Kinetics of ozonation. 2. Amino acids and model compounds in water and comparisons to rates in nonpolar solvents

Abstract

Absolute rates of reaction of amino acids and model compounds with ozone have been measured in aqueous buffer solutions. For the less reactive amino acids and for amines, the rates of reaction are proportional to the amount of free (Le., unprotonated) amine present and therefore are relatively slow below p H 7. The rates of reaction of the corresponding amides are also slow. Rates of reaction of the more reactive amino acids at pH 7.0 are in the order of cysteine > tryptophan > methionine > tyrosine > histidine. 3-Hexenoic acid, a model of a polyunsaturated fatty acid (PUFA), is similar in reactivity to methionine or tyrosine. These data imply that the more reactive amino acids, whether free or in polypeptides, must be considered as possible targets for the reaction of ozone in vivo, along with the more usually considered PUFA. Which of these types of target molecules might receive the most damage in vivo probably depends on accessibility and cellular architecture, and these factors, plus the possibility of co-oxidation and other complexities, require further study.

Peptide ozonolysis: Product structures and relative reactivities for oxidation of tyrosine and histidine residues

“…Non-additivity could arise from the local competition for ozone between Tyr and His in angiotensin II or a slight change of secondary structure caused by the replacement of Phe, Tyr and/or His with Ala. For peptides, the effect is minor as the sum of the rate coefficients for the isolated residues differs from the angiotensin II rate coefficient by less than 50%. Reaction rates have been measured in other studies for residues within proteins [16] and for free amino acids [23]. Unfortunately, the rate coefficients for His and Tyr residues in larger proteins determined by Berlett et al [16] are not directly comparable to our work owing to a lack of information on the reactant concentrations.…”

“…Unfortunately, the rate coefficients for His and Tyr residues in larger proteins determined by Berlett et al [16] are not directly comparable to our work owing to a lack of information on the reactant concentrations. Pryor et al [23] report second-order rate coefficients for the free amino acid histidine, ranging from 38 M-1s-1 at pH 2 to 2.0 ϫ 105 M-1s-1 at pH 7. The rate coefficient for the free amino acid at a pH 3.5 is approximately equal to that of the His residue in DRVAIHPA in an unbuffered solution.…”

“…Spectrophotometric studies have shown that the amino acids Met, Trp, Tyr, Cys, His, and Phe are most susceptible to oxidation by ozone in aqueous solutions [23][24][25][26]. The reaction rates depend on solution conditions such as pH, as well as the precise method of ozonolysis and detection employed [12,23,24].…”

“…The reaction rates depend on solution conditions such as pH, as well as the precise method of ozonolysis and detection employed [12,23,24]. These studies have been performed mainly with UV absorbance, which permits characterization of carbonyls formed by side-chain oxidation and fluorescence-quenching that permits characterization of Trp oxidation.…”

Determination of rapid chlorination rate constants by a stopped-flow spectrophotometric competition kinetics method

“…(1) the reaction related to OCl À could be neglected due to a much lower reactivity of OCl À than HOCl (Gerritsen and Margerum, 1990;Rebenne et al, 1996); and (2) the protonated amine was unreactive towards HOCl/OCl À , just as it does not react with other electrophilic oxidants such as ozone and ferrate (Pryor et al, 1984;Qiang and Adams, 2004;Sharma et al, 2006). Therefore, the reaction of DPD with FC occurred predominantly between the neutral (i.e.…”