PHOTOCHEMISTRY OF 2‐MERCAPTOPYRIDINES. PART 1. AN EPR AND SPIN‐TRAPPING INVESTIGATION USING 5,5‐DIMETHYLl‐PYRROLINE N ‐OXIDE IN AQUEOUS AND TOLUENE SOLUTIONS *

Abstract We have employed EPR and the spin trap 5,5‐dimethyi‐1‐pyrroline N ‐oxide (DMPO) to investigate the photochemistry of pyridine‐2‐thione (2‐S‐PyrH), N ‐hydroxypyridine‐2‐thione (2‐S‐PyrNOH), and its sodium salt, (2‐S‐PyrNONa), and disulfides, 2,2′‐dithiobis (pyridine N ‐oxide) [(2‐S‐PyrN→O) 2 ] and 2,2′‐dithiodipyridine [(2‐S‐Pyr),]. We have found that upon UV irradiation they generate aromatic thil radicals, 2–'SPyr and 2–'S‐PyrN → O, detected as DMPO adducts, DMPO/2–'S‐Pyr ( 1 ) and DMPO/2–'S‐PyrN→O (2). In aqueous solution (pH 7) hyperfine splitting constants (hfsc) were determined to be for 1: a N = 14.92 G, a H β = 16.57 G, and for 2: a N = 14.78 G, a H β = 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 β = 11.40 G, a H γ = 1.18 G), while in aerated toluene DMPO/.O 2 H was formed (adduct 3b, a N = 12.74 G, a H β = 10.41 G, a H γ = 1.295 G). In both systems adduct 1 was also observed. Because compounds possessing the pyridine‐2‐thione moiety show antifungal, antibacterial and anticancer properties, it is likely that the ability to photogenerate free radicals may be pertinent to their biological activity.