No abstract

Statistical analysis of data from crystal structures extracted from the Cambridge Structural Database (CSD) has shown that S and Se atoms display a similar tendency towards specific types of interaction if they are part of a fragment that corresponds to the side chains of cysteine (Cys), methionine (Met) selenocysteine (Sec) and selenomethionine (Mse). The most numerous are structures with C-HÁ Á ÁSe and C-HÁ Á ÁS interactions ($80%), notably less numerous are structures with SeÁ Á ÁSe and SÁ Á ÁS interactions ($5%), and SeÁ Á Á and SÁ Á Á interactions are the least numerous. The results of quantum-chemical calculations have indicated that C-HÁ Á ÁSe ($À0.8 kcal mol À1 ) and C-HÁ Á ÁS interactions are weaker than the most stable parallel interaction ($À3.3 kcal mol À1 ) and electrostatic interactions of / type ($À2.6 kcal mol À1 ). Their significant presence can be explained by the abundance of CH groups compared with the numbers of Se and S atoms in the crystal structures, and also by the influence of substituents bonded to the Se or S atom that further reduce their possibilities for interacting with species from the environment. This can also offer an explanation as to why O-HÁ Á ÁSe ($À4.4 kcal mol À1 ) and N-HÁ Á ÁSe interactions ($À2.2 kcal mol À1 ) are less numerous. Docking studies revealed that S and Se rarely participate in interactions with the amino acid residues of target enzymes, mostly because those residues preferentially interact with the substituents bonded to Se and S. The differences between Se and S ligands in the number and positions of their binding sites are more pronounced if the substituents are polar and if there are more Se/S atoms in the ligand.