## 3 Results and Conclusions  
The fully optimized molecular structures i.e. optimal conformations of platinum and tin chelates are shown in Figs 1-5. The evaluated PM3 (with our preliminary parametrization for the Pt atom: USS\(=\)45.550, Upp\(=\)65.894, UDD\(=\)79.000, BETAS\(=\)2.780, BETAP\(=\)-2.006, ZS\(=\)2.000, ZP\(=\)2.000, ZD\(=\)5.884, ALP\(=\)1.690, GSS\(=\)10.190, GSP\(=\)7.235, GPP\(=\)2.100, GP\(=\)1.000, HSP\(=\)0.503 according to the package scheme [13]) and ab initio binding energies of complexes under considerations are listed in Table 2.  
In the [Pt(NH\({}_{3}\))\({}_{2}\)(guanine)]\({}^{2+}\) complex the two NH\({}_{3}\) ligands are coordinated in the plane of the guanine ring (Fig. 1) and the cation is bonded to the N7 atom even when we start from the structure involving guanine ligand coordinated via N7 and O6 atoms. It means that the second bond breaks during the optimization process. This structure is different from other calculations [19]. However, there may also occur some hydrogen bonding stabilization effect as the result of interaction of C - O... H (i.e. with the proton of the ammine group where O... H distance is of 1.05 A). This result indicates that platinum via free coordination site can bind with other N atom forming intrastrand or interstrand crosslinks [7] being one of possible modes of the specific interaction. Rather essential deformation of the guanine molecule was found with the -NH\({}_{2}\) group being out of the molecular plane. The ab initio Pt - N7 bond length is 2.002 A and for the Pt - N amounts to 2.122 A, whereas the distance Pt - O6 is equal to 2.663 A and O-Pt-N angle is 109\({}^{o}\) and 92\({}^{o}\), thus this complex has T shape. The platinum atom charge q(Pt), according to the [PERSON] population, amounts to 1.304. In the case of the dimethyltin [Sn(CH\({}_{3}\))\({}_{2}\)(guanine)]\({}^{2+}\) complex guanine ligand is coordinated through N7 and O6 atoms as a chelate ligand (Fig. 2). The tin complex has distorted tetrahedral coordination with C1-Sn-C2, N-Sn-C1, N-Sn-C2, O-Sn-C1, O-Sn-C2, and O-Sn - N angles are equal from 106\({}^{\rm o}\) to 112\({}^{\rm o}\). The bond lengths are: Sn - N7: 2.201 A, Sn - O6: 2.113 A, Sn - C: 2.105 A and the tin atomic charge is q(Sn) = 1.632. However, interaction of the [Sn(CH\({}_{3}\))\({}_{2}\)]\({}^{2+}\) cation with guanine can lead to the unexpected product, the complex in which one methyl group is transferrred to the O6 atom of the guanine molecule (Fig. 3). This stable conformation was found when optimizing geometry of this moiety at the all-valence PM3 level with one CH\({}_{3}\) group connected to the O6 atom and Sn(CH\({}_{3}\))\({}^{2+}\) bonded covalently to the N7 atom (Fig. 3). The ab initio binding energy for this conformation, which may be a transition complex, is greater by 10.23 eV in comparison with those for the structure depicted in Fig. 2. In this case the the Sn - N7 bond length is 2.264 A, and that of Sn-C(H\({}_{3}\)) amounts to 2.150 A. This result indicates that diorgano and triorganotin compounds can interact not only with nucleotide bases and phosphate groups of DNA, but also as an alkylating antitumor agents. \begin{table}
\begin{tabular}{c c c} \hline
**COMPLEX** & **PM3** & **AB INITIO** \\ \hline [Pt(NH\({}_{3}\))\({}_{2}\)(guanine)]\({}^{2+}\) & 11.13 & 5.86 \\ [Pt(NH\({}_{3}\))\({}_{3}\)(guanine)]\({}^{2+}\) & 5.92 & 5.63 \\ [Sn(CH\({}_{3}\))\({}_{2}\)(guanine)]\({}^{2+}\) & 6.48 & 8.30 \\ [Sn(CH\({}_{3}\))\({}_{3}\)(guanine)]\({}^{+}\) & 2.28 & 3.62 \\ \hline \end{tabular}
\end{table} 
Table 2: Binding energies [eV] for some model complexes of tin-DNA fragments  
Figure 2: Molecular structure of the [Sn(CH\({}_{3}\))\({}_{2}\)(guanine)]\({}^{2+}\) complex. Figure 4: Molecular structure of the square planar [Pt(NH\({}_{3}\))\({}_{3}\)(guanine)]\({}^{2+}\) complex. The guanine ligand is perpendicular to the coordination plane. Figure 3: The unusual resulting molecular structure of the [Sn(CH\({}_{3}\))\({}_{2}\) (guanine)]\({}^{2+}\) complex.