Electronic Structure of a Transient Histidine Radical in Liquid Aqueous Solution: EPR Continuous-Flow Studies and Density Functional Calculations

Theoretical analysis of the OH-initiated atmospheric oxidation reactions of imidazole

Imidazoles are present in Earth's atmosphere in both the gas-phase and in aerosol particles, and have been implicated in the formation of brown carbon aerosols. The gas-phase oxidation of imidazole (C3N2H4) by hydroxyl radicals has been shown to be preferentially initiated via OH-addition to position C5, producing the 5-hydroxyimidazolyl radical adduct. However, the fate of this adduct upon reaction with O2 in the atmospheric gas-phase is currently unknown. We employed an automated approach to investigate the reaction mechanism and kinetics of imidazole's OH-initiated gas-phase oxidation, in the presence of O2 and NOx. The explored mechanism included reactions available to first-generation RO2 radicals, as well as alkoxyl radicals produced from RO2 + NO reactions. Product distributions were obtained by assembling and solving a master equation, under conditions relevant to the Earth's atmosphere. Our calculations show a complex, branched reaction mechanism, which nevertheless converges to yield two major closed-shell products: 4H-imidazol-4-ol (4H-4ol) and N,N'-diformylformamidine (FMF). At 298 K and 1 atm, we estimate the yields of 4H-4ol and FMF from imidazole oxidation initiated via OH-addition to position C5 to be 34 : 66, 12 : 85 and 2 : 95 under 10 ppt, 100 ppt and 1 ppb of NO respectively. This work also revealed O2-migration pathways between the α-N-imino peroxyl radical isomers. This reaction channel is fast for the first-generation RO2 radicals, and may be important during the atmospheric oxidation of other unsaturated organic nitrogen compounds as well.

Enhanced cyanogen chloride formation after UV/PS and UV/H2O2 pre-oxidation and chlorination in natural river water

Ultraviolet/persulfate (UV/PS) and Ultraviolet/hydrogen peroxide (UV/H₂O₂) have attracted much attention in recent years as advanced oxidation processes for water treatment. However, it is not all clear how these two methods affect the formation of cyanogen chloride (CNCl) in the subsequent water chlorination process. In this study, it was found that both UV/H₂O₂ and UV/PS pre-oxidation promoted the formation of CNCl in six actual water samples collected from urban rivers. Glycine, uric acid, arginine and histidine were investigated as the model compounds to explore the effects of different methods on the production of CNCl. The results showed that compared with chlorination alone, pre-oxidation by UV/H₂O₂ and UV/PS can reduce the production of CNCl for glycine and uric acid by up to 95% during post-chlorination process. However, they can greatly promote the formation of CNCl for arginine and histidine by up to 120-fold. In a more detailed investigation, pre-oxidation of histidine formed highly reactive intermediates to chlorine, leading to increased CNCl formation and chlorine consumption. The results showed that the precursors of CNCl was altered after pre-oxidation, and need to be re-evaluated.

Hydroxyl-Radical Reaction Pathways for the Fast Photochemical Oxidation of Proteins Platform As Revealed by 18O Isotopic Labeling

Fast photochemical oxidation of protein (FPOP) has become an important mass spectrometry-based protein footprinting approach. Although the hydroxyl radical (•OH) generated by photolysis of hydrogen peroxide (H2O2) is most commonly used, the pathways for its reaction with amino-acid side chains remain unclear. Here, we report a systematic study of •OH oxidative modification of 13 amino acid residues by using 18O isotopic labeling. The results differentiate three classes of residues on the basis of their oxygen uptake preference toward different oxygen sources. Histidine, arginine, tyrosine, and phenylalanine residues preferentially take oxygen from H2O2. Methionine residues competitively take oxygen from H2O2 and dissolved oxygen (O2), whereas the remaining residues take oxygen exclusively from O2. Results reported in this work deepen the understanding of •OH labeling pathway on a FPOP platform, opening new possibilities for tailoring FPOP conditions in addressing many biological questions in a profound way.

