Modulation Of Hydrophobic Interactions Research Articles

Modulation of hydrophobic interactions by proximally immobilized ions.

The structure of water near non-polar molecular fragments or surfaces mediates the hydrophobic interactions that underlie a broad range of interfacial, colloidal and biophysical phenomena. Substantial progress over the past decade has improved our understanding of hydrophobic interactions in simple model systems, but most biologically and technologically relevant structures contain non-polar domains in close proximity to polar and charged functional groups. Theories and simulations exploring such nanometre-scale chemical heterogeneity find it can have an important effect, but the influence of this heterogeneity on hydrophobic interactions has not been tested experimentally. Here we report chemical force microscopy measurements on alkyl-functionalized surfaces that reveal a dramatic change in the surfaces' hydrophobic interaction strengths on co-immobilization of amine or guanidine groups. Protonation of amine groups doubles the strength of hydrophobic interactions, and guanidinium groups eliminate measurable hydrophobic interactions in all pH ranges investigated. We see these divergent effects of proximally immobilized cations also in single-molecule measurements on conformationally stable β-peptides with non-polar subunits located one nanometre from either amine- or guanidine-bearing subunits. Our results demonstrate the importance of nanometre-scale chemical heterogeneity, with hydrophobicity not an intrinsic property of any given non-polar domain but strongly modulated by functional groups located as far away as one nanometre. The judicious placing of charged groups near hydrophobic domains thus provides a strategy for tuning hydrophobic driving forces to optimize molecular recognition or self-assembly processes.

Polymorphism of β2-Microglobulin Amyloid Fibrils Manifested by Ultrasonication-enhanced Fibril Formation in Trifluoroethanol

The polymorphic property of amyloid structures has been focused on as a molecular basis of the presence and propagation of different phenotypes of amyloid diseases, although little is known about the molecular mechanism for expressing diverse structures from only one protein sequence. Here, we have found that, in combination with an enhancing effect of ultrasonication on nucleation, β(2)-microglobulin, a protein responsible for dialysis-related amyloidosis, generates distinct fibril conformations in a concentration-dependent manner in the presence of 2,2,2-trifluoroethanol (TFE). Although the newly formed fibrils all exhibited a similar needle-like morphology with an extensive cross-β core, as suggested by Fourier transform infrared absorption spectra, they differed in thioflavin T intensity, extension kinetics, and tryptophan fluorescence spectra even in the same solvents, representing polymorphic structures. The hydrophobic residues seemed to be more exposed in the fibrils originating at higher concentrations of TFE, as indicated by the increased binding of 1-anilinonaphthalene-8-sulfonic acid, suggesting that the modulation of hydrophobic interactions is critical to the production of polymorphic amyloid structures. Interestingly, the fibrils formed at higher TFE concentrations showed significantly higher stability against guanidium hydrochloride, the perturbation of ionic strength, and, furthermore, pressurization. The cross-β structure inside the fibrils seems to have been more idealized, resulting in increased stability when nucleation occurred in the presence of the alcohol, indicating that a weaker contribution of hydrophobic interactions is intrinsically more amenable to the formation of a non-defective amyloid structure.

Effects of β-cyclodextrin and β-cyclodextrin polymer addition and temperature on the modulation of hydrophobic interactions in aqueous solutions of two hydrophobically modified biopolymers

Novel information about the effects of temperature and addition of hydroxypropyl-β-cyclodextrin (HP-β-CD) or a charged β-cyclodextrin polymer (poly(β-CDN +)) on the rheological features of aqueous solutions of hydrophobically modified hydroxyethylcellulose (HM-HEC) and hydrophobically modified dextran (HM-dextran) is given. The viscosity results for the HM-HEC/poly(β-CDN +) system demonstrated that the addition of this cosolute sets up bridges between adjacent polymer chains, which lead to a strong enhancement of the viscosity. Furthermore, a shear thickening effect is registered at fairly high shear rates, and this feature is influenced by temperature. On the other hand, addition of HP-β-CD monomers to the system generates decoupling of associations via inclusion complex formation with the polymer hydrophobic tails. For the HM-dextran solutions, the interaction with the cosolutes is weak and only a slight increase of the viscosity by addition of poly(β-CDN +) is observed. In contrast to the HM-HEC solutions, addition of HP-β-CD monomers to the aqueous solutions of HM-dextran deactivates the intramolecular hydrophobic associations and thereby the compact coil structure expands and this leads to viscosity enhancement. Elevated temperature induces higher chain mobility and the formation of weaker network associations. The bridges formed in the HM-HEC/poly(β-CDN +) system are especially sensitive to temperature changes.

