles, oxygen stimulates hydroxylation reactions. We propose that in the presence of both oxygen and nanoparticles, oxygen attaches first to acetanilide OH-adducts, and then nanoparticles catalyze the oxidation of peroxyl type radicals, which does not lead to the formation of hydroxylation products.