FACILE AND SELECTIVE ELECTROSTATIC STABILIZATION OF URACIL N(1)(-) ANION BY A PROXIMATE PROTONATED AMINE - A CHEMICAL IMPLICATION FOR WHY URACIL N(1) IS CHOSEN FOR GLYCOSYLATION SITE

A question of how uracil nitrogen N(1) is selectively activated in enz ymes (e.g., for deglycosylation in uracil-DNA glycosylase) has been ut terly overlooked, to which we have addressed by a model study with 6-( (1,4,7,10-tetraazacyclododecyl)methyl)uracil (HL, cyclen-attached urac il). The uracil N(1)H of the diprotonated cyclen-attached uracil (HL . 2H(+)) is easily deprotonated to be N(1)(-) anion form (L-. 2H(+)) in aqueous solution. The deprotonation constant (pK(a)) of 7.14 for HL . 2H(+) reversible arrow L-. 2H(+) + H+ was determined by potentiometri c pH titration at 25 degrees C with I = 0.10 (NaClO4). The unusually l ow deprotonation constant (cf. pK(a) = 9.9 for 3-methyluracil) is due to the electrostatic stabilization of the N(1)(-) anion by a proximate secondary ammonium cation of the diprotonated cyclen at physiological pH. The X-ray crystal structure of HL . 2H(+) as its dipicrate reveal ed that the uracil N(1)H is linked by a hydrogen bond network to one o f the cyclen secondary ammonium cation through a water. Crystals of HL . 2H(+).(picrate)(2) . H2O (C25H32N12O17) are triclinic, space group P (1) over bar (no. 2) with a = 9.295(4) Angstrom, b = 19.67(1) Angstr om, c = 8.886(6) Angstrom, alpha = 94.36(3)degrees, beta = 102.95(4)de grees, gamma = 87.04(4)degrees, V = 1576(8) Angstrom(3), Z = 2, R = 0. 054, and R-W = 0.081. The electrostatic stabilization of uracil N(1)(- ) anion is reassessed by a comparative study with a zinc(II) complex w ith the cyclen-attached uracil, where Zn2+ in the cyclen cavity strong ly binds to the uracil N(1)(-) (localized) anion. The deprotonation of N(1)H of HL (1 mM) occurred below pH 5 by the effect of equimolar Zn2 +, a stronger acid than two protons. Crystals of the zinc(II) complex (C13H23N6O2Zn . ClO4 . H2O) are triclinic, space group P (1) over bar (no. 2) with a = 9.461(3) Angstrom, b = 13.156(4) Angstrom, c = 8.687( 2) Angstrom, alpha = 101.21(2)degrees, beta = 103.55(2)degrees, gamma = 73.21(2)degrees, V = 997(0) Angstrom(3), Z = 2, R = 0.063, and R-W = 0.093. For comparison, we also have investigated the uracil N(1) acid ity with an ethylenediamine-attached uracil and an isomeric cyclen-att ached (at C(5)) uracil. The present example of electrostatic stabiliza tion of N(1)(-) anion may explain the facile uracil N(1)-alkyl (e.g., glycosyl) bond formation and cleavage in enzymes.