Perovskites as Catalysts in Advanced Oxidation Processes for Wastewater Treatment

Advanced oxidation processes (AOPs), based on the formation of highly reactive radicals are able to degrade many organic contaminants present in effluent water. In the heterogeneous AOPS the presence of a solid which acts as catalyst in combination with other systems (O3, H2O2, light) is required. Among the different materials that can catalyse these processes, perovskites are found to be very promising, because they are highly stable and exhibit a high mobility of network oxygen with the possibility of forming vacancies and to stabilize unusual oxidation states of metals. In this review, we show the fundaments of different kinds of AOPs and the application of perovskite type oxides in them, classified attending to the oxidant used, ozone, H2O2 or peroxymonosulfate, alone or in combination with other systems. The photocatalytic oxidation, consisting in the activation of the perovskite by irradiation with ultraviolet or visible light is also revised.

Atmospheric oxidation reactions of imidazole initiated by hydroxyl radicals: kinetics and mechanism of reactions and atmospheric implications

The atmospheric oxidation mechanism of imidazole initiated by hydroxyl radicals is investigated via OH-addition and H-abstraction pathways by quantum chemistry calculations at the M06-2X/aug-cc-pVTZ level of theory coupled with reaction kinetics calculations using statistical Rice–Ramsperger–Kassel–Marcus (RRKM) theory and transition state theory (TST).

Surface Facet of CuFeO2 Nanocatalyst: A Key Parameter for H2O2 Activation in Fenton-Like Reaction and Organic Pollutant Degradation

The development of efficient heterogeneous Fenton catalysts is mainly by "trial-and-error" concept and the factor determining H₂O₂ activation remains elusive. In this work, we demonstrate that suitable facet exposure to elongate O-O bond in H₂O₂ is the key parameter determining the Fenton catalyst's activity. CuFeO₂ nanocubes and nanoplates with different surface facets of {110} and {012} are used to compare the effect of exposed facets on Fenton activity. The results indicate that ofloxacin (OFX) degradation rate by CuFeO₂ {012} is four times faster than that of CuFeO₂ {110} (0.0408 vs 0.0101 min^-1). In CuFeO₂ {012}-H₂O₂ system, OFX is completely removed at a pH range 3.2-10.1. The experimental results and theoretical simulations show that ^•OH is preferentially formed from the reduction of absorbed H₂O₂ by electron from CuFeO₂ {012} due to suitable elongation of O-O (1.472 Å) bond length in H₂O₂. By contrast, the O-O bond length is elongated from 1.468 to 3.290 Å by CuFeO₂ {110} facet, H₂O₂ tends to be dissociated into -OH group and passivates {110} facet. Besides, the new formed ≡Fe²⁺* on CuFeO₂ {012} facet can accelerate the redox cycle of Cu and Fe species, leading to excellent long-term stability of CuFeO₂ nanoplates.

Electron transfer vs. proton-coupled electron transfer as the mechanism of reaction between amino acids and triplet-excited benzophenones revealed by time-resolved CIDNP

We use proportionality between NMR signal enhancements in geminate products and hyperfine coupling constants in transient radicals to elucidate reaction mechanism in detail.

Protonation-modulated localization of excess electrons in histidine aqueous solutions revealed by ab initio molecular dynamics simulations: anion-centered versus cation-centered localization

In this work, we present an ab initio molecular dynamics simulation study on the interaction of an excess electron (EE) with histidine in its aqueous solution. Two different configurations of histidine (imidazole group protonated or not) are considered to reflect its different existing forms in neutral or slightly acidic surroundings. The simulation results indicate that localizations of EEs in different aqueous histidine solutions are quite different and are strongly affected by protonation of the side chain imidazole group and are thus pH-controlled. In neutral aqueous histidine solution, an EE localizes onto the carboxyl anionic group of the amino acid backbone after a relatively lengthy diffuse state, performing just like in an aliphatic amino acid solution. But in weakly acidic solution in which the side chain imidazole group is protonated, an EE undergoes a short lifetime diffuse state and finally localizes on the protonated imidazole group. We carefully examine these two different localization dynamics processes and analyze the competition between different dominating groups in their corresponding electron localization mechanisms. To explain the difference, we investigate the frontier molecular orbitals of these two systems and find that their energy levels and compositions are important to determine these differences. These findings can provide helpful information to understand the interaction mechanisms of low energy EEs with amino acids and even oligopeptides, especially with aromatic rings.

