Radiation-induced degradation of hydroxyl-containing amino acids in aqueous solutions

The set of data presented in this work shows that the radiolysis of hydroxyl-containing amino acids in aqueous solutions results in their degradation with -C-C- bond cleavage, which leads to both the decarboxylation of parent molecules and the removal of side substituents. The probability of these reactions mainly depends on the pH of the solution. Aminyl radicals or aminium radical cations, which are formed in the interaction of serine and threonine with ^·OH radicals via the mechanism of electron or hydrogen-atom transfer from the amino groups of the parent substances, play an important role in these degradation processes.     

An efficient synthesis of dehydroamino acids and dehydropeptides from O-Cbz and O-Eoc derivatives of serine and threonine

A simple and efficient method for the synthesis of dehydroamino acids from serine and threonine is reported. Various O-Cbz and O-Eoc derivatives of serine and threonine are prepared using CbzCl and EocCl, respectively, and are subjected to an anti-selective elimination on treatment with K₂CO₃ in DMF at 65 °C to afford dehydroalanine and dehydroamino butyric acid derivatives, respectively, in excellent yields. The high stability of these carbonate derivatives of serine and threonine allows their use in normal peptide synthesis as protected serine and threonine residues. Peptides synthesized by incorporating O-Cbz or O-Eoc derivatives undergo ready elimination under the reported conditions, to give the corresponding dehydropeptides in excellent yields. The reaction conditions are mild enough not to cause the racemization of other stereogenic centers present in the peptide.     

Effects of coenzyme Q<Subscript>0</Subscript>, ascorbic acid,and its glycoside on the radiation-induced dephosphorylation of organic phosphates in aqueous solutions

The effects of coenzyme Q₀, ascorbic acid, and 2-O-α-D-glucopyranosylascorbic acid on the radiation-induced dephosphorylation of 1-glycerophosphate, glucose-1-phosphate, and glucose-6-phosphate in deaerated aqueous solutions at pH 7 were studied by means of continuous radiolysis. It was found that the test compounds efficiently suppressed the radiation-induced dephosphorylation of organic phosphates in equimolar concentrations by interactions with water radiolysis products. At an organic phosphate to additive concentration ratio of 100: 1, ascorbic acid and coenzyme Q₀ can suppress the radiation-induced dephosphorylation of the initial substances in aqueous solutions by the reduction and oxidation of hydroxyl-containing carbon-centered radicals, respectively.     

Silylgruppenwanderung und CO2-Eliminierung beim massenspektrometrischen Zerfall von N-Trifluoracetyl-α-aminosäretrimethylsilylestern

The molecular ions of N-trifluoroacetyl α-amino acid trimethylsilyl esters exhibit a characteristic elimination of CO₂, in contrast to other amino acid derivatives and apparently caused by migration of the ester trimethylsilyl group to the oxygen atom of the N-trifluoroacetyl function. Fragmentation of the [M – CO₂]⁺˙ ions gives rise to a series of intense peaks, especially for the aliphatic amino acid derivatives. In the case of the isomers leucine and isoleucine, different base peaks are formed for the 20 eV spectra. Amino acids which can easily split off a group in their β-position possibly fragment by synchronous elimination of CO₂ and this group. With serine, threonine and cysteine a concurrent ester silyl migration to the oxygen of the β-function is observed, accompanied by the expulsion of CO₂.     

Radiation-induced transformations of threonine and its derivatives in aqueous solutions

It was found that the presence of the α,β-aminoalcohol group in the molecules of the hydroxyl-containing amino acid threonine and its derivatives is responsible for their ability to undergo free-radical degradation with C-C bond cleavage. This degradation occurs by the fragmentation of the nitrogen- and carbon-centered radicals of the initial substances.     

Reactions of aminyl radicals during radiolysis and photolysis of aqueous solutions of amino alcohols and their derivatives

It has been found that the radiolysis of the aqueous solutions of α,β-amino alcohols leads to the formation of degradation products of the parent substances. The experimental data suggest that the degradation process includes the stage of the formation of aminyl radicals, which undergo decomposition with the simultaneous cleavage of -C-C- and -O-H bonds through a five-membered transition state. The radiation-induced degradation of amino alcohols is enhanced in an alkaline medium, in which the amino group is deprotonated, and is blocked via the etherification of the hydroxyl group in the parent substances or the introduction of reducing agents.     

