X-ray crystal structure of tetrakis(1-methylcytosine)copper(II) perchlorate dihydrate: effect of 1-methyl substitution on cytosine on the spectral and redox behaviour Pitchumony Tamil Selvi a , Mariappan Murali a , Mallayan Palaniandavar a, *, Martin Ko ¨ ckerling b , Gerald Henkel b a Department of Chemistry, Bharathidasan University, Tiruchirappalli 620024, Tamilnadu, India b FB6-Festko ¨rperchemie, Gerhard-Mercator-Universita ¨t Duisburg, Lotharstrabe 1, D-47057 Duisburg, Germany Received 20 December 2001; accepted 9 May 2002 Abstract Copper(II) tetrakis-complexes of cytosine (cyt), 1-methylcytosine (1-mcyt) and cytidine (cyd) have been isolated and their spectral and electrochemical properties investigated. The X-ray crystal structure of tetrakis(1-mcyt)copper(II) perchlorate dihydrate has been successfully determined. The co-ordination geometry around copper in the complex corresponds to square-based 4 /4? co- ordination. In addition to the preferential Cu  /N3 bonds, there is significant interaction between copper(II) and the exocyclic O2 of 1-mcyt rings. The mutually cis 1-mcyt rings are present in a head-tail-head-tail arrangement, which is stabilised by a network of bifurcated hydrogen-bonding between the exocyclic amine hydrogen atoms and the oxygen atoms of the adjacent carbonyl groups. The aqueous solution spectra of the complexes are slightly different from solid state spectra revealing that the solid state structures undergo slight changes on dissolution in water. The electronic and EPR spectral and electrochemical results are consistent with the retention of the solid state structure even in solution. The EPR spectra exhibit N-superhyperfine lines corresponding to the coordination of four N3 atoms of cytosines. The higher g  values indicate decreased covalency in the metal  /ligand bond and the range of g  /A  quotient (119  /121 cm) confirms the presence of CuN 4 square-planar co-ordination geometry even in solution. The plot of i pc and E 1/2 values versus 1-mcyt concentration for the electrochemical titration of Cu(ClO 4 ) 2 with 1-mcyt reveals an inflection point indicating the formation of 1:4 species in solution. The trend in E 1/2 values of the complexes shows that the incorporation of electron releasing methyl group/ribose moiety at N1 position of cytosine ring enhances the stabilisation of Cu(II), in spite of the steric demand from O /C2. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Cu(II) complexes; 1-Methylcytosine; Crystal structures; EPR and electronic spectra; Redox chemistry; Metal /nucleobase interaction 1. Introduction There is considerable interest in the interaction of metal ions with nucleic acids in view of the speculation that metal ions, which play a crucial role at some stage of gene expression (replication, transcription, transla- tion), would promote the interaction of proteins with nucleic acids through the formation of ternary com- plexes [1  /6]. Also metal ions have profound effect upon the conformational stability of nucleic acids in solution and in their absence the hydrogen bonds in nucleic acids are disrupted causing destabilisation of its structure [7 / 11]. The role of metal ions in inducing metal carcino- genesis [12], mutagenesis [12] and antineoplasticity of metallo-antitumor drugs [13], especially those of Pt(II) [13], is particularly well known. Further, metal ions are used in purifying nucleic acids [14,15] and in probing the structure and biochemistry of nucleic acids [14 /18]. So the study of models for metal ion interaction with nucleic acids gains importance and serves to provide insight into metal binding sites, mode of interaction and solution geometry of nucleic acids. Despite a wealth of structural information derived from X-ray data [19] and * Corresponding author. Tel.:  /91-431-660 353; fax:  /91-431-660 245 E-mail addresses: palani@bdu.ernet.in, palani51@sify.com. (M. Palaniandavar). Inorganica Chimica Acta 340 (2002) 139  /146 www.elsevier.com/locate/ica 0020-1693/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0020-1693(02)01091-5