MOLECULAR-ORBITAL STUDY OF THE STRUCTURE AND INTERACTIONS OF YLIDENE RHODANINES

Semiempirical (with the AM1 Hamiltonian) and ab initio (mainly with th e 6-31G and LANL2DZ + + (d,p) basis sets) molecular oribital calculat ions show the predominant tautomer of gas phase methylidene rhodanine is 2-thioxo-4-thiazolidinone in agreement with earlier work on other t ypes of rhodanines. Inclusion of solvation effects in the AM1 calculat ions corroborates with this tautomer is also preferred in aqueous solu tion. Energy-optimized bond lengths and angles show good agreement wit h those for a crystalline benzylidene rhodanine. The geometry of a hyd rogen bonded dimer matches closely the crystalline state arrangement. The two N-H ... O hydrogen bonds that form in the dimer provide a stab ilization energy of about -10 kcal/mol. The interactions of methyliden e rhodanine with a calcium ion are modeled with basis sets as large as LANL2DZ + + (d,p). The preferred binding site is near the carbonyl ox ygen for the neutral rhodanine and near the nitrogen and thione sulfur in a bidentate arrangement for the rhodanine anion. Implications for the interaction of benzylidene rhodanines with various possible protei n and metalloprotein receptors are discussed.