Abstract
In the attack of dithiothreitol (DTT) monoanion at flavin C(4a) to form the intermediate A,S-acetal, a for general-acid catalysis at N(5) was determined to be 0.58, 0.49, and 0.34 for 3-methyl- (3), 7-chloro- (4), and 7,8-dichlororiboflavin (6), respectively. Thus -{da/do) = pxy' = 0.29. Because Bransted plots were linear for these flavins, da/dpKHA = px = 0. Linear Hammett plots were observed for hydronium ion (p = 0.1) and water (p = 3.24) in the general-acid-catalyzed attack step, in the breakdown of the TV,5-acetal (p = 3.21) and for flavin Ed values (p = 4.64). In assigning o values substituents at C(7) and C(8) of flavin were taken to be meta and para, respectively, with regard to the reaction center at N(5) and C(4a). With other general acids, including acetic acid, phosphate monoanion and glycine ethyl ester, Hammett plots for attack showed upward curvature and -{dp/do) = py' = -3.6. It is shown that preassociation hydrogen-bonding pathways involving the species ~· and F1H+-A" cannot account for the observed general-acid-catalyzed rates. The attack step thus appears to be truly concerted, with proton transfer and S-C-N bond rearrangements occurring simultaneously. With use of the treatment proposed by Jencks and Jencks4 for reaction coordinate diagrams and the structure-reactivity parameters px, pf, and pxy', the transition state for the ester assisted addition of DTT to 4 occurs at a = 0.49 and = 0.41 with level lines of 0° and 134° glycine ethyl p„at the transition state. The minimum energy path (reaction coordinate) through the transition state bisects the level lines and has a slope of 0.41 with respect to the x'= a axis of a transformed reaction diagram. This result also implies concerted general-acid catalysis in attack. Structure-reactivity effects for breakdown of the TV,5-acetal suggest an early transition state with little S-S bond formation or C-S bond breakage. The effective molarity of 53 M for DTT compared with mercaptoethanol implies that some entropy must be lost in this early transition-state reaction. In Appendix I the pK for 3 protonated at N(5) was estimated to be -6.6.