Heterogeneous activation of H2O2 by defect-engineered TiO(2-x) single crystals for refractory pollutants degradation: A Fenton-like mechanism

The heterogeneous catalyst plays a key role in Fenton-like reaction for advanced oxidation of refractory pollutants in water treatment. Titanium dioxide (TiO2) is a typical semiconductor with high industrial importance due to its earth abundance, low cost and no toxicity. In this work, it is found that TiO2 can heterogeneously activate hydrogen peroxide (H2O2, E°=1.78 eV), a common chemical oxidant, to efficiently generate highly-powerful hydroxyl radical, OH (E(0)=2.80 eV), for advanced water treatment, when its crystal shape, exposed facet and oxygen-stoichiometry are finely tuned. The defect-engineered TiO2 single crystals exposed by high-energy {001} facets exhibited an excellent Fenton-like activity and stability for degrading typical refractory organic pollutants such as methyl orange and p-nitrophenol. Its defect-centered Fenton-like superiority is mainly attributed to the crystal oxygen-vacancy, single-crystalline structure and exposed polar {001} facet. Our findings could provide new chance to utilize TiO2 for Fenton-like technology, and develop novel heterogeneous catalyst for advanced water treatment.

EPR studies on the kinetics of the α-hydroxyethyl radical generated by Fenton-like chemistry

Flow systems, either stopped or continuous, have long been at the core of kinetic studies of chemical reactions. Such flow systems need to be coupled with appropriate spectroscopic or otherwise techniques for the detection of the chemical species studied. If paramagnetic species are formed or consumed during the investigated reaction, electron paramagnetic resonance (EPR) with its nanomolar sensitivity can be the spectroscopic method of choice. However, not much literature is available on the application of EPR to quantitatively study kinetics of chemical reactions in the liquid state. Herein, we report the characterisation of the commercially available mixing resonator ER 4117 MX from Bruker using the TEMPO-dithionite reaction as a standard. Furthermore, this setup was used to study the kinetics of the Fenton-like system of TiCl3/H2O2 and ethanol forming theα-hydroxyethyl radical. Potential contributions of reactions with O2, H2O2, Ti(3+/4+), and self-recombination in the decay of theα-hydroxyethyl radical were investigated and the bimolecular decay was shown to be the dominant decay pathway, with a decay rate constant of 6.6×10(8) M(-1) s(-1). This study shows the effectiveness and capabilities of EPR as a direct, sensitive and in-situ method in kinetic studies.

Histidine residue mediates radical-induced hinge cleavage of human IgG1

Hydroxyl radicals induce hinge cleavage in a human IgG1 molecule via initial radical formation at the first hinge Cys(231) followed by electron transfer to the upper hinge residues. To enable engineering of a stable monoclonal antibody hinge, we investigated the role of the hinge His(229) residue using structure modeling and site-directed mutagenesis. Direct involvement of His(229) in the reaction mechanism is suggested by a 75-85% reduction of the hinge cleavage for variants in which His(229) was substituted with either Gln, Ser, or Ala. In contrast, mutation of Lys(227) to Gln, Ser, or Ala increased hinge cleavage. However, the H229S/K227S double mutant shows hinge cleavage levels similar to that of the single H229S variant, further revealing the importance of His(229). Examination of the hinge structure shows that His(229) is capable of forming hydrogen bonds with surrounding residues. These observations led us to hypothesize that the imidazole ring of His(229) may function to facilitate the cleavage by forming a transient radical center that is capable of extracting a proton from neighboring residues. The work presented here suggests the feasibility of engineering a new generation of monoclonal antibodies capable of resisting hinge cleavage to improve product stability and efficacy.

