Role of the orientation of –OH groups in the sensitivity and selectivity of the interaction of M2+ with ribosyl- and galactosyl-imino-conjugates

Abstract

Three glyco-conjugates, viz., L₁, L₂ and L₃, which differ either in their carbohydrate moiety or in their aromatic moiety or both, were synthesized and characterized and were shown to have β-anomeric form based on ¹H NMR and optical rotation studies. Metal ion interaction studies carried out in solution by emission and absorption spectral techniques exhibited selectivity towards Cu²⁺ in HEPES buffer and a two fold higher sensitivity for L₂ as compared to L₁. The composition of the complexed species has been established based on ESI MS. Dinuclear-Cu(II) complexes of all these conjugates have been synthesized and characterized based on analytical and spectral methods including FTIR, ¹H NMR, FAB MS, EPR, ORD, CD and magnetism, and the structures of 1 and 3 have been established based on single crystal XRD. The structures revealed subtle differences present in the orientation of the –OH groups and also their ion binding preferences both at the molecular level as well as at the lattice levels. In the dinuclear-Cu(II) complexes, while C3-O⁻ of ribosyl acts as a bridging moiety in 1, it is the C2-O⁻ of galactosyl that bridges in 3 and the Cu₂O₂ cores are stabilized by two intra-complex H-bond interactions formed using C4-OH in the case of 1 and C3-OH in the case of 3. While the glyco-moiety is poised perpendicular to the average plane of the Cu₂O₂ core in 1, this is in plane in the case of 3.