Reactions of an aromatic σ,σ-biradical with amino acids and dipeptides in the gas phase

Gas-phase reactivity of a positively charged aromatic σ,σ-biradical (N-methyl-6,8-didehydroquinolinium) was examined toward six aliphatic amino acids and 15 dipeptides by using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR) and laser-induced acoustic desorption (LIAD). While previous studies have revealed that H-atom and NH₂ abstractions dominate the reactions of related monoradicals with aliphatic amino acids and small peptides, several additional, unprecedented reaction pathways were observed for the reactions of the biradical. For amino acids, these are 2H-atom abstraction, H₂O abstraction, addition — CO₂, addition — HCOOH, and formation of a stable adduct. The biradical reacts with aliphatic dipeptides similarly as with aliphatic amino acids, but undergoes also one additional reaction pathway, addition/C-terminal amino acid elimination (addition — CO — NHCHR_C). These reactions are initiated by H-atom abstraction by the biradical from the amino acid or peptide, or nucleophilic addition of an NH₂ or a HO group of the amino acid or peptide at the radical site at C-6 in the biradical. Reactions of the unquenched C-8 radical site then yield the products not observed for related monoradicals. The biradical reacts with aromatic dipeptides with an aromatic ring in N-terminus (i.e., Tyr-Leu, Phe-Val, and Phe-Pro) similarly as with aliphatic dipeptides. However, for those aromatic dipeptides that contain an aromatic ring in the C-terminus (i.e., Leu-Tyr and Ala-Phe), one additional pathway, addition/N-terminal amino acid elimination (addition — CO — NHCHR_N), was observed. This reaction is likely initiated by radical addition of the biradical at the aromatic ring in the C-terminus. Related monoradicals add to aromatic amino acids and small peptides, which is followed by Cα-Cβ bond cleavage, resulting in side-chain abstraction by the radical. For biradicals, with one unquenched radical site after the initial addition, the reaction ultimately results in the loss of the N-terminal amino acid. Similar to monoradicals, the C-S bond in amino acids and dipeptides was found to be especially susceptible to biradical attack.     

EPR study of gamma induced radicals in amino acid powders

To better understand the composite character of amino acids EPR spectra, the radiolysis and reactions which occurred after irradiation of amino acids, a comparative EPR study of a few simple amino acids has been made in order to identify qualitatively and quantitatively the different radiation-induced radicals in amino acid powders. A spin-trapping methodology has been developed and carried out on irradiated glycine, alanine and valine.     

Highly lipophilic benzoxazoles with potential antibacterial activity

A series of lipophilic 2-substituted 5,7-di-tert-butylbenzoxazoles was prepared in average yields by the reaction of 3,5-di-tert-butyl-1,2-benzoquinone with amino acids and dipeptides bearing N-terminal glycine. Dipeptides having other N-terminal amino acids undergo oxidative deamination. 5,7-Di-tert-butylbenzoxazoles have shown activity against Mycobacterium tuberculosis and some nontuberculous strains where isoniazid has been inactive. Antifungal activity was mediocre.     

Preparation of Enantiomerically Pure Compounds Employing Anodic Oxidations of Carboxylic Acids – A Late Review of Research Done in the 1980ies

There are widely unknown enantiopure building blocks and non‐conventional transformations described in this old work that could become useful in today's diversity‐oriented organic synthesis world. Coupling and mixed couplings of functionalized CF₃‐substituted chiral radicals by Kolbe electrolysis of carboxylic acids lead to hexafluoro‐hexane‐2,5‐diol and to butyro‐ and valerolactone derivatives with functional‐group relationships that normally require components with reactivity umpolung. Oxidative decarboxylation of amino‐acid and peptide derivatives by Hofer‐Moest electrolyses provide entry into the synthetic use of chiral acyliminium‐ion intermediates. Chiral oxazoline and thioazoline building blocks (from serine, threonine, and cysteine) are accessible for substitutions and cycloadditions. The stereochemical course of oxidative CO₂H replacement in serine by nucleophilically introduced groups with retention of configuration is discussed.     

Electrochemical activation and carboxylation of fluorine-containing aromatic imines

On the basis of a study of the processes involved in the electrochemical activation of a series of fluorine-containing aromatic imines with and without the presence of carbon dioxide it was shown that the initial stage of such processes is the formation of imine radical-anions, which can both undergo spontaneous transformations and react with CO₂ with the formation of fluorine-containing amino acids. A correlation was established between the effects of the electronic structure of the imines on the electrochemical characteristics of their reduction and on the stability of the radical-anions. A relationship was established between the parameters of electrochemical activation of the imines in the presence of CO₂ and the yields of the amino acids. Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 44, No. 5, pp. 265–271, September–October, 2008.     

Use of Diphenyliodonium Bromide in the Synthesis of Some N-Phenyl α-Amino Acids

The N-phenyl methyl esters 4 of glycine, alanine, valine, leucine, isoleucine, phenylalanine, methionine, proline, serine, threonine, tyrosine, aspartic acid, and glutamic acid have been synthesized in good to excellent yields using diphenyliodonium bromide, AgNO₃, and a catalytic amount of CuBr starting from the relevant amino acid ester. The chiral integrity of the amino acids 5 was maintained during these reactions, which were confirmed by the synthesis of dipeptide for each N-phenyl amino acid. The structures of the new compounds were confirmed by the analysis of their IR, ¹H, and ¹³C NMR spectra in addition to CHN microanalysis or high-resolution mass spectrometry for the new N-phenyl amino acids 5 and the esters 4.     

