The Relevance of Carbohydrate Hydrogen-Bonding Cooperativity Effects: A Cooperative 1,2-trans-Diaxial Diol and Amido Alcohol Hydrogen-Bonding Array as an Efficient Carbohyrate-Phosphate Binding Motif in Nonpolar Media

摘要

Carbohydrats with suitably positioned intramolecularly hydrogenbonded hydroxyl and amide groups have the potential to act as efficient bidentate phosphate binders by taking advantage of #sigma#-and/or #sigma#,#pi#-H-bonding cooperativity in nonolar solvents. Donor-donor 1,2-trans-diaxial amido alochol (1) and diol (3), in which one of the donor centres is cooperative, are very efficient carbohyrate-phsophate binding motifs. We have proven and quantified the key role of hydrogen-bonding centres indirectly involved in complexation, which serve to generate an intramolecular H-bond (six-membered cis H-bond) in 1 and 3. This motif enhances the donor nature of the H-bonding centres that are directly involved in complexation. A comparison of th thermodynamic parameters of the complexes formed between phophate and a cooperative (1-Phos) or anti-cooperative (2-Phos) bidentate H-bonded motif of a carbohydrate has allowed us to quantify the energietic advantage of H-bonding cooperativity in CDCl_3 and CDCl_3/CCl_4 (1:1.3) (#DELTA##DELTA#Gdeg=-2.2 and -2.0 kcal mol~(-1), respectively). The solvent dependences of the entropy and enthalpy contributions ot binding provide a valuable example of the delicate balance between entropy and enthalpy that can arise for a single process, providing effective cooperative binding in terms of #DELTA#Gdeg.