Effects of β-Cyclodextrin Addition and Temperature on the Modulation of Hydrophobic Interactions in Aqueous Solutions of an Associative Alginate

Novel information about the effects of beta-cyclodextrin (beta-CD) addition and temperature on structural and rheological features of semidilute solutions of alginate and its hydrophobically modified analogue (HM-alginate) is given. Enhanced turbidity is observed for the HM-alginate solutions at high levels of beta-CD addition and low temperatures. The viscosity results revealed cross-linking of the alginate chains at high beta-CD concentrations and low temperatures. Rheological results for the HM-alginate solutions demonstrated that high levels of beta-CD addition and elevated temperatures promoted decoupling of the hydrophobic polymer-polymer associations via inclusion complex formation between beta-CD cavities and the hydrophobic side chains of the polymer. Analysis of small-angle neutron scattering (SANS) results from HM-alginate solutions in the presence of beta-CD suggested that the polymer chains are locally stretched at all of the considered levels of beta-CD and temperatures. The SANS results revealed association structures. The general picture that emerges is that beta-CD addition and temperature can be combined to tune the intensity of the hydrophobic interactions and to cross-link the unmodified alginate.

Modulation of hydrophobic interactions in denatured whey proteins by transglutaminase enzyme

The role of enzyme crosslinking in mediating formation of hydrophobic association in chemically denatured whey protein isolates (WPI) is examined. WPI samples denatured with dithiothreitol (DTT) and incubated at 50 °C with and without transglutaminase enzyme show dramatic differences in viscosity, with the viscosity of the sample exposed to enzyme being lower by several orders of magnitude than the sample without enzyme. Upon further exposure of both samples to sodium dodecyl sulfate (SDS) to eliminate hydrophobic interactions, we observe no change in the viscosity of the sample previously treated with enzyme, suggesting this sample to have minimal hydrophobic associations. In contrast, the sample without enzyme shows a dramatic drop in viscosity indicating it to have had substantial hydrophobic associations. A similar trend but to a lesser extent is observed at a higher WPI concentration. These results taken together suggest that the formation of enzyme catalyzed ε-(γ-glutamyl)lysine bonds attenuates hydrophobic interactions through steric hindrance and formation of compact molecules that limits exposure of the hydrophobic moieties.

Modulation of Hydrophobic Interactions in Associative Polymers Using Inclusion Compounds and Surfactants

We report the modulation of the solution rheology of a comblike, hydrobhobically modified alkali-soluble emulsion (HASE) associative polymer through addition of α- and β-cyclodextrins (CDs). The ring-shaped CDs with hydrophobic inner cores interact with the pendant macromonomer segments of the associative polymer containing hydrophobic end groups, leading to reduction in polymer solution viscosity and dynamic moduli by several orders of magnitude. We find no interactions between the CDs and the polymer backbone as substantiated by the fact that an analogous parent polymer without hydrophobes reveals no changes in the solution rheology in the presence of CDs. In contrast, the CDs encapsulate the hydrophobic groups on the associative polymer. This is confirmed by the complexation between the CD and a surfactant modified to resemble the hydrophobic macromonomer of the associative polymer as observed using 1H NMR, differential scanning calorimetry (DSC), and thermal gravimetric analysis (TGA). The stoichiomet...

Structure and activity of a chimeric interleukin-8-melanoma-growth-stimulatory-activity protein.

A 72-amino-acid chimeric protein, Chi1, was constructed from the N-terminal part of interleukin 8, IL-8-(1-53), and the C-terminal part of melanoma growth stimulatory activity, MGSA-(54-72). Chi1 protein showed receptor-binding specificity and biological activity similar, but not identical to IL-8 and decidedly different from MGSA. The structure of Chi1 was determined in solution by two-dimensional NMR and molecular-dynamics calculations. The structure resembled the structures of MGSA and IL-8 closely, containing a triple-stranded beta-sheet in the IL-8 part and an amphipathic alpha-helix in the MGSA part. Chi1 formed dimers at millimolar concentrations via the first strand from the N-terminus, analogous to IL-8 and MGSA. In contrast to the latter molecules, however, the alpha-helix of Chi1 did not pack against the beta-sheet part, but was an independent structural element. This structural difference could be explained mainly by the modulation of hydrophobic interactions between the helix and the rest of the protein in Chi1 as compared to IL-8 and MGSA. It is concluded that tight helix packing is not required for receptor binding and biological activity of Chi1.