Biomimetic cleavage of RNA models promoted by a dinuclear Zn(II) complex in ethanol. Greater than 30 kcal/mol stabilization of the transition state for cleavage of a phosphate diester.

The cleavage of a series of seven substituted aryl 2-hydroxypropyl phosphates (1a-g) promoted by a dinuclear Zn(II) complex (3:Zn(II)2:(-OCH2CH3)) was investigated in ethanol at pH 9.0 +/- 0.2 and 25 degrees C. The kinetics for appearance of the product phenols follow very strong saturation behavior for all substrates where the dissociation constant of the bound complex has an upper limit of Km = 3 x 10(-7) M and the k(cat)(max corr.) values (corrected for triflate inhibition) range from 168 to 3 s(-1). A partial s(s)pH/log k(cat)(max corr). profile for the 3:Zn(II)2:(-OCH2CH3)-catalyzed reaction of le (3-methoxyphenyl 2-hydroxypropyl phosphate) is bell-shaped, plateauing from 7.9-10, and is fit to a two kinetically important ionizations having s(s)pKa values of 7.22 and 10.9. The Brønsted plot of log (k(cat)(max corr.)) vs. the s(s)pKa values for the phenols shows a break at about 14.3 with two beta(lg), values of -1.12 and 0.0. This is analyzed in terms of a change in rate limiting step from cleavage of the phosphate to a conformational change where the binding of the phosphate changes from one P-O- ----Zn(II) interaction to a Zn(II)----O-P-O---Zn(II) double activation. An energetics calculation comparing the ethoxide promoted cleavage of 1a-g with the 3:Zn(ll)2:(-OEt) promoted reaction indicates that the complex, 3:Zn(II)2, stabilizes the ethoxide plus substrate transition state for the cleavage of 1a-g by between 33 and 36 kcal/mol. The origins of the large stabilization are discussed in terms of the effect of the medium on the various rate and equilibrium constants involved.