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Biomedical subjects

K D Asmus

Publications and source records attributed to K D Asmus.

24 records · Page 2Linked to original sources
PubMed1981

The reactions of the 2'-deoxyadenosine electron adduct in aqueous solution. The effects of the radiosensitizer p-nitroacetophenone. A pulse spectroscopic and pulse conductometric study.

The reactions of the one electron adduct of 2'-deoxyadenosine dA(-), in aqueous solutions have been studied using pulse radiolysis techniques with optical absorption and conductivity detection. The dA(-) radical anion itself shows a weak and featureless optical absorption at greater than 300 nm. It reacts rapidly with H20 (k equals 6 x 10(4)M(-1)s(-1); t1/2 equals 210 ns) to yield at least three different protonated structures (dAH., dA'H., dA"H.). In neutral solutions the most important of these (dAH., lambda(max) equals 315 nm) decays by a first order process (t1/2 approximately equal to 9 microseconds). In basic solutions dAH. undergoes on OH- catalysed rearrangement into another neutral radical (dA"'H., lambda(max) equals 355 nm; k(dAH. plus OH-) equals 1.7 x 10(8)M(-1)s(-1)). p-Nitroacetophenone (PNAP) reacts rapidly with the protonated electron adducts of 2'-deoxyadenosine (k equals 5 x 10(9)M(-1)s(-1)). An electron transfer occurs with dAH to yield PNAP and a reoxidized form of 2'-deoxyadenosine. As indicated by its pK value of 8.8 the latter is not, however, simply a repaired 2'-deoxyadenosine molecule, but is suggested to include the elements of water. Species dA"'H.(and dA'H.) react with PNAP in a process which is clearly not an electron transfer but likely an addition reaction.

Acetophenones↗
PubMed1981

Oxidation of methionine by X2.- in aqueous solution and characterization of some S therefore X three-electron bonded intermediates. A pulse radiolysis study.

The primary steps of the oxidation of methionine, Met, by X2.- (X = Cl; Br, I, SCN) have been investigated using pulse radiolysis techniques. In principle, the mechanism follows the same pattern which has been established for the OH radical induced oxidation. It is characterized by a primary attack at the sulphur atom and the formation of sulphur-centred radical cations S+. and S therefore (+) S as key intermediates. At pH greater than pKa of the carboxyl group these can then oxidize the amino function intramolecularly, which subsequently leads to irreversible decarboxylation. An additional important intermediate is a S therefore X radical with a three-electron bond between sulphur and halide. This radical is linked to the OH . radical induced mechanism through the equilibrium S therefore X + Met in equilibrium with S therefore (+) S + X-, and in addition exists in the equilibria X2.- + Met in equilibrium with S therefore X + X-, S therefore X in equilibrium with S+. + X- and S therefore X in equilibrium with Met + X.. The S therefore X- species absorb at 410, 400, and 390 nm for X = I, Br, and Cl, respectively. Absolute rate constants have been measured for the reactions S therefore (+) S + I- (k = 1.0 x 10(10) mol-1 ls-1, pH 1.4), Br2.- + Met (k = 2.5 x 10(9), 1.7 x 10(9), and 2.0 x 10(9) mol-1 ls-1 at pH 1, 5, and 11, respectively) and Cl2.- + Met (k = 3.9 x 10(9) mol-1 ls-1, pH 1). Methionine is also oxidized by (SCN)2.- whereas any significant oxidation by I2.- is not indicated. N-acetylmethionine, a model compound for a sulphur-containing peptide, and some other methionine derivatives are oxidized by X2.- in the same way, that is, through electrophilic addition at the sulphur function. The results require reinterpretation of some data published in the literature and are discussed in view of a 'selective free radical attack'.

Anions↗
PubMed1981

Tl2+ and Ag2+ metal-ion-induced oxidation of methionine in aqueous solution. A pulse radiolysis study.

Oxidation of methionine by Tl2+ and Ag2+ occurs with bimolecular rate constants of 2.5 x 10(9) and 3.3 x 10(8) mol-1 ls-1, respectively, and lead to sulphur-centred radical cations as primary intermediates. The overall mechanism is in principle the same as for OH. and X2.- (X = halide) induced oxidations with however one significant difference. Thus no oxidation of the amino group by S+. including subsequent decarboxylation is observed for methionine in its zwitterionic form while this readily occurs in the OH. and X2.- initiated mechanism. The reason is suggested to be complexing of the amino function by the metal ions which effectively lowers the reduction potential of the amino group. The sulphur-centred radical cation rather decays by a mostly first-order deprotonation process to yield CH2-S-CH2 approximately and CH3-S-CH approximately radicals of methionine. They absorb with lambda max = 290 mm and epsilon = 3000 +/- 600 mol-1 l cm-1, and exhibit reducing properties.

Free Radicals↗
PubMed1978

Chemical aspects of radiosensitization. Reaction of sensitizers with radicals produced in the radiolysis of aqueous solutions of nucleic acid components.

The effects of oxidants (sensitizers) in the radiolysis of aqueous solutions of pyrimidine and purine bases have been investigated. Their influence on the nature of the permanent radiolysis products and on the kinetics of disappearance of transient intermediates is reported. Particular attention has been paid to the chemical fates of carbocationic intermediates which can be produced from radical-sensitizer interaction.

Adenine↗
PubMed1976

Kinetics of nitroxyl radical reactions. A pulse-radiolysis conductivity study.

Absolute rate-constants for the reaction of the nitroxyl free radicals TAN and TMPN with radiation-chemically-formed radicals and ions have been determined. k(TAN + X) (in M(-1) sec(-1)=4-0 X 10(9) (for X = OH-), 2-9 X 10(10) (eaq-), 8-0 X 10(9) (H-), 7-2 X 10(8) (-CH2OH), 4-0 X 10(8) (CH3CHOH), 4-3 X 10(8) ((CH3)2COH) 2-8 X 10(8) (-CH2(CH3)2COH), 5-9 X 10(7) (glucose radical), 4-0 X 10(8) (c-C5H9-), and k(TMPN + X)=3-4 X 10(9) (OH-), 7-8 X 10(9) (eq-), 4-9 X 10(9) (H-), 4-4 X 10(8) (-CH2OH), 4-9 X 10(8) (CH3CHOH), 3-6 X 10(8) ((CH3)2COH), 1-5 X 10(8) (-CH2(CH3)2COH), 4-9 X 10(7) (glucose radical), 4-3 X 10(8) (c-C5H9-). Direct measurements by means of a pulse-radiolysis conductivity technique were based on the formation and destruction of charged species in these reactions within certain pH ranges. It is indicated that the radiosensitizing nitroxyls undergo both redox and addition reactions.

Cyclic N-Oxides↗