Spontaneous drying of non-polar deep-cavity cavitand pockets in aqueous solution

Abstract

There are many open questions regarding the hydration of solvent-exposed non-polar tracts and pockets in proteins. Although water is predicted to de-wet purely repulsive surfaces and evacuate crevices, the extent of de-wetting is unclear when ubiquitous van der Waals interactions are in play. The structural simplicity of synthetic supramolecular hosts imbues them with considerable potential to address this issue. To this end, here we detail a combination of densimetry and molecular dynamics simulations of three cavitands, coupled with calorimetric studies of their complexes with short-chain carboxylates. Our results reveal the range of wettability possible within the ostensibly identical cavitand pockets-which differ only in the presence and/or position of the methyl groups that encircle the portal to their non-polar pockets. The results demonstrate the ability of macrocycles to template water cavitation within their binding sites and show how the orientation of methyl groups can trigger the drying of non-polar pockets in liquid water, which suggests new avenues to control guest complexation.

Fig. 1|. Chemical structures and illustrations of the deep-cavity cavitand hosts examined.

This figure demonstrates the relative positioning of the methyl functional units encircling the portal of hosts 2 and 3 that can trigger de-wetting of their non-polar binding pockets. (a) Chemical structures of octa-acid (OA, 1), tetra-endo-methyl octa-acid (TEMOA, 2), and tetra-exo-methyl octa-acid (TEXMOA), 3); (b) Space-filling representations of 1, 2 and 3 showing the structural differences of adding methyl groups (colored pink) to the endo-position (2) and exo-position (3) of the non-polar pocket.

Fig. 2|. Determination of cavitand volumes by measurement of densities of hosts in aqueous solution.

The results in this figure demonstrate host 2 is substantially larger than either hosts 1 or 3 in aqueous solution, providing experimental evidence of de-wetting of host 2’s non-polar pocket. Inverse of the experimentally determined solution density plotted as a function of the host mass fraction at 25°C. The symbols for hosts 1–3 are identified in the legend. The lines indicate linear fits to the experimental data. Error bars in the mass fraction and density indicate one standard deviation.

Fig. 3|. Molecular simulation characterization of cavitand pocket hydration and volume.

These figures characterize the hydration state of the non-polar pockets of hosts 1–3 and the role of de-wetting on determining the their partial molecular volumes. (a) Hydration number probability distribution, p(n), as a function of the number of waters within the cavitand’s non-polar pocket, n, determined from simulations at 25 °C and 1 bar. The symbols for hosts 1–3 are identified in the figure legend. (b) Cavitand partial molar volumes, $\overline{\nu }\left(n\right)$, as a function of the number of hydration waters in the non-polar pocket determined from simulation. The macroscopic partial molar host volume is determined by the weighted average $\overline{\nu }=\sum p\left(n\right)\overline{\nu }\left(n\right)$. The symbols correspond to results for 1, 2, and 3 as defined in (a). The thick, dashed, red line corresponds to the results for 1 shifted up by Δ = 81 cm³/mol. The error bars in both figures indicate one standard deviation.

Fig. 4|. Molecular simulation evaluation of cavitand pocket drying thermodynamics.

The results in this figure enable evaluation of the enthalpic and entropic contributions to cavitand pocket de-wetting, enabling assessment of the role of de-wetting on guest binding. Free energy of drying (ΔG_dry = −kTlnp(0)) hosts 1–3 as a function of temperature determined from simulation. The symbols for hosts 1–3 are identified in the figure legend. The lines indicate fits of the drying free energies to the expression ΔG_dry = ΔH_dry − TΔS_dry, assuming the enthalpy and entropy are constant. The error bars indicate one standard deviation.