(E)-1-Alkyl-4-[2-(alkylsulfonyl)-1-ethenyl]pyridinium Salts: Reaction with Thiol Groups Giving Rise to Chromophoric (E)-1-Alkyl-4-[2-(alkylsulfanyl)-1-ethenyl]pyridinium Salts

Abstract

(E)-1-Alkyl-4-[2-(alkylsulfonyl)-1-ethenyl]pyridinium salts were synthesized in two steps. These sulfones were stable at pH 7.3 and underwent a nucleophilic vinylic substitution (SNV) with mercaptans, including thiouracile, to give the corresponding 4-(thiovinyl)pyridinium salts. The X-ray diffraction structure of (E)-1-methyl-4-[2-(ethylsulfanyl)-1-ethenyl]pyridinium iodide indicated conjugation of the sulfur with the pyridinium ring. (Z)-1-Methyl-4-[2-(methylsulfanyl)-1-ethenyl]pyridinium iodide, prepared from the corresponding thioether by reaction with methyl iodide in diethyl ether, underwent isomerization to the E isomer in a first-order reaction in deuterated [D6]DMSO with an activation energy of 14 kcal mol−1. At pH 7, the (E)-1-methyl-4-[2-(methylsulfonyl)-1-ethenyl]pyridinium iodide (19) reacted specifically with thiols. The reaction of this sulfone with glutathione in a TES buffer at pH 7 was a second-order reaction (k=4100 M−1 s−1 at 30 °C) and gave the corresponding substitution product with an intense long wavelength absorption band (λmax=360 nm, e=27 500 M−1 cm−1). The modification of different enzymes of known structure with 19 showed the high selectivity of this reagent towards thiol groups and its usefulness in the quantitative determination of free thiol groups in proteins.