Microcalorimetric investigation on the formation of supramolecular nanoassemblies of associative polymers loaded with gadolinium chelate derivatives

In this study, isothermal titration microcalorimetry (ITC) and molecular modeling were used to investigate the mechanism of formation of supramolecular nanoassemblies prepared by mixing aqueous solutions of two associative polymers (i.e. polymerised beta-CD (p beta-CD) and dextran grafted with lauryl side chains (MD)). Their capacity to entrap a contrast agent for magnetic resonance imaging (a gadolinium, (Gd3+) derivative) has been determined by the same methods. ITC experiments have been employed to evaluate the stoichiometry of interaction (N), association constants (K) and thermodynamic parameter variation associated with complexation between hosts and guests involved in this system. The inclusion compounds studied were: as hosts, beta-CD and p beta-CD, and as guests, MID, adamantyl amine, and a Gd3+ complex functionalized with adamantane. It has been demonstrated that p beta-CD cavities tend to interact more favourably with MID (K = 25,000M(-1)) than with adamantly amine (K = 3650 M-1) and Gd3+ complex (K = 1460 M-1), forming 1:1 complexes, as also confirmed by molecular modeling. Noteworthy, the Gd3+ derivatives, although incorporated in the supramolecular nanoassemblies (by inclusion into the P-CD cavities of p beta-CD), did not destabilize the p beta-CD-MD inclusion complexes, probably because the interaction between p beta-CD and MD was stronger. Finally, the analysis of thermodynamic parameters revealed that the interaction between MD and p beta-CD was entropy driven (vertical bar Delta H vertical barvertical bar T Delta S vertical bar). (C) 2009 Elsevier B.V. All rights reserved.