Robust and straightforward chemo-enzymatic enantiopure dipeptide syntheses and diketopiperazines thereof

We have explored the scope of the synthetic route towards d-phenylglycyl diketopiperazines, involving a penicillin acylase catalysed formation of d-phenylglycyl dipeptides of l-amino acids with functional groups in the side chain. The synthesis of dipeptides from serine, threonine, glutamic acid, glutamine and methionine was successful. In contrast, aspartic acid, asparagine and cysteine only afforded trace amounts of dipeptides while no dipeptide was detected with arginine, lysine and tyrosine. Isolated dipeptide yields varied from 10% to 76%. The dipeptides were successfully converted into their corresponding enantiopure diketopiperazines by chemical esterification and cyclization under alkaline conditions, in 35–43% yield. In the case of glutamic acid, the procedure yielded the diketopiperazine with an esterified side chain. Remarkably with glutamine, the amide function in the side chain was transformed into an ester moiety, resulting in the same diketopiperazine as with glutamic acid.     

Radiation-induced decomposition of cyclotrimethylenetrinitramine in aqueous solutions

Radiation-induced decomposition of cyclotrimethylenetrinitramine, a representative of heterocyclic nitramines, in dilute aqueous solutions saturated with argon, air, and N₂O has been investigated. Dose dependence curves have been obtained, and the initial yields of formation of nitrite ions and hydrogen peroxide have been measured, making up 0.122 and 0.049 in deaerated solutions, 0.050 and 0.065 in aerated solutions, and 0 and 0.074 μmol/J in N₂O-saturated solutions, respectively. The main radiolysis initial steps involve the reaction of dissociative electron capture with nitro-group elimination in the form of nitrite ion and the reaction of hydrogen atom abstraction from the methylene group by hydroxyl radicals and atomic hydrogen.     

Effect of ascorbic acid and its derivatives on the radiation-chemical transformations of hydroxyl-containing organic compounds

The effect of ascorbic acid, 5,6-O-isopropylidyl-2,3-O-dimethylascorbic acid, and 2-O-glucopyranosylascorbic acid on the formation of main radiolysis products of ethanol and aqueous ethanol, ethylene glycol, α-methylglucopyranoside, and maltose solutions was studied by means of continuous radiolysis. The obtained results indicate that ascorbic acid effectively reacts with the carbon-centered hydroxyl-containing radicals derived from the substrates, thus decreasing the yield of their recombination and fragmentation products. It was found that the interaction of ascorbic acid and its derivatives with the carbon-centered radicals during the radiolysis of deaerated ethanol and its aqueous solutions may occur via both reducing and oxidizing mechanisms and that ascorbic acid in the aerated solutions acts as a hydrogen donor, reducing mainly the HO ₂ ^· radical to hydrogen peroxide.     

Radical-driven dissociation of odd-electron peptide radical ions produced in 157 nm photodissociation

Odd-electron a+1 radical ions generated in the 157 nm photodissociation of peptide ions were investigated in an ion trap mass spectrometer. To localize the radical, peptide backbone amide hydrogens were replaced with deuterium. When the resulting radical ions underwent hydrogen elimination, no H/D scrambling was obvious, suggesting that without collisional activation, the radical resides on the terminal α-carbon. Upon collisional excitation, odd-electron radical ions dissociate through two favored pathways: the production of a-type ions at aromatic amino acids via homolytic cleavage of backbone C_α-C(O) bonds and side-chain losses at nonaromatic amino acids. When aromatic residues are not present, nonaromatic residues can also lead to a-type ions. In addition to a-type ions, serine and threonine yield c _n−1 and a _n−1+1 ions where n denotes the position of the serine or threonine. All of these fragments appear to be directed by the radical and they strongly depend on the amino acid side-chain structure. In addition, thermal fragments are also occasionally observed following cleavage of labile Xxx-Pro bonds and their formation appears to be kinetically competitive with radical migration.     

