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    HYDROPHOBE Challenge: A Joint Experimental and Computational Study on the Host–Guest Binding of Hydrocarbons to Cucurbiturils, Allowing Explicit Evaluation of Guest Hydration Free-Energy Contributions

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    The host–guest complexation of hydrocarbons (22 guest molecules) with cucurbit[7]­uril was investigated in aqueous solution using the indicator displacement strategy. The binding constants (10<sup>3</sup>–10<sup>9</sup> M<sup>–1</sup>) increased with guest size, pointing to the hydrophobic effect and dispersion interactions as driving forces. The measured affinities provide unique benchmark data for the binding of neutral guest molecules. Consequently, a computational blind challenge, the HYDROPHOBE challenge, was conducted to allow a comparison with state-of-the-art computational methods for predicting host–guest affinity constants. In total, three quantum-chemical (QM) data sets and two explicit-solvent molecular dynamics (MD) submissions were received. When searching for sources of uncertainty in predicting the host–guest affinities, the experimentally known hydration energies of the investigated hydrocarbons were used to test the employed solvation models (explicit solvent for MD and COSMO-RS for QM). Good correlations were obtained for both solvation models, but a rather constant offset was observed for the COSMO data, by ca. +2 kcal mol<sup>–1</sup>, which was traced back to a required reference-state correction in the QM submissions (2.38 kcal mol<sup>–1</sup>). Introduction of the reference-state correction improved the predictive power of the QM methods, particularly for small hydrocarbons up to C5