PHOTOCHEMISTRY OF 2-MERCAPTOPYRIDINES .1. AN EPR AND SPIN-TRAPPING INVESTIGATION USING 5,5-DIMETHYL-1-PYRROLINE N-OXIDE IN AQUEOUS AND TOLUENE SOLUTIONS

We have employed EPR and the spin trap 5,5-dimethyl-1-pyrroline N-oxid e (DMPO) to investigate the photochemistry of pyridine-2-thione (2-S-P yrH), N-hydroxypyridine-2-thione (2-S-PyrNOH), and its sodium salt, (2 -S-PyrNONa), and disulfides, 2,2'-dithiobis (pyridine N-oxide) [(2-S-P yrN-->O)(2)] and 2,2'-dithiodipyridine [(2-S-Pyr)(2)]. We have found t hat upon UV irradiation they generate aromatic thiyl radicals, 2-S-.-P yr and 2-S-.-PyrN-->O, detected as DMPO adducts, DMPO/2-S-.-Pyr (1) an d DMPO/2-S-.-PyrN-->O (2). In aqueous solution (pH 7) hyperfine splitt ing constants (hfsc) were determined to be for 1: a(N) = 14.92 G, a(H) (beta) = 16.57 G, and for 2: a(N) = 14.78 G, a(H)(beta) = 16.05 G. In toluene hfsc were 13.09 G, 13.93 G for 1, and 13.25 G, 12.04 G for 2. Irradiation of 2-S-PyrH and DMPO in aerated pH 7 buffer generated the DMPO/O-.(2)- radical (3a, a(N) = 14.10 G, a(H)(beta) = 11.40 G, a(H)(g amma) = 1.18 G), while in aerated toluene DMPO/(O2H)-O-. was formed (a dduct 3b, a(N) = 12.74 G, a(H)(beta) = 10.41 G, a(H)(gamma) = 1.295 G) . In both systems adduct 1 was also observed. Because compounds posses sing the pyridine-2-thione moiety show antifungal, antibacterial and a nticancer properties, it is likely that the ability to photogenerate f ree radicals may be pertinent to their biological activity.