Radical trapping properties of imidazolyl nitrones

The ability of ten imidazolyl nitrones to directly scavenge free radicals (R(*)) generated in polar ((*)OH, O(*)(2)(-), SO(*)(3)(-) cysteinyl, (*)CH(3)) or in apolar (CH(3)-(*)CH-CH(3)) media has been studied. When oxygen or sulfur-centered radicals are generated in polar media, EPR spectra are not or weakly observed with simple spectral features. Strong line intensities and more complicated spectra are observed with the isopropyl radical generated in an apolar medium. Intermediate results are obtained with (*)CH(3) generated in a polar medium. EPR demonstrates the ability of these nitrones to trap radicals to the nitrone C(alpha) atom (alpha radical adduct) and to the imidazol C(5) atom (5-radical adduct). Beside the nucleophilic addition of the radical to the C(alpha) atom, the EPR studies suggest a two-step mechanism for the overall reaction of R(*) attacking the imidazol core. The two steps seem to occur very fast with the (*)OH radical obtained in a polar medium and slower with the isopropyl radical prepared in benzene. In conclusion, imidazolyl nitrones present a high capacity to trap and stabilize carbon-centered radicals.

Bicarbonate and active site zinc modulate the self-peroxidation of bovine copper-zinc superoxide dismutase

Peroxidation reactions of copper-zinc superoxide dismutase (CuZn-SOD1) or its zinc-depleted form (CuE-SOD1) that likely also involve a component of bicarbonate buffer have been implicated in the pathophysiology of the neurodegenerative diseases amyotrophic lateral sclerosis (ALS), Alzheimer's Disease and Parkinson's Disease. Neither removal of the zinc ion nor adding bicarbonate had large effects on the self-peroxidation reaction of bovine SOD1, but the combination of zinc-deficiency and added bicarbonate caused major changes to the spin trapped SOD1-centred free radical. Removal of the active site zinc ion greatly decreased the formation of an unassigned SOD1-centred free radical in the reaction with the inorganic peroxide peroxynitrite. The results suggest that under cellular conditions ( approximately 5 mM bicarbonate) zinc-deficient SOD1 peroxidation could play a pathogenic role in neurodegenerative diseases.

Tryptophan-based radical in the catalytic mechanism of versatile peroxidase from Bjerkandera adusta

Versatile peroxidase (VP) from Bjerkandera adusta is a structural hybrid between lignin (LiP) and manganese (MnP) peroxidase. This hybrid combines the catalytic properties of the two above peroxidases, being able to oxidize typical LiP and MnP substrates. The catalytic mechanism is that of classical peroxidases, where the substrate oxidation is carried out by a two-electron multistep reaction at the expense of hydrogen peroxide. Elucidation of the structures of intermediates in this process is crucial for understanding the mechanism of substrate oxidation. In this work, the reaction of H(2)O(2) with the enzyme in the absence of substrate has been investigated with electron paramagnetic resonance (EPR) spectroscopy. The results reveal an EPR signal with partially resolved hyperfine structure typical of an organic radical. The yield of this radical is approximately 30%. Progressive microwave power saturation measurements indicate that the radical is weakly coupled to a paramagnetic metal ion, suggesting an amino acid radical in moderate distance from the ferryl heme. A tryptophan radical was identified as a protein-based radical formed during the catalytic mechanism of VP from Bjerkandera adusta through X-band and high-field EPR measurements at 94 GHz, aided by computer simulations for both frequency bands. A close analysis of the theoretical model of the VP from Bjerkandera sp. shows the presence of a tryptophan residue near to the heme prosthetic group, which is solvent-exposed as in the case of LiP and other VPs. The catalytic role of this residue in a long-range electron-transfer pathway is discussed.

