Experimental and theoretical approach to hydrogen-bonded diastereomeric interactions in a model complex

Binding affinities of (R,R)-1,2-cyclohexanediamine (R) to (R,R)-1,2-cyclopentanediol (R-5) and (S,S)-1,2-cyclopentanediol (S-5) and to the corresponding cyclohexanediols (R-6 and S-6) have been measured in benzene and in CCl4 at 298 K by microcalorimetry, and unexpected differences between the diastereomeric complexes are observed. Long time scale (0.1 mu s) molecular-dynamics simulations of the two smaller diastereomeric complexes, R/R-5 and R/S-5, in a simplified solvent model are reported. A direct free energy calculation gives results in good agreement with the experimental values measured in benzene for the first pair, but nearly identical results for the second pair, which is at variance with experiment. A systematic analysis of the dependence of simulation results on model parameters is performed, and no possibility is found to improve the enantioselectivity by parameter tuning. Other possible causes for discrepancies are specific solute-solvent or solvent-solvent interactions, electronic charge redistribution effects, or formation of clusters of more than two molecules. Owing to the long time scales reached, a well-converged picture of the dynamics is obtained, and the species present at equilibrium can be studied in detail. The average lifetime of the complex is found to be about 200 ps, whereas that of a hydrogen bond is only about 5 ps. Besides the unbound state, the dominant species observed in the simulations for both diastereomeric pairs are singly hydrogen-bonded complexes, with a clear preference for a O to N over the N to O hydrogen bond. Many other hydrogen-bonding patterns (bridged, double) are also observed in minor amounts.