A Cucurbit7uril-Ferrocene Complex Achieves Avidin-Biotin Affinity by Overcoming Enthalpy-Entropy Compensation

摘要

1.Introduction The design and characterization of synthetic host-guest pairs still represents an open challenge in supramolecular chemistry. Avidin-biotin complex is the strongest biological glue, achieving an extraordinarily high affinity of 1015 M-1[1,2] Cooperative, multiple, non-covalent interactions are essential for realizing such strong complexation and indeed the binding site of avidin is composed of an array of polar and aromatic residues, all of which cooperatively contribute to optimize biotin recognition and binding. Thus, several aromatic amino acid residues form a rigid "hydrophobic box" around the binding site and a number of polar residues stabilize the complex through a network of multiple hydrogen bonds. This complex structure induces a large negative enthalpy change (△H°) resulting from the formation of multiple hydrogen bonds, as well as robust van der Waals contacts inside the "hydrophobic box".[3] At the same time, a large negative entropy change (△S°) is expected due to the severe restriction of the biotin motion upon comptcxation with avidin. This effect is, however, cancelled by a large, positive entropy of dcsolvation, eventually making the overall entropy of complexation nearly zero.[3]