Rate constant determination for the reaction of sulfhydryl species with the hydrated electron in aqueous solution

Abstract

The techniques of pulse radiolysis and absorption spectroscopy have been used to determine hydrated electron reaction rate constants with the sulfhydryls mercaptoethanol, 2-aminoethanethiol, cysteine, and penicillamine and the disulfides cystine and penicillamine disulfide, over the pH range 5.0-13.0 in aqueous solution. These values were found to be strongly dependent on the acid-base properties of the sulfhydryl compounds, with the rate constants dependent on the state of protonation of both the amino and mercapto groups, as well as the overall charge in these compounds. Rate constants for reaction of individual sulfhydryl and disulfide species have been determined using literature ionization constants. These values are contrasted to previous literature data.

Rational Design of a Selective Covalent Modifier of G Protein βγ Subunits

“…HEDS, also having a net neutral charge with a hydroxyl group in the place of amine of cystamine/selenocystamine, inhibited G␤␥ with an IC 50 of 137 mM, more than 1000 times that of cystamine. Given that the pK a of the leaving group, 2-mercaptoethanol, is 9.50 (Mezyk, 1995), just 0.90 above that of cysteamine, we conclude that the loss of the positive charge is responsible for most of the loss of reactivity. Although steric effects of the carboxyl group, in addition to the charge, might explain the lesser activity of L-cystine, this cannot be the case with HEDS.…”

“…Cystamine is also known to react with cysteines, but in this case, to form mixed disulfides (Gilbert, 1990). The pK a of selenocysteamine is 5.50 (Arnold et al, 1986), whereas the pK a of cysteamine is 8.60 (Mezyk, 1995). Cystamine indeed inhibited G␤␥ in the ELISA, but with an IC 50 of approximately 100 M (5000 times that of selenocystamine) (Fig.…”

“…4A). L-Cystine, with a net neutral charge, because of a carboxyl group in addition to each amine of cystamine, inhibited G␤␥ with IC 50 of 1 mM, 10 times that of cystamine, even though the pK a of L-cysteine is 8.33 (Mezyk, 1995), a little lower than that of cysteamine. HEDS, also having a net neutral charge with a hydroxyl group in the place of amine of cystamine/selenocystamine, inhibited G␤␥ with an IC 50 of 137 mM, more than 1000 times that of cystamine.…”

Reversible Hydrogen Transfer Reactions of Cysteine Thiyl Radicals in Peptides: the Conversion of Cysteine into Dehydroalanine and Alanine, and of Alanine into Dehydroalanine

“…Thus, the formation of 5 during the photo-irradiation of 1b in the presence of CH 2 Cl 2 indicates the possibility for disproportionation of CysS • outside the cage. 50,51 …”

Acoustic Droplet Vaporization and Propulsion of Perfluorocarbon‐Loaded Microbullets for Targeted Tissue Penetration and Deformation

“…To illustrate the selective PFH-droplet immobilization strategy developed herein, fluorescently tagged PFH emulsions, stabilized by a negatively charged surfactant, were electrostatically immobilized onto the cysteamine-modified inner gold surfaces ( Figure SI1 in the Supporting Information). The exposed amino group (pK a 8.6) of the cysteamine [39] is positively charged for the prescribed experimental settings (i.e., pH range 7.4-8.0) and thus electrostatically binds to emulsions stabilized with an anionic phosphate fluorosurfactant (pK a 7.2). Emulsions were strongly negative with a measured zeta potential of À46 mV in phosphate-buffered saline.…”