Interaction of methyl beta-D-xylopyranoside with metal ions: density functional theory study of cationic and neutral bridging and pendant complexes

Density functional theory calculations on complexes of 4C1, 1C4 and 2SO ring conformations of methyl beta-D-xylopyranoside 1 with divalent metal cations, M = Mg2+, Ca2+, Zn2+, and Cd2+, are presented. Bridging and pendant cationic, M(H2O)41]2+ and M(H2O)(5)1]2+, as well as neutral complexes, M(OH)2(H2O)(2)1] and M(OH)2(H2O)(3)1], and neutral complexes involving a doubly deprotonated sugar, M(H2O)(4)1(2-)], are considered. In aqueous and chloroform solution the stability of cationic and pendant neutral complexes is greatly diminished compared with gas-phase results. In contrast, bridging neutral complexes M(OH)2(H2O)(2)1] and those of type M(H2O)(4)1(2-)], are stabilized with increasing solvent polarity. Solvation also profoundly influences the preferred binding position and ring conformation. Compared with complexes of bare metal cations, additional ligands, e.g., H2O or OH-, significantly reduce the stability of 1C4 ring complexes. Irrespective of the cation, the most stable structure of bridging complexes M(H2O)(4)1]2+ results from coordination of the metal to O3 and O4 of methyl beta-D-xylopyranoside in its 4C1 ring conformation.