Validating a Strategy for Molecular Dynamics Simulations of Cyclodextrin Inclusion Complexes through Single-Crystal X-ray and NMR Experimental Data: A Case Study
A theoretical and experimental study about the formation and structure of the inclusion complex (?)-menthyl-O-?-D-glucopyranoside 1 with ?-cyclodextrin (?-CD) 2 is presented as paradigmatic case study to test the results of molecular dynamics (MD) simulations. The customary methodological approach?the use of experimental geometrical parameters as restraints for MD runs?is logically reversed and the calculated structures are a posteriori compared with those obtained from NMR spectroscopy in D2O solution and single crystal X-ray diffraction so as to validate the simulation procedure. The guest molecule 1 allows for a broad repertoire of intermolecular interactions (dipolar, hydrophobic, hydrogen bonds) concurring to stabilize the host?guest complex, thus providing the general applicability of the simulation procedure to cyclodextrin physical chemistry. Many starting geometries of the host?guest association were chosen, not assuming any a priori inclusion. The simulation protocol, involving energy minimization and MD runs in explicit water, yielded four possible inclusion geometries, ruling out higher-energy outer adducts. By analysis of the average energy at room temperature, the most stable geometry in solution was eventually obtained, while the kinetics of formation showed that it is also kinetically favored. The reliability of such geometry was thoroughly checked against the NOE distances via the pair distribution functions, that is, the statistical distribution of intermolecular distances among selected diagnostic atoms calculated from the MD trajectories at room temperature. An analogous procedure was adopted both with implicit solvent and in vacuo. The most stable geometry matched that found with explicit solvent but major differences were observed in the relative stability of the metastable complexes as a consequence of the lack of hydration on the polar moiety of the guest. Finally, a control set of geometrical parameters of the thermodynamically favored complex matched the corresponding one obtained from the X-ray structure, while local conformational differences were indicative of packing effects.