The dinuclear Zn(II) complex catalyzed cyclization of a series of 2-hydroxypropyl aryl phosphate RNA models: Progressive change in mechanism from rate-limiting P-O bond cleavage to substrate binding

Am ethoxide-bridged dinuclear Zn(II) complex of 1,3-[N,N'-bis(1,5,9-triazacyclododecane)]propane (1-Zn(II)(2):(-OCH3)) was prepared, and its catalysis of the cyclization of a series of 2-hydroxypropyl aryl phosphates (4a-g) was investigated in methanol at (s)(s)pH 9.8, T= 25 degrees C by stopped-flow spectrophotometry. An X-ray diffraction structure of the hydroxide analogue of 1-Zn(II)(2):(-OCH3), namely 1-Zn(II)(2):(-OH), reveals that each of the Zn(II) ions is coordinated by the three N's of the triazacyclododecane units and a bridging hydroxide. The cyclizations of substrates 4a-g reveal a progressive change in the observed kinetics from Michaelis-Menten saturation kinetics for the poorer substrates (4-OCH3 (4g); 4-H (4f); 3-OCH3 (4e); 4-Cl (4d); 3-NO2, (4c)) to second-order kinetics (linear in 1-Zn(II)(2):(-OCH3)) for the better substrates (4NO(2),3-CH3 (4b); 4-NO2, (4a)). The data are analyzed in terms of a multistep process whereby a first formed complex rearranges to a reactive complex with a doubly activated phosphate coordinated to both metal ions. The kinetic behavior of the series is analyzed in terms of change in rate-limiting step for the catalyzed reaction whereby the rate-limiting step for the poorer substrates (4g-c) is the chemical step of cyclization of the substrate, while for the better substrates (4b,a) the rate-limiting step is binding. The catalysis of the cyclization of these substrates is extremely efficient. The k(cat)/K-M values for the catalyzed reactions range from 2.75 x 10(5) to 2.3 x 10(4) M-1 s(-1), providing an acceleration of 1 x 10(8) to 4 x 10(9) relative to the methoxide reaction (k(2)(OCH3), which ranges from 2.6 x 10(-3) to 5.9 x 10(-1) M-1 s(-1) for 4a-g). At a (s)(s)pH of 9.8 where the catalyst is maximally active, the acceleration for the substrates ranges from (1 - 4) x 10(12) relative to the background reaction at the same (s)(s)pH. Detailed energetics calculations show that the transition state for the catalyzed reaction comprising 1-Zn(II)(2), methoxide, and 4 is stabilized by about -21 to -23 kcal/mol relative to the transition state for the methoxide reaction. The pronounced catalytic activity is attributed to a synergism between a positively charged catalyst that has high affinity for the substrate and for the transition state,for cyclization, and a medium effect involving a reduced polarity/dielectric constant that complements a reaction where an. oppositely charged reactant and catalyst experience charge dispersal in the transition state.