Three-State, Conformational Probe for Hydrophobic, π-Stacking Interactions in Aqueous and Mixed Aqueous Solvent Systems:  Anisotropic Solvation of Aromatic Rings

The conformational equilibrium of α,α‘-m-xylylene-N,N‘-bis-2-phenylpyridinium (1a) is analyzed as a three-state system and is herein proposed as a minimalist conformational probe to assay the stability of hydrophobic aromatic clusters in aqueous and mixed aqueous solvents. VT NMR spectrometry and computation elucidate the dynamic behavior of the conformational distribution of 1a. In this analysis the effect of diamagnetic anisotropy of the phenyl rings on the central m-xylene ring plays a vital role. α,α‘-m-Xylylene-N,N‘-bis-2-methylpyridinium (1b) serves as a spectrometric and calculational reference. Low aqueous concentrations of MeOD-d₄, EtOD-d₆, iPrOD-d₈, HFIP-d₂, HFA-d₂, and DMSO-d₆ perturb the distribution of conformers of 1a observed in D₂O. With fluoroalkanol as cosolvent, the conformer (C) that hides the most hydrogen atom SASA is the most stable. With alkanol as cosolvent, the conformer (F) that exposes the most hydrogen atom SASA is most stable. The work suggests a solvent parameter based on conformation that places water between alkanol and fluoroalkanol. Studies indicate that perturbations from pure aqueous solvation by HFIP or HFA (2−10 mol %) enhance the hydrophobicity (solvophobicity) of the solute (or lipophobicity of the solvent) primarily by increasing ΔH of solvation. Results are discussed in terms of anisotropic solvation of aromatic rings, perturbations by cosolvent, and exalted hydrophobic interactions between aqueous solvent and solute when the solute bears charge. Significance of the work is drawn to the solvent effect on peptide conformation.