Hydrophobic interactions, theoretical

Separations in hydrophobic interaction chromatography have been modeled as a function of the ionic strength of the buffer and of the hydrophobicity of the column, and tested using the elution of lysozyme and ovalbumin from octyl-, butyl- and phenyl-Sepharose phases.2 The theoretical framework used preferential interaction analysis, a theory competitive to solvophobic theory. Solvophobic theory views protein-surface interaction as a two-step process. In this model, the protein appears in a cavity in the water formed above the adsorption site and then adsorbs to the phase, with the free energy change... 

Proteins Theoretical aspects Ion Exchange, Hydrophobic Interaction, Reversed-Phase [28]... 

The theoretical treatment of the hydrophobic effect is limited to pure aqueous systems. To describe chromatographic separations in RPC Horvath and Melander developed the solvophobic theory [47]. In this theory, no special assumptions are made about the properties of solute and solvent, and besides hydrophobic interaction electrostatic and other specific interactions are included. The theory has been valuable to describe the retention of nonpolar [48], polar [49], and ionizable [50] solutes in RPC. The modulation of selectivity via secondary equilibria (variation of pH, ion pair formation [51]) can also be described. On the other hand, it is not a problem to find examples of dispersive interactions in literature, e.g., separation of carotinoids with a long chain (C30) RP gives a higher selectivity compared to standard RP C18 cyclohexanols are preferentially retarded on cyclohexyl-bonded phases compared to phases with linear-bonded alkyl groups. 

Stabilization of a P-hairpin structure can be achieved in two ways, promoting a stable (or restricted) turn structure (as done with mimetics) or linking the two arms either chemically, or, more naturally, by hydrophobic interactions. In an approach to utilizing both methods, a D-Pro-Gly linkage was used to stabilize a left-handed turn (type I or II ) and various charged and hydrophobic residues were used to stabilize the molecule and enhance the interaction between arms. I252"254 Examples of these peptides studied in nonaqueous solution by IR, VCD and NMR spectroscopy exhibit characteristics of well-formed hairpins. 255 Alternatively, in aqueous solution, IR, VCD, and ECD results for related peptides agree with the NMR interpretation of conformations characterized as hairpins stabilized at the turn and frayed at the ends. 256 These latter results also have a qualitative match with theoretical simulations. Recently, examples of hydrophobically stabilized hairpins studied by NMR spectroscopy have avoided use of a nonnatural amino acid. 257,258 ... 

The relationship between odour quality and chemical structure is of considerable practical and theoretical interest. A numt r of methods have been used to determine quantitatively the relationships between the structure of a molecule and its odour quality (7). Though quantitative results were not obtained, a number of interesting theories were present in that the intermolecular interaction in olfaction involved electrostatic attraction, hydrophobic bonding, van der Waals forces, hydrogen bonding, and dipole-dipole interactions. Hydrophobic interactions also appeared to be a major force for substrate binding in olfaction. It had previously been shown that lipophilicity and water solubility were factors diat significandy influenced the odour thresholds of the pyrazines (8),... 

Several other studies have also been made in an attempt to account theoretically for the phase transition in terms different from those of the Flory-Huggins theory. Otake et al. [55] thus proposed a theoretical model that takes hydrophobic interaction into account in explaining the thermally induced discontinuous volume collapse of hydrogels. In addition, Prausnitz et al. [56] proposed a lattice model, an improvement of which was made to explain the swelling curves of gels consisting of /V,/V -methylenebis(acrylamide) (MBA)-crosslinked copolymers of AAm with [(methacrylamide)propyl]trimethyl-ammonium chloride (MAPTAC) [57],... 

Fig. 5.3 Comparison of the theoretical and experimental 3D structure (ribbon representation) of the putative nitroreductase, one of the targets of CASP6 competition. The energy expression which was used in theoretical calculations takes into account the physical interactions (such as hydrogen bonds, hydrophobic interactions, etc.) as well as an empirical potential deduced from representative proteins experimental structures deposited in the Brookhaven Protein Data Bank (no bias towards the target protein), (a) Predicted by Kolinski and Bujnicki [11] by the Monte Carlo method, and (b) determined experimentally by X-ray diffraction [12]. Both structures in atomic resolution differ (r.m.s.) by 2.9A. Reproduced by courtesy of Professor Andrzej Kolinski...

Studies on the interaction between oppositely charged polyelectrolytes date back to 1896 when Kossel389 precipitated egg albumin with protamine. Since that time extensive studies have been made on pairs of strong polyelectrolytes, pairs of strong and weak polyelectrolytes, pairs of weak polyelectrolytes, as well as on amphoteric complexes. However, the theoretical considerations of intermacromolecular interactions between polyelectrolytes were only based on extremely simplified model systems. However, even in the case of such systems, there are many unsolved problems such as the determination of the local dielectric constant in domains of macromolecular chains, the evaluation of other secondary binding forces, especially hydrophobic interactions, and so on. 

In addition to above phenomena, there is the sizable contribution of hydrophobic effects in our nanoparticulate system. As the opposite charges of interacting molecules are neutralized, and hydrophobicity rises, the particle is instantaneously formed. This is supported by our observations on the effect of ionic strength (Fig. 18), leading to an enhancement of release when the salt concentration is lowered. Hydrophobic interactions, in addition to ionic forces, were identified as an important mechanism for drug release from ion exchange resin [64], from the CT/TPP complex [62] and from theoretical calculations of forces involved in the assembly of polyelectrolytes [65]. 

The similarities between non-ionic micelles and globular proteins (Nemethy, 1967 Schott, 1968 Jencks, 1969) render micelles potentially useful as models for the investigation of hydrophobic interactions. Indeed, the stability of non-ionic micelles has been treated theoretically in terms of hydrophobic interactions (Poland and Scheraga, 1965). Since the critical micelle concentration is related to the degree and nature of the hydrophobic interactions of the amphiphile, its valne in the presence of additives and at different temperatures can be nsed as a quantitative measure of the effect of these variables on the hydrophobic interactions. In spite of the similarities between proteins and micelles, considerable caution is warranted in extrapolating the results obtained from micellar models to the more complex protein systems. 

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