4-(Chloroacetyl)pyridinium Salt: A New Chromophoric and Solvatochromic Reagent of the Thiol Group

N-(4‘-chlorobenzyl)-4-(chloroacetyl)pyridinium
chloride was prepared from the
N-(4‘-chlorobenzyl)-4-acetylpyridinium chloride by chlorination with sulfuryl chloride.
This chloro ketone had a half life of 150 min at
pH 7.3 and reacted specifically with thiols. The reaction product
with pentanethiol crystallized as an enol whose
structure was determined by X-ray diffraction. The structure
indicated conjugation of the sulfur with the pyridinium
ring. In solution this enol was in equilibrium with the
corresponding ketone and with the hydrate or the hemiketal
according to the solvent. The enol was the major species in DMSO
while in water the ketone and the hydrate were
major. This solvent dependent equilibrium was reflected in the
variation of the absorption coefficient at the maximum
wavelength at around 400 nm according to the solvent. The
absorption maximum wavelength of the thioether, a
negative solvatochrome, depended on the solvent and showed a close to
linear relationship with the polarity index
of Snyder. The enolic hydroxyl group has a
pKa of 7.4 and the α thioether ketone a
pKa of 7.9. The chloro ketone
is a useful reagent to explore the environment of reactive cysteine
residue in proteins, giving information on the
polarity of the environment by the position of the absorption maximum
wavelength and the pKa of the enolic
hydroxyl
group.