Journal of Physical Chemistry B, Vol.121, No.13, 2864-2872, 2017
Analysis of Molecular Interaction of Drugs within beta-Cyclodextrin Cavity by Solution-State NMR Relaxation
The prime focus of the present study is to employ NMR relaxation measurement to address the intermolecular interactions, as well as motional dynamics, of drugs, viz., paracetamol: and aspirin, encapsulated within the beta-cydodextrin (beta-CD) cavity. In this report, we have attempted to demonstrate the applicability of nonselective (R-1(ns)), selective (R-1(se)), and bi-selective (R-1(bs)) spin-lattice relaxation rates to infer dynamical parameters, for example, the molecular rotational correlation times (tau(c)) and cross-relaxation rates (sigma(ij)) of the encapsulated drugs. Molecular rotational correlation times of the free drugs were calculated using the selective relaxation rate in the fast molecular motion time regime (omega(2)(H)tau(2)(c)) << 1 and R-1(ns)/R-1(se) approximate to 1.500), whereas that of the 1:1 complexed drugs were found from the ratio of R-1(ns)/R-1(se) in the intermediate motion time regime (omega(2)(H)tau(2)(c) similar to 1 and R-1(ns)/R-1(se) approximate to 1.054), and these values were compared with each other to confirm the formation of inclusion complexes. Furthermore, the cross-relaxation rates were used to evaluate the intermolecular proton distances. Also, density functional theory calculations were performed to determine the minimum energy geometry of the inclusion complexes and the results compared with those from experiments. The report, thus, presents the possibility of utilizingNMR relaxation data, a more cost-effective experiment, to calculate internuclear distances in the case of drug-supramolecule complexes that are generally obtained by extremely time consuming two-dimensional nuclear Overhauser enhancement-based methods. A plausible mode of insertion of the drug molecules into the beta-CD cavity has also been described based on experimental NMR relaxation data analysis.