Spectrophotometric measurement of mercaptans with 4,4 '-dithiodipyridine

In a preceding study, 4,4'-dithiodipyridine (DTDP) was shown to be superior to 5,5'-dithio-bis (2-nitrobenzoic acid) (Ellman's reagent) for spectropho tometric measurement of thiol groups in aqueous solution. (i) Sensitivity i s higher because a larger absorbance increase is seen at a given thiol conc entration. (ii) The intrinsic reactivity of thiolate anions for DTDP is muc h higher than for Ellman's reagent; thus, the reaction can be carried out a t pH greater than or equal to 4.5 instead of at pH 8.0. In the present stud y, these advantages of DTDP were exploited for spectrophotometric measureme nt of thiols in organic solvent. DTDP was found to quantitatively react wit h nonpolar thiols when triethylamine was used as catalyst, intense Light ab sorption (between 344 and 360 nn) was seen when the reaction was terminated with acetic acid, and the spectrophotometric responses were independent of the nonthiol portions of the mercaptans. The determination limit (10 x the standard deviation of the reagent blank) was 3 muM, and the upper limit wa s similar to 40 muM On a typical spectrophotometer. The thiol contents of t he mercaptans were independently verified by a modification of standard iod ometry in which toluene/butanol or chloroform/butanol was included to disso lve nonpolar mercaptans. (C) 2001 Academic Press.