On the mechanism of hydrophobic association of nanoscopic solutes

The hydration behavior of two planar nanoscopic hydrophobic solutes in liquid water at normal temperature and pressure is investigated by calculating the potential of mean force between them at constant pressure as a function of the solute-solvent interaction potential. The importance of the effect of weak attractive interactions between the solute atoms and the solvent on the hydration behavior is clearly demonstrated. We focus on the underlying mechanism behind the contrasting results obtained in various recent experimental and computational studies on water near hydrophobic solutes. The length scale where crossover from a solvent separated state to the contact pair state occurs is shown to depend on the solute sizes as well as on details of the solute-solvent interaction. We find the mechanism for attractive mean forces between the plates is very different depending on the nature of the solute-solvent interaction which has implications for the mechanism of the hydrophobic effect for biomolecules.     

A study of association by hydrogen bonding in the system phenol-CCl4 by the nuclear magnetic relaxation method

The system phenol-CCl₄ was studied by measuring proton and deuteron magnetic relaxation rates and self-diffusion coefficients at 25°C. From these data intermolecular relaxation rates have been calculated. By means of an association parameter A, association of phenol molecules with respect to various parts of the molecule has been established. Closest distances of approach between protons and configurations of maximum occurrence probabilities are reported.     

An application of 1H nuclear magnetic resonance to the determination of hydrophobic interactions in peptides

Intermolecular hydrophobic interactions between the indole or phenyl moieties of the peptides containing L -tryptophan (L -Trp) or L -phenylalanine (L -Phe) residues and the apolar isopropyl groups of the peptides containing L -leucine (L -Leu) or L -valine (L -Val) in aqueous solutions have been detected by ¹H NMR by monitoring the observed changes in the proton magnetic resonance parameters of the methyl resonances of the peptides containing L -Leu or L -Val residues. The ¹H NMR data indicate that intermolecular hydrophobic interactions are responsible for the observed changes in the proton magnetic resonance parameters of the methyl resonances. For example, when a solution of glycylglycylleucine (Gly-Gly-L -Leu) in deuterium oxide was mixed with glycylglycyltryptophan (Gly-Gly-L -Trp), the methyl protons of Gly-Gly-L -Leu exhibited large diamagnetic ring current shifts. However, when glycylglycylglycine (Gly-Gly-Gly) was substituted for Gly-Gly-L -Trp in the NMR experiment, the methyl resonances did not show any upfield or downfield shift, thereby demonstrating that the observed upfield shifts are not due to bulk susceptibility differences between solutions. The C-terminus peptides containing L -Leu or L -Val residues bind to the aromatic L -Trp or L -Phe peptides better than the N-terminus L -Leu or L -Val peptides. The C-terminus Gly-Gly-L -Leu binds better than the C-terminus glycylglycylvaline (Gly-Gly-L -Val). The strength and specificity of hydrophobic interactions in several small peptides are discussed.     

A nuclear magnetic relaxation study of the system benzene-methanol

The system benzene-methanol was investigated at 25°C by the nuclear magnetic relaxation method. The total relaxation rates are separated into an intramolecular and an intermolecular part. The rotational and translational mobility of the components were studied. Strong OH...OH association has been confirmed.Work done at Professor Hertz's laboratory in Karlsruhe on a leave from the Quaid-i-Azam University.     

Confinement of a liquid crystal to small droplets and its effect on nuclear magnetic relaxation

Abstract

The spatial dependence of the orientation of the molecular director and of the nematic order parameter is obtained by minimization of the Landau–de Gennes free energy of the nematic liquid crystal confined in a spherical droplet. Special attention is given to the vicinity of the nematic–isotropic transition. The influence of the resulting nematic structure, large liquid crystal–polymer interface and restricted molecular diffusion on the nuclear magnetic relaxation is analysed. The translationally-induced molecular reorientation and the liquid crystal–polymer cross relaxation are discussed in particular. The possibility of an indirect study of the molecular anchoring on the polymer surface is demonstrated.     

Complex formation of Fe(III) with tartaric acid by the method of nuclear magnetic relaxation

An investigation was made of the system Fe(III)-tartaric acid at an equimolar ratio of the components by the method of nuclear magnetic relaxation. The formation of monomer and dimer complexes FeH_ntar and Fe₂(H_ntar)₂ (n = 3–0), as well as a mixed ligand complex Fe₂(H₃tar)(H₂tar), was established, and their stability constants were determined. The diagrams of the distribution of complex forms as a function of the pH were calculated according to the values of the stability constants. Questions of the monomer dimer transition of the investigated complexes in solution are discussed.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 23, No. 4, pp. 492–497, July–August 1987.     

On the theory of the effective nuclear magnetic relaxation time T2e and its application to polymer melts

A general expression for the magnetization decay of a multipulse group is derived. This formula is applied to a three-component model of molecular motions in polymer melts. The influence of the several components on the magnetization decay is discussed. The relation of the effective nuclear magnetic relaxation time T_2e to the Anderson-Weiss formula is also shown, and an analytical expression for the transverse relaxation in melts is derived. Finally T_2e is compared with the relaxation time in the rotating frame T_1ρ in the melt. The theoretical results for T_2e are tested with measurements of frequency dependence in polyethylene melts.     

