INTERACTIONS BETWEEN [AUX4](-) (X = CL, CN) AND CYTOSINE AND GUANINE MODEL NUCLEOBASES - SALT FORMATION WITH (HEMI-) PROTONATED BASES, COORDINATION, AND OXIDATIVE-DEGRADATION OF GUANINE

Complexes of composition AuCl3L with L= 1,9-dimethylguanine (1,9-DimeG ) (1), 2-amino-6-methoxy-9-methylpurine ((OMe)-Me-6-9-MeG) (2), 9-meth ylguanine (9-MeGH) (3), and 9-ethylguanine (9-EtGH) (4) were prepared from Na[AuCl4] or H[AuCl4] and L in water at pH 3.2 +/- 0.5. The X-ray crystal structures of 1 and 2 were determined. In both complexes, Au( III) binding is via N(7) of the purine ring. The coordination sphere o f Au(III) is completed by three Cl ligands in a square plane and a wea ker intermolecular contact with an adjacent molecule (Au ... Cl, 3.59( 5) Angstrom (1) and 3.48(1) Angstrom (2)), thereby making each Au atom approximately square-pyramidal. In addition, three salts containing t he fully or hemiprotonated model nucleobases 7,9-dimethylguanine (7,9- DimeGH), 9-MeGH and 1-methylcytosine (1-MeC) were X-ray structurally c haracterized: [7,9-DimeGH] [AuCl4] (5), [(9-MeGH(2)) . (9-MeGH) ] [Au( CN)(4)] (6), and [ ( 1-MeCH) . (1-MeC) ] [Au(CN)(4)] . 2H(2)O (7). Alt hough poorly soluble in water, compounds 1-4 redissolve in a slow proc ess that takes weeks to months with destruction of the purine skeleton and formation of guanidinium and parabanic acid derivatives as well a s other products, depending upon the conditions applied. (C) 1998 Else vier Science S.A. All rights reserved.