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/-O₂,- radical (3a, a_N= 14.10 G, a_H^β= 11.40 G, a_H^γ= 1.18 G), while in aerated toluene DMPO/.O₂H 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.