MEC-N3)(5)(1-MEC-02)(3)(])(4-MEC=1-METHYLCYTOSINE) - STRUCTURAL MODELFOR DNA CROSS-LINKING AND DNA REWINDING BY ZN(II)()(1)

A trinuclear zinc complex, [Zn-3(OH)(2)(1-MeC)(8)](NO3)(4).5H(2)O (1-M eC = 1-methylcytosine) has been isolated and studied by X-ray crystall ography. It is composed of a Zn(OH)Zn(OH)Zn backbone, to which eight 1 -MeC nucleobases are bound, five via N(3) and three via O(2). The trin uclear cation can be divided into three parallel Zn(1-MeC-N3)(1-MeC-O2 ) ''layers'' which are connected by two mu-OH-groups and capped by two (1-MeC-N3) ligands. Within a layer, the two 1-MeC rings are planar (o r close to planar) around tetrahedral Zn ions, which is possible only because one of the two binding sites utilized is an exocyclic one (O(2 )). This O(2) oxygen atom is at the some time involved in a H bond to NH2(4) of the N(3) bound 1-MeC. Crystal data are as follows: monoclini c system, space group P2(1)/m, a = 12.825(3), b = 20.193(4), and c = 1 3.076(4) Angstrom, beta = 99.68(2), Z = 2. The trinuclear complex repr esents a feasible structural model for metal cross-links which occur i n DNA melting in the presence of Zn(II) ions and which probably are re sponsible for DNA renaturation. Zn-modified guanine, cytosine (G-C) an d adenine, thymine (A-T) base pairs are proposed which are formed acco rding to the same principle as ZnC2 entities: coordination to an unpro tonated endocyclic N atom of one base (C-N(3), A-N(1)) and an exocycli c O atom of the second base (G-O(6), T-O(4)) with retention of one H b ond.