A new tyrosyl radical on Phe208 as ligand to the diiron center in Escherichia coli ribonucleotide reductase, mutant R2-Y122H. Combined x-ray diffraction and EPR/ENDOR studies

The R2 protein subunit of class I ribonucleotide reductase (RNR) belongs to a structurally related family of oxygen bridged diiron proteins. In wild-type R2 of Escherichia coli, reductive cleavage of molecular oxygen by the diferrous iron center generates a radical on a nearby tyrosine residue (Tyr122), which is essential for the enzymatic activity of RNR, converting ribonucleotides into deoxyribonucleotides. In this work, we characterize the mutant E. coli protein R2-Y122H, where the radical site is substituted with a histidine residue. The x-ray structure verifies the mutation. R2-Y122H contains a novel stable paramagnetic center which we name H, and which we have previously proposed to be a diferric iron center with a strongly coupled radical, Fe(III)Fe(III)R.. Here we report a detailed characterization of center H, using 1H/2H -14N/15N- and 57Fe-ENDOR in comparison with the Fe(III)Fe(IV) intermediate X observed in the iron reconstitution reaction of R2. Specific deuterium labeling of phenylalanine residues reveals that the radical results from a phenylalanine. As Phe208 is the only phenylalanine in the ligand sphere of the iron site, and generation of a phenyl radical requires a very high oxidation potential, we propose that in Y122H residue Phe208 is hydroxylated, as observed earlier in another mutant (R2-Y122F/E238A), and further oxidized to a phenoxyl radical, which is coordinated to Fe1. This work demonstrates that small structural changes can redirect the reactivity of the diiron site, leading to oxygenation of a hydrocarbon, as observed in the structurally similar methane monoxygenase, and beyond, to formation of a stable iron-coordinated radical.

Selective Cu2+/ascorbate-dependent oxidation of alzheimer's disease beta-amyloid peptides

This review summarizes tandem mass spectrometric investigations on the selectivity of metal-catalyzed oxidation of beta-amyloid peptide (betaAP) and related sequences. A remarkable feature of the Cu(2+)/ascorbate-dependent oxidation of these peptides is the switch from predominantly His oxidation in the neurotoxic peptide betaAP1-40 to predominantly Tyr oxidation in the nonneurotoxic reverse sequence betaAP40-1. Within betaAP1-40, His(13) and His(14) of the high-affinity Cu(2+)-binding site are most sensitive to oxidation. Eventually, the oxidation of one or both of these His residues could result in a less redox-active betaAP-Cu(2+) complex, lowering the incidence of betaAP-Cu(2+)-dependent Fenton-type reactions for the benefit of surrounding biological tissue.

Electronic structure of the cysteine thiyl radical: a DFT and correlated ab initio study

The electronic structure and the unusual EPR parameters of sulfur-centered alkyl thiyl radical from cysteine are investigated by density functional theory (DFT) and correlated ab initio calculations. Three geometry-optimized, staggered conformations of the radical are found that lie within 630 cm(-1) in energy. The EPR g-values are sensitive to the energy difference between the nearly-degenerate singly occupied orbital and one of the lone-pair orbitals (excitation energies of 1732, 1083, and 3429 cm(-1) from Multireference Configuration Interaction calculations for the structures corresponding to the three minima), both of which are almost pure sulfur 3p orbitals. Because of the near degeneracy, the second order correction to the g tensor, which is widely used to analyze g-values of paramagnetic systems, is insufficient to obtain accurate g-values of the cysteine thiyl radical. Instead, an expression for the g tensor must be used in which third order corrections are taken into account. The near-degeneracy can be affected to roughly equal extents by changes in the structure of the radical and by hydrogen bonds to the sulfur. The magnitude of the hyperfine coupling constants for the beta protons of the cysteine thiyl radical is found to depend on the structure of the radical. On the basis of a detailed comparison between experimental and calculated g-values and hyperfine coupling constants an attempt is made to identify the structure of thiyl radicals and the number of hydrogen bonds to the sulfur.

