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

Abstract

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-d 4, EtOD-d 6, iPrOD-d 8, HFIP-d 2, HFA-d 2, and DMSO-d 6 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.

Graphene Quantum Dot Layers with Energy-Down-Shift Effect on Crystalline-Silicon Solar Cells

“…Recovery of the aromatic system led to π-stacking of neighboring GQDs, which eventually increased the size effect with red shift of PL peaks. 42,43 In the drop casting and drying method ( Figure S5d−f), the annealing process was also treated and the same PL spectra behavior was conformed. However, the degree of aggregation could not be controlled, which caused the multiple peak with larger red shift.…”

Helicogenicity of solvents in the conformational equilibrium of oligo(m-phenylene ethynylene)s: Implications for foldamer research

“…As with biological macromolecules (9), solvent-induced equilibrium shifting from the folded to the unfolded state involves disruption of these noncovalent interactions through either competitive solvation or changes in the bulk properties of the medium. In the foldamer field, the impact of solvent on foldable chains has been addressed only recently, and of these studies, only a limited scope of solvents has been explored (7,(10)(11)(12)(13)(14)(15)(16)(17). This fact is surprising considering the ease with which this experimental variable can be modulated and the information that can be obtained about the nature of the driving forces involved in the folding reaction.…”

Quantitative Four‐State Conformational Analysis by Ring Current NMR Anisotropy: A Family of Molecules Capable of Intramolecular π‐Stacking

“…Compounds 1 and 4 were used in recent work in which ab initio calculations of chemical shifts were used for the first time with modeling and NMR experiments in order to quantify multi-conformer, dynamic equilibria. [25,26] The results of the current study put some of the conclusions of the previous work in question. These incongruities are summarized and discussed in the conclusion section.…”

“…[26] Fluorinating the conformational solvent probe in this work rigorously tested the level of involvement of these dispersive interactions. Graphs 6.1Ϫ6.3 show that fluorination did not change the solvent-dependence of the conformers, thus edge-wise dispersive interactions could not be responsible for the solvent effect on conformation in 1Ϫ3.…”