Single-Molecule Force Spectroscopy Reveals Cation-$\pi$ Interactions in Aqueous Media Are Highly Affected by Cation Dehydration

Cation-$\pi$ interactions underlie many important processes in biology and materials science. However, experimental investigations of cation-$\pi$ interactions in aqueous media remain challenging. Here, we studied the cation-$\pi$ binding strength and mechanism by pulling two hydrophobic polymers with distinct cation binding properties, i.e., poly-pentafluorostyrene and polystyrene, in aqueous media using single-molecule force spectroscopy and nuclear magnetic resonance measurement. We found that the interaction strengths linearly depend on the cation concentrations, following the order of ${\mathrm{Li}}^{+}<{\mathrm{NH}}_4^{+}<{\mathrm{Na}}^{+}<{\mathrm{K}}^{+}$. The binding energies are $0.03–0.23\mathrm{kJ}{\mathrm{mol}}^{-1}{\mathrm{M}}^{-1}$. This order is distinct from the strength of cation-$\pi$ interactions in gas phase and may be caused by the different dehydration ability of the cations. Taken together, our method provides a unique perspective to investigate cation-$\pi$ interactions under physiologically relevant conditions.

Figure 1

Pulling collapsed polymer nanospheres in water using the SMFS method. (a) Illustration of the experimental design. Inset is polymer structures. (b) Typical force-extension curves for PPFS and PS. WLC fittings to the elastic stretching part are shown in orange. Dashed line is the base line. Upper inset illustrates the whole unfolding process of the nanosphere. (c) The histogram of plateau force for PPFS and PS collected in one experiment. (d) Illustration of the hydration shell theory.

Figure 2

The data analysis of plateau forces for pulling PPFS nanospheres in salt solutions. (a), (b), (c), and (d) are plateau forces in the NaCl, LiCl, KCl, and ${\mathrm{NH}}_4\mathrm{Cl}$ solution, respectively. The error bars represent the standard deviation. The lines are linear fittings to the data. The slope ($k$), intercept ($b$), and linear correlation coefficient ($\eta$) are listed in the figure, respectively. Here, the unit M denotes $\mathrm{mol}/\mathrm{kg}$ water and similarly hereinafter. (e) The surface tension of the salt solution at different concentrations. The line is the linear fitting to the data with slope ($k$) and intercept ($b$). (f) Four independent lines of Eq. (3) based on the data from Figs. 2.

Figure 3

Determining cation-$\pi$ binding strengths from SMFS data. (a), (b), (c), and (d) are concentration-dependent plateau forces for the NaCl, LiCl, KCl, and ${\mathrm{NH}}_4\mathrm{Cl}$ solutions, respectively. (e) The difference between the slopes of salt concentration dependent plateau forces of PS and PPFS. (f) The cation-$\pi$ binding energy averaged by the number of monomers and cation concentrations.

Figure 4

NMR studies of the interaction between hydrated ${\mathrm{Li}}^{+}$ and PS. (a) A schematic illustration of dehydrated ${\mathrm{K}}^{+}$, ${\mathrm{Na}}^{+}$ and hydrated ${\mathrm{Li}}^{+}$, ${\mathrm{NH}}_4^{+}$ entering the PS nanosphere. (b) The proton 1D spectrum for PS, $\mathrm{PS}+\mathrm{LiCl}$, and $\mathrm{PS}+\mathrm{NaCl}$. (c) Spectrum of a $100\text{−}\mathrm{\mu }\mathrm{s}$ H-H spin diffusion encoded HETCOR experiment. Measurement was performed at a 20 kHz magic angle spinning at 600 MHz magnetic field. (d) Spectrum of ${}^1\mathrm{H}\text{−}{}^7\mathrm{Li}$ CP (contact time of $400\mathrm{\mu }\mathrm{s}$) at 20 kHz magic angle spinning.