A Systematic Investigation and Insight into the Formation Mechanism of Bilayers of Fatty Acid/Soap Mixtures in Aqueous Solutions

Vesicles are the most common form of bilayer structures in fatty acid/soap mixtures in aqueous solutions; however, a peculiar bilayer structure called a “planar sheet” was found for the first time in the mixtures. In the past few decades, considerable research has focused on the formation theory of bilayers in fatty acid/soap mixtures. The hydrogen bond theory has been widely accepted by scientists to explain the formation of bilayers. However, except for the hydrogen bond, no other driving forces were proposed systematically. In this work, three kinds of weak interactions were investigated in detail, which could perfectly demonstrate the formation mechanism of bilayer structures in the fatty acid/soap mixtures in aqueous solutions. (i) The influence of hydrophobic interaction was detected by changing the chain length of fatty acid (C_<i>n</i>H_2<i>n</i>+1COOH), in which <i>n</i> = 10 to 18, the phase behavior was investigated, and the phase region was presented. With the help of cryogenic transmission electron microscopy (cryo-TEM) observations, deuterium nuclear magnetic resonance (²H NMR), and X-ray diffraction (XRD) measurements, the vesicles and planar sheets were determined. The chain length of C_<i>n</i>H_2<i>n</i>+1COOH has an important effect on the physical state of the hydrophobic chain, resulting in an obvious difference in the viscoelasticity of the solution samples. (ii) The existence of hydrogen bonds between fatty acids and their soaps in aqueous solutions was demonstrated by Fourier transform infrared (FT-IR) spectroscopy and molecule dynamical simulation. From the pH measurements, the pH ranges of the bilayer formation were at the p<i>K</i>_a values of fatty acids, respectively. (iii) Counterions can be embedded in the stern layer of the bilayers and screen the electrostatic repulsion between the COO^– anionic headgroups. FT-IR characterization demonstrated a bidentate bridging coordination mode between counterions and carboxylates. The conductivity measurements provided the degree of counterion binding (β = 0.854), indicating the importance of the counterions

Carboxyl–Peptide Plane Stacking Is Important for Stabilization of Buried E305 of Trichoderma reesei Cel5A

Hydrogen bonds or salt bridges are usually formed to stabilize the buried ionizable residues. However, such interactions do not exist for two buried residues D271 and E305 of Trichoderma reesei Cel5A, an endoglucanase. Mutating D271 to alanine or leucine improves the enzyme thermostability quantified by the temperature <i>T</i>₅₀ due to the elimination of the desolvation penalty of the aspartic acid. However, the same mutations for E305 decrease the enzyme thermostability. Free energy calculations based on the molecular dynamics simulation predict the thermostability of D271A, D271L, and E305A (compared to WT) in line with the experimental observation but overestimate the thermostability of E305L. Quantum mechanical calculations suggest that the carboxyl–peptide plane stacking interactions occurring to E305 but not D271 are important for the carboxyl group stabilization. For the protonated carboxyl group, the interaction energy can be as much as about −4 kcal/mol for parallel stacking and about −7 kcal/mol for T-shaped stacking. For the deprotonated carboxyl group, the largest interaction energies for parallel stacking and T-shaped stacking are comparable, about −7 kcal/mol. The solvation effect generally weakens the interaction, especially for the charged system. A search of the carboxyl–peptide plane stacking in the PDB databank indicates that parallel stacking but not T-shaped stacking is quite common, and the most probable distance between the two stacking fragments is close to the value predicted by the QM calculations. This work highlights the potential role of carboxyl amide π–π stacking in the stabilization of aspartic acid and glutamic acid in proteins