Radiolytic Modification of Basic Amino Acid Residues in Peptides: Probes for Examining Protein−Protein Interactions

Protein footprinting utilizing hydroxyl radicals coupled with mass spectrometry has become a powerful technique for mapping the solvent accessible surface of proteins and examining protein-protein interactions in solution. Hydroxyl radicals generated by radiolysis or chemical methods efficiently react with many amino acid residue side chains, including the aromatic and sulfur-containing residues along with proline and leucine, generating stable oxidation products that are valuable probes for examining protein structure. In this study, we examine the radiolytic oxidation chemistry of histidine, lysine, and arginine for comparison with their metal-catalyzed oxidation products. Model peptides containing arginine, histidine, and lysine were irradiated using white light from a synchrotron X-ray source or a cesium-137 gamma-ray source. The rates of oxidation and the radiolysis products were primarily characterized by electrospray mass spectrometry including tandem mass spectrometry. Arginine is very sensitive to radiolytic oxidation, giving rise to a characteristic product with a 43 Da mass reduction as a result of the loss of guanidino group and conversion to gamma-glutamyl semialdehyde, consistent with previous metal-catalyzed oxidation studies. Histidine was oxidized to generate a mixture of products with characteristic mass changes primarily involving rupture of and addition to the imidazole ring. Lysine was converted to hydroxylysine or carbonylysine by radiolysis. The development of methods to probe these residues due to their high frequency of occurrence, their typical presence on the protein surface, and their frequent participation in protein-protein interactions considerably extends the utility of protein footprinting.

Role of radical formation at tyrosine 193 in the allene oxide synthase domain of a lipoxygenase-AOS fusion protein from coral

Coral allene oxide synthase (cAOS), a fusion protein with 8R-lipoxygenase in Plexaura homomalla, is a hemoprotein with sequence similarity to catalases. cAOS reacts rapidly with the oxidant peracetic acid to form heme compound I and intermediate II. Concomitantly, an electron paramagnetic resonance (EPR) signal with tyrosyl radical-like features, centered at a g-value of 2.004-2.005, is formed. The radical is identified as tyrosyl by changes in EPR spectra when deuterated tyrosine is incorporated in cAOS. The radical location in cAOS is determined by mutagenesis of Y193 and Y209. Upon oxidation, native cAOS and mutant Y209F exhibit the same radical spectrum, but no significant tyrosine radical forms in mutant Y193H, implicating Y193 as the radical site in native cAOS. Estimates of the side chain torsion angles for the radical at Y193, based on the beta-proton isotropic EPR hyperfine splitting, A(iso), are theta(1) = 21 to 30 degrees and theta(2) = -99 to -90 degrees. The results show that cAOS can cleave nonsubstrate hydroperoxides by a heterolytic path, although a homolytic course is likely taken in converting the normal substrate, 8R-hydroperoxyeicosatetraenoic acid (8R-HpETE), to product. Coral AOS achieves specificity for the allene oxide formed by selection of the homolytic pathway normally, while it inactivates by the heterolytic path with nonoptimal substrates. Accordingly, with the nonoptimal substrate, 13R-hydroperoxyoctadecadienoic acid (13R-HpODE), mutant Y193H is inactivated after turning over significantly fewer substrate molecules than required to inactivate native cAOS or the Y209F mutant because it cannot absorb oxidizing equivalents by forming a radical at Y193.

Histidinyl radical formation in the self-peroxidation reaction of bovine copper-zinc superoxide dismutase

