version date: 1 December 2006 Estradiol (7) 8,6A trans-diethylstilbestrol(6 3A cis-diethylstilbestrol (8) 5,9A Fig 5 Comparing molecule format and interatomic distances in hormone estradiol(7)and its active and inactive derivatives, respectively, trans-diethylstilbestrol (6) and cis-diethy stilbestrol(8) Conformational analysis and minimal energy The Chem3D program was employed for these studies, but others like Molecular Modeling Pro, Chem Site, Alchemy, Sybyl, Hyperchem, ChemX, CAChe, and Weblab Viewer are also available In conformational analysis of molecules, the bond rotation changes the dihedral angles and, consequently, the corresponding steric energy due to spatial overlaying of non- linked atoms and rotation torsional barriers The molecules drawn three-dimensionally are not necessarily in the most stable conformation generating a certain structure causes molecular distortions with unfavorable lengths, angles, and dihedral angles. Non-linked atoms also interact in the same spatial regions, generating steric and electrostatic repulsion. Correction of the molecule distortions may be achieved by energy minimization through two mathematical models(i) molecular mechanics or(ii) quantum mechanics. Unpredictable interactions related to superimposing molecular orbitals, electronic density distribution, or steric influence can be solved by computational methods. The optimized geometry of a molecule results from the interaction <www.iupac.org/publications/cd/medicinalchemistry/>6 H CH3 OH H H HO HO OH trans-diethylstilbestrol (6) Estradiol (7) OH HO cis-diethylstilbestrol (8) 5,9 Å 9,3 Å 8,6 Å Fig. 5 Comparing molecule format and interatomic distances in hormone estradiol (7) and its active and inactive derivatives, respectively, trans-diethylstilbestrol (6) and cis-diethylstilbestrol (8). Conformational analysis and minimal energy The Chem3D program was employed for these studies, but others like Molecular Modeling Pro, Chem Site, Alchemy, Sybyl, Hyperchem, ChemX, CAChe, and Weblab Viewer are also available. In conformational analysis of molecules, the bond rotation changes the dihedral angles and, consequently, the corresponding steric energy due to spatial overlaying of non￾linked atoms and rotation torsional barriers. The molecules drawn three-dimensionally are not necessarily in the most stable conformation. Generating a certain structure causes molecular distortions with unfavorable lengths, angles, and dihedral angles. Non-linked atoms also interact in the same spatial regions, generating steric and electrostatic repulsion. Correction of the molecule distortions may be achieved by energy minimization through two mathematical models (i) molecular mechanics or (ii) quantum mechanics. Unpredictable interactions related to superimposing molecular orbitals, electronic density distribution, or steric influence can be solved by computational methods. The optimized geometry of a molecule results from the interaction <www.iupac.org/publications/cd/medicinal_chemistry/> version date: 1 December 2006