Radiolysis of Aqueous Solutions of Poly(ethylene oxide) at 77 K

Low-temperature (77 K) γ-radiolysis of aqueous solutions of poly(ethylene oxide) was studied by ESR spectroscopy. Trapped electrons, hydroxyl radicals, and -CH₂-ĊH-O- radicals were identified as principal paramagnetic products of radiolysis. It was shown that an increase in the polymer concentration in solution led to a growth in number of trapped electrons and -CH₂-ĊH-O- macroradicals. The decay of hydroxyl radicals occurring at 110–115 K was accompanied by the formation of -CH₂-ĊH-O- macroradicals and partial recombination of hydroxyl radicals. Action of visible light resulted in virtually quantitative transformation of trapped electrons into -CH₂-ĊH-O- macroradicals. The radiation-chemical yields of the species trapped at 77 K were estimated and the scheme of radiation-induced chemical processes was suggested.__________Translated from Khimiya Vysokikh Energii, Vol. 39, No. 4, 2005, pp. 243–249.Original Russian Text Copyright © 2005 by Zakurdaeva, Nesterov, Feldman.     

Influence of Various Solid Catalysts on the Destruction Kinetics of Sodium Lauryl Sulfate in Aqueous Solutions by DBD

The destruction kinetics of sodium lauryl sulfate (anionic surfactant) in water solutions as well as the formation kinetics of destruction products under the action of an oxygen dielectric barrier discharge at atmospheric pressure both in the presence and in the absence of TiO₂, NiO and Ag₂O catalysts was studied. As it turned out in all cases the main decomposition products were carboxylic acids, aldehydes and carbon dioxide. The catalysts application was shown to result both in the increase of the decomposition efficiency and in a change in the ratio and yields of decomposition products.     

Abiogenesis and photostimulated heterogeneous reactions in the interstellar medium and on primitive earth: Relevance to the genesis of life

Heterogeneous reactions occur on solid photocatalyst particles of semiconductors and dielectric materials. When irradiated with suitable UV/visible light energy these particles generate electrons and holes, which on the surface are poised to undergo reductive and oxidative chemistry with a variety of organics and light gases. Various such particles have been identified in Interstellar Space, specifically in molecular–dust clouds, comets and meteorites. In this article, we examine briefly the nature of these dust clouds and then describe some basic aspects of heterogeneous photocatalysis, a methodology that has been shown useful in transforming organic substrates into smaller molecules and in the synthesis of potential biomolecules. Various types of gas/solid heterogeneous reactions involving mostly small molecules in gas/solid systems find a relationship to abiogenesis. For example, the decomposition of H₂O and CO₂ in the presence of CH₄ yields H₂CO; methane is photoconverted into ethane, propane, ethylene and other hydrocarbons and is photooxidized to alcohols and carbon dioxide; photofixation of CO₂ occurs to yield formaldehyde, formic acid, methanol and methane; and finally photofixation of molecular nitrogen N₂ can take place to produce NH₃ and N₂H₂. Not least is the synthesis of glycine, alanine, aspartic acid and serine from CH₄ and NH₃ over platinized titania. The relevance of heterogeneous photocatalysis to abiogenesis is discussed. It is argued that the physical conditions available in the interstellar medium are propitious to generate such biomolecules as amino acids and others, albeit this assertion necessitates laboratory simulations. Recent laboratory experiments involving very simple photoinduced processes are encouraging.     

The chemical effects of ionizing radiations on fluorinated ethers

The state of knowledge on the mechanism of the radiolysis of poly(perfluoroethers) (PFPE) with the poly(trimethylene oxide), poly(propylene oxide) and poly(ethylene-methylene oxide) structures is reviewed and the results of recent investigations performed using EPR, radiothermal luminescence (RTL), electron scavenging, pulse radiolysis and UV-Vis matrix spectroscopy are reported. The radiolytic degradation of the poly(perfluoroethers) is based on excited species formed presumably via the cation-electron geminate recombination. The excited ether moieties undergo homolytic CO and CC bond cleavage with formation of a variety of neutral free radicals which by coupling and cage disproportionation accounts for the major chemical changes detected in the PFPE chains. As a consequence of the bond cleavages a linear decrease of the average molecular weights with increasing the radiation dose is observed. The CF splitting is also detected from the related free radicals and branching units in the chains; however their incidence is of minor importance with respect of chain scissions and it is significant only in the linear PFPEs. The role of ionic intermediates has been analyzed by matrix EPR and UV-Vis spectroscopy, pulse radiolysis, RTL spectroscopy and electron scavenging experiments: the results strongly suggest that ionic reactions are participating to the radiolysis mechanism but the upper limit estimate of their contribution to free radical yields is <20% for the Demnum™ and Krytox™ and <30% for the more polar Fomblin™ Z. An insight of the stability and decomposition modes of the elusive radical-cations and radical-anions of the perfluoroethers associated to the ionic mechanism was obtained through low-temperature electron loss and electron capture selective experiments coupled with matrix EPR spectroscopy and M.O. calculations; the results suggest that both species are intrinsically unstable at 77 K, the favored decomposition modes being the CO splitting for the anions and the cleavage of the C-C bonds adjacent to oxygen for the cations.