In the absence of suitable oxidizable substrates, the peroxidase reaction of copper-zinc superoxide dismutase (SOD) oxidizes SOD itself, ultimately resulting in its inactivation. A SOD-centered free radical adduct of 2-methyl-2-nitrosopropane (MNP) was detected upon incubation of SOD with the spin trap and a hydroperoxide (either H(2)O(2) or peracetic acid). Proteolysis by Pronase converted the anisotropic electron paramagnetic resonance (EPR) spectrum of MNP/(center dot)SOD to a nearly isotropic spectrum with resolved hyperfine couplings to several atoms with non-zero nuclear spin. Authentic histidinyl radical (from histidine + HO(center dot)) formed a MNP adduct with a very similar EPR spectrum to that of the Pronase-treated MNP/(center dot)SOD, suggesting that the latter was centered on a histidine residue. An additional hyperfine coupling was detected when histidine specifically (13)C-labeled at C-2 of the imidazole ring was used, providing evidence for trapping at that atom. All of the experimental spectra were convincingly simulated assuming hyperfine couplings to 2 nearly equivalent nitrogen atoms and 2 different protons, also consistent with trapping at C-2 of the imidazole ring. Free histidinyl radical consumed oxygen, implying peroxyl radical formation. MNP-inhibitable oxygen consumption was also observed when cuprous SOD but not cupric SOD was added to a H(2)O(2) solution. Formation of 2-oxohistidine, the stable product of the SOD-hydroperoxide reaction, required oxygen and was inhibited by MNP. These results support formation of a transient SOD-peroxyl radical.

Reductive damage in directly ionized DNA: saturation of the C5=C6 bond of cytosine in d(CGCG)(2) crystals

Electron paramagnetic resonance (EPR) was used to study an oligodeoxynucleotide duplex of d(CGCG)(2) that is known to crystallize in Z-form. After X irradiation at 4 K, EPR data were collected on single crystals and polycrystalline samples as a function of annealing temperature and dose. A radical produced by the net gain of a hydrogen atom at C6 and a proton at N3, Cyt(C6+H, N3+H(+))(+*), is identified. This radical had not been positively identified in polymeric DNA previously. The Cyt(C6+H, N3+H(+))(+*) makes up about 4% of the total radical population at 4 K, increasing to about 10-15% after the DNA is annealed to 240 K. There appears to be neither an increase nor a decrease in the absolute concentration of Cyt(C6+H, N3+H(+))(+*) upon annealing from 4 K to 240 K. Additionally, the presence of another radical, one due to the net gain of hydrogen at C5 of cytosine, the Cyt(C5+H)(*), is implicated. Together, these two radicals appear to account for 60-80% of the reduced species in DNA that has been irradiated at 4 K and annealed to 240 K.

Enzyme catalysis of 1,2-amino shifts: the cooperative action of B6, B12, and aminomutases

Ab initio molecular orbital theory is used to investigate 1,2-amino shifts catalyzed by aminomutases, coenzyme B12, and vitamin B6 (in the form of pyridoxal 5'-phosphate or PLP). Our calculations suggest essential catalytic roles for each of B12, B6, and the enzyme in aminomutase-catalyzed reactions. In the first place, coenzyme B12 provides a source of abstracting radicals, allowing the rearrangement reaction to take place on the radical surface. The involvement of radicals is supported by comparison of experimental and theoretical electron paramagnetic resonance parameters. Next, B6 allows the enzyme to lower the barrier height by introducing a double bond (allowing a low-energy intramolecular rearrangement pathway) and by providing a suitable site for partial protonation (preventing overstabilization of the reaction intermediate which could lead to enzyme inactivation). The PLP hydroxyl group is also identified as an important participant in these reactions. Finally, the enzyme holds the various reaction components in place and is the source of acidic functional groups that can provide partial protonation.

Mechanisms of metal-catalyzed oxidation of histidine to 2-oxo-histidine in peptides and proteins

The metal-catalyzed oxidation of histidine (His) to 2-oxo-histidine (2-oxo-His) represents an important pathway of protein oxidation in vivo and in vitro. In the pharmaceutical literature this pathway has received less attention. However, this fact may not necessarily represent reality as, in some cases, the analysis of His oxidation in proteins may be compromised by aggregation and precipitation of the target protein. For predicting the susceptibility of His residues in proteins it is important to understand in detail how protein sequence and conformation control the mechanisms of His oxidation to 2-oxo-His, reviewed in this paper.