New insights on the mechanisms of the pH-independent reactions of benzo[a]pyrenes 7,8-diol 9,10-epoxides.
The rates and products of the reactions of (+/-)-7beta,8alpha-dihydroxy-9beta,10beta-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (1) and (+/-)-7beta,8alpha-dihydroxy-9alpha,10alpha-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (2) in water and dioxane-water mixtures have been determined over a pH range wider than that of earlier studies. This study provides additional insight on the mechanisms of the pH-independent reactions of 1 and 2. The rate profile for reaction of 1 shows acid-catalyzed hydrolysis at pH <5, a rate plateau at pH 5-9.5, a negative inflection at pH 10-11.5, and a rate increase at pH >11.5. The rate decrease between pH 10 and pH 11.5 is accompanied by a decrease in the yield of tetrols from 60% (pH 8) to 29% (pH 11.2) and is interpreted to be the result of a partial change in mechanism brought about by attack of hydroxide ion acting as a base to deprotonate a carbocation intermediate and regenerate 1 at pH >10, thus reducing the contribution of the pathway for tetrol formation in which water attacks the carbocation. The rate profile for the reaction of 2 exhibits only a single rate plateau at intermediate pH, along with increases in rate at low and high pH because of second-order reactions of 2 with H+ and HO-, respectively. The lack of a rate depression at pH >10 and the product studies for the reaction of 2 in dilute sodium azide solutions suggest that the tetrol-forming reactions of the pH-independent reaction of 2 are concerted or near-concerted.
Doan
L
L
Department of Chemistry and Biochemistry, University of Maryland, Baltimore County, Baltimore, Maryland 21250, USA.
Lin
B
B
Yagi
H
H
Jerina
D M
DM
Whalen
D L
DL
D015123:7,8-Dihydro-7,8-dihydroxybenzo(a)pyrene 9,10-oxide / Q000737:chemistry* / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D006878:Hydroxides / Q000378:metabolism; D008956:Models, Chemical; D013329:Structure-Activity Relationship
D006878:Hydroxides; D015123:7,8-Dihydro-7,8-dihydroxybenzo(a)pyrene 9,10-oxide; C031356:hydroxide ion