(E)-1-Alkyl-4-[2-(alkylsulfonyl)-1-ethenyl]pyridinium salts: reaction withthiol groups giving rise to chromophoric (E)-1-Alkyl-4-[2-(alkylsulfanyl)-1-ethenyl]pyridinium salts

(E)-1-Alkyl-4-[2-(alkylsulfonyl)-1-ethenyl]pyridinium salts were synthesize d in two steps. These sulfones were stable at pH 7.3 and underwent a nucleo philic vinylic substitution (SNV) with mercaptans, including thiouracile, t o give the corresponding 4-(thiovinyl)-pyridinium salts. The X-ray diffract ion structure of (E)-1-methyl-4-[2-(ethylsulfanyl)-1-ethenyl]pyridinium iod ide indicated conjugation of the sulfur with the pyridinium ring. (Z)-1-Met hyl-4-[2-(methylsulfanyl)-1-ethenyl]pyridinium iodide, prepared from the co rresponding thioether by reaction with methyl iodide in diethyl ether, unde rwent isomerization to the E isomer in a first-order reaction in deuterated [D-6]DMSO with an activation energy of 14 kcal mol(-1). At pH 7, the (E)-1 -methyl-C[2-(methylsulfonyl)-1-ethenyl]pyridinium iodide (19) reacted speci fically 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 degre es C) and gave the corresponding substitution product with an intense long wavelength absorption band (lambda(max) = 360nm, epsilon = 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 usefulne ss in the quantitative determination of free thiol groups in proteins.