Dynamic enolate recognition in aqueous solution by zinc(II) in a phenacyl-pendant cyclen complex: Implications for the role of zinc(II) in class II aldolases

A zinc(II) complex of 1-(4-bromophenacyl)-1,4,7,10-tetraazacyclododecane, Z nL, has been synthesized to mimic the active center of class II aldolases. Its X-ray crystal structure showed that zinc(II) remains atop the four nitr ogen atoms of cyclen (average Zn-N distance 2.170 Angstrom) to bind with th e carbonyl oxygen (Zn-O distance of 2.159(3) Angstrom) and one H2O (Zn-O di stance of 2.130(3) Angstrom). Crystal data: monoclinic, space group P2(1)/n (No. 14), with a = 12.221(8) Angstrom, b = 11.052(7) Angstrom, c = 18.194( 9) Angstrom, beta = 97.12(3)degrees, V = 2438(5) Angstrom(3), Z = 4, R = 0. 041, and R-w = 0.046. The potentiometric pH titration of ZnL in aqueous sol ution disclosed dissociation of one proton with pK(a) = 8.41 at 25 degrees C and I = 0.1 (NaClO4), which yields a mixture of a hydroxide-bound complex (ZnL-OH-) and an enolate-bound complex (ZnH-L-1) in a 3:1 ratio, as determ ined by UV spectrophotometric and C-13 and H-1 NMR titrations. This is the first quantitative assessment of enolate formation promoted by a proximate zinc(II) ion near neutral pH in aqueous solution. The enolate-bound complex ZnH-L-1 independently isolated by treatment of ZnL with an equivalent amou nt NaOMe in acetonitrile was fully characterized to compare with ZnL. The e quilibrium between the ZnL-OH- and ZnH-L-1 varied with temperature (15, 25, and 35 degrees C), from which thermodynamic parameters of Delta G = 3.0 kJ mol(-1), Delta H = 8.7 kJ mol(-1), and Delta S = 19 7 mol(-1) K-1 at 25 de grees C were determined. Because of the facile enolization by zinc(II), the methylene hydrogen atoms (adjacent to the carbonyl) of ZnL were readily ex changed by deuterium under physiological conditions. The half-life at 25 de grees C for this H-D exchange at pD 7 (20 mM MOPS buffer) was determined to be 25 min by H-1 NMR measurements. Implications of the present results for the role of zinc(II) in the active center of class II aldolases are discus sed.