Potential of mean force between hydrophobic solutes in the Jagla model of water and implications for cold denaturation of proteins

Using the Jagla model potential we calculate the potential of mean force (PMF) between hard sphere solutes immersed in a liquid displaying water-like properties. Consistent estimates of the PMF are obtained by (a) umbrella sampling, (b) calculating the work done by the mean force acting on the hard spheres as a function of their separation, and (c) determining the position dependent chemical potential after calculating the void space in the liquid. We calculate the PMF for an isobar along which cold denaturation of a model protein has previously been reported. We find that the PMF at contact varies non-monotonically, which is consistent with the observed cold denaturation. The Henry constant also varies non-monotonically with temperature. We find, on the other hand, that a second (solvent separated) minimum of the PMF becomes deeper as temperature decreases. We calculate the solvent-solvent pair correlation functions for solvents near the solute and in the bulk, and show that, as temperature decreases, the two pair correlation functions become indistinguishable, suggesting that the perturbation of solvent structure by the solute diminishes as temperature decreases. The solvent-solute pair correlation function at contact grows as the temperature decreases. We calculate the cavity correlation function and show the development of a solvent-separated peak upon decrease of temperature. These observations together suggest that cold denaturation occurs when the solvent penetrates between hydrophobic solutes in configurations with favorable free energy. Our results thus suggest that cold denatured proteins are structured and that cold denaturation arises from strong solvent-solute interactions, rather than from entropic considerations as in heat denaturation.     

Quantifying the hydrophobic effect. 1. A computer simulation-molecular-thermodynamic model for the self-assembly of hydrophobic and amphiphilic solutes in aqueous solution

Surfactant micellization and micellar solubilization in aqueous solution can be modeled using a molecular-thermodynamic (MT) theoretical approach; however, the implementation of MT theory requires an accurate identification of the portions of solutes (surfactants and solubilizates) that are hydrated and unhydrated in the micellar state. For simple solutes, such identification is comparatively straightforward using simple rules of thumb or group-contribution methods, but for more complex solutes, the hydration states in the micellar environment are unclear. Recently, a hybrid method was reported by these authors in which hydrated and unhydrated states are identified by atomistic simulation, with the resulting information being used to make MT predictions of micellization and micellar solubilization behavior. Although this hybrid method improves the accuracy of the MT approach for complex solutes with a minimum of computational expense, the limitation remains that individual atoms are modeled as being in only one of two states-head or tail-whereas in reality, there is a continuous spectrum of hydration states between these two limits. In the case of hydrophobic or amphiphilic solutes possessing more complex chemical structures, a new modeling approach is needed to (i) obtain quantitative information about changes in hydration that occur upon aggregate formation, (ii) quantify the hydrophobic driving force for self-assembly, and (iii) make predictions of micellization and micellar solubilization behavior. This article is the first in a series of articles introducing a new computer simulation-molecular thermodynamic (CS-MT) model that accomplishes objectives (i)-(iii) and enables prediction of micellization and micellar solubilization behaviors, which are infeasible to model directly using atomistic simulation. In this article (article 1 of the series), the CS-MT model is introduced and implemented to model simple oil aggregates of various shapes and sizes, and its predictions are compared to those of the traditional MT model. The CS-MT model is formulated to allow the prediction of the free-energy change associated with aggregate formation (gform) of solute aggregates of any shape and size by performing only two computer simulations-one of the solute in bulk water and the other of the solute in an aggregate of arbitrary shape and size. For the 15 oil systems modeled in this article, the average discrepancy between the predictions of the CS-MT model and those of the traditional MT model for gform is only 1.04%. In article 2, the CS-MT modeling approach is implemented to predict the micellization behavior of nonionic surfactants; in article 3, it is used to predict the micellization behavior of ionic and zwitterionic surfactants.     

Effect of perturbation of the atomic basis on nuclear magnetic shielding constants for small molecules

We have calculated the components of the paramagnetic part of the magnetic shielding tensor for nuclei in molecules of LiH, HF, and H₂O within the uncoupled variant of Hartree-Fock-Roothaan perturbation theory, taking into account the dependence of the original basis set of Slater-type AO's (STO's) on the perturbation parameter. We have shown that it is necessary to take into account such a dependence when calculating the components of the magnetic shielding tensor in minimal basis sets of STO's. We have carried out a comparative analysis of the data obtained with results of other approaches.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 24, No. 5, pp. 527–532, September–October, 1988     

Ethanol determination in frozen fruit pulps: an application of quantitative nuclear magnetic resonance

This study reports the chemical composition of five types of industrial frozen fruit pulps (acerola, cashew, grape, passion fruit and pineapple fruit pulps) and compares them with homemade pulps at two different stages of ripening. The fruit pulps were characterized by analyzing their metabolic profiles and determining their ethanol content using quantitative Nuclear Magnetic Resonance (qNMR). In addition, principal component analysis (PCA) was applied to extract more information from the NMR data. We detected ethanol in all industrial and homemade pulps; and acetic acid in cashew, grape and passion fruit industrial and homemade pulps. The ethanol content in some industrial pulps is above the level recommended by regulatory agencies and is near the levels of some post‐ripened homemade pulps. This study demonstrates that qNMR can be used to rapidly detect ethanol content in frozen fruit pulps and food derivatives. Copyright © 2015 John Wiley & Sons, Ltd.     

Diffusion coefficients of small molecules at the gas-solid interface as measured by the nuclear magnetic resonance pulsed field gradient method

The nuclear magnetic resonance pulsed field gradient method has been used to measure the diffusion coefficients for some low molecular weight compounds adsorbed at the gas-solid interface. The systems studied are ammonia adsorbed upon graphite, methane adsorbed upon graphite, neopentane adsorbed upon graphite and neopentane adsorbed upon titanium dioxide. Results are compared with values obtained from quasi-elastic neutron scattering where available.