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Hydrophobic interactions guide protein folding, multidomain protein assembly, receptor-ligand binding, membrane formation, and cellular transportation. On the macroscale, hydrophobic interactions consist of the aggregation of "oil-like" objects in water by minimizing the interfacial energy. However, studies of the hydration behavior of small hydrophobic molecules have shown that the microscopic (~1 nm) hydration mechanism differs fundamentally from its macroscopic counterpart. Theoretical studies over the last two decades have pointed to an intricate dependence of molecular hydration mechanisms on the length scale. The microscopic-to-macroscopic crossover length scale is critically important to hydrophobic interactions in polymers, proteins, and other macromolecules. Accurate experimental determination of hydration mechanisms and interaction strengths directly influence our understanding of protein folding. In this Account, we discuss our recent measurements of the hydration energies of single hydrophobic homopolymers as they unfold. We describe in detail our single molecule force spectroscopy technique, the interpretation of the single polymer force curve, and how it relates to the hydration free energy of a hydrophobic polymer. Specifically, we show how temperature, side-chain sizes and solvent conditions, affect the driving force of hydrophobic collapse. The experiments reveal that the size of the nonpolar polymer side-chains changes the thermal signatures of hydration. The sizes of the polymer side-chains bridge the length scale where theories had predicted a transition between entropically driven microscopic hydration and enthalpically driven macroscopic hydrophobic hydration. Our experimental results revealed a crossover length scale of approximately 1 nm, similar to the results from recent theoretical studies. Experiments that probe solvent dependency show that the microscopic polymer hydration is correlated with macroscopic interfacial tension. Consistent with theoretical predictions, the solvent conditions affect the microscopic and macroscopic hydrophobic strengths in similar ways. Although the extended polymers and proteins span hundreds of nanometers, the experiments show that their hydration behavior is determined by the size of a single hydrophobic monomer. As the hydrophobic particle size decreases from the macroscopic to the microscopic regime, the scaling relationship changes from a dependence on interfacial area to a dependence on volume. Therefore, under these conditions, the driving force for the aggregation of hydrophobic molecules is reduced, which has significant implications for the strength of hydrophobic interactions in molecular systems, particularly in protein folding.

The hydrophobic interaction is significantly responsible for driving protein folding and self-assembly. To understand it, the thermodynamics, the role of water structure, the dewetting process surrounding hydrophobes, and related aspects have undergone extensive investigations. Here, we examine the hypothesis that polymer-solvent interfacial free energy is adequate to describe the energetics of the collapse of a hydrophobic homopolymer chain at fixed temperature, which serves as a much simplified model for studying the hydrophobic collapse of a protein. This implies that changes in polymer-solvent interfacial free energy should be directly proportional to the force to extend a collapsed polymer into a bad solvent. To test this hypothesis, we undertook single-molecule force spectroscopy on a collapsed, single, polystyrene chain in water-ethanol and water-salt mixtures where we measured the monomer solvation free energy from an ensemble average conformations. Different proportions within the binary mixture were used to create solvents with different interfacial free energies with polystyrene. In these mixed solvents, we observed a linear correlation between the interfacial free energy and the force required to extend the chain into solution, which is a direct measure of the solvation free energy per monomer on a single chain at room temperature. A simple analytical model compares favorably with the experimental results. This knowledge supports a common assumption that explicit water solvent may not be necessary for cases whose primary concerns are hydrophobic interactions and hydrophobic hydration. [ABSTRACT FROM AUTHOR]

Amyloid fibril diseases are characterized by the abnormal production of aggregated proteins and are associated with many types of neuro- and physically degenerative diseases. X-ray diffraction techniques, solid-state magic-angle spinning NMR spectroscopy, circular dichroism (CD) spectroscopy, and transmission electron microscopy studies have been utilized to detect and examine the chemical, electronic, material, and structural properties of amyloid fibrils at up to angstrom spatial resolution. However, X-ray diffraction studies require crystals of the fibril to be analyzed, while other techniques can only probe the bulk solution or solid samples. In the work reported here, apertureless near-field scanning infrared microscopy (ANSIM) was used to probe the secondary structure of individual amyloid fibrils made from an in vitro solution. Simultaneous topographic and infrared images of individual amyloid fibrils synthesized from the #21-31 peptide fragment of β2-microglobulin were acquired. Using this technique, IR spectra of the amyloid fibrils were obtained with a spatial resolution of less than 30 nm. It is observed that the experimental scattered field spectrum correlates strongly with that calculated using the far-field absorption spectrum. The near-field images of the amyloid fibrils exhibit much lower scattering of the IR radiation at approximately 1630 cm-1. In addition, the near-field images also indicate that composition and/or structural variations among individual amyloid fibrils were present. [ABSTRACT FROM AUTHOR]

Abstract: To date, the calcareous tubes of serpulid marine worms have not been studied extensively in a biomineralization context. The structure and composition of the tube shell and adhesive cement of the marine tubeworm Hydroides dianthus were studied using a variety of characterization techniques, including powder XRD, FTIR, SEM, EDX, and AFM. The tube and cement were determined to be inorganic–organic composite materials, consisting of inorganic aragonite (CaCO3) and Mg-calcite ((Ca0.8Mg0.2)CO3) crystals, and both soluble and insoluble organic matrices (SOM and IOM). SEM imaging revealed a variety of crystal morphologies. AFM nanoindentation of the inorganic components yielded Young’s moduli of ∼20GPa in the wet state, and ∼50GPa in the dry state. Amino acid analysis of the SOM indicated substantial amounts of acidic and non-polar neutral amino acids. Part of the insoluble organic tube lining was identified as being composed of collagen-containing fibres aligned in a criss-crossed structure. The SOM and organic tube lining were found to contain carboxylated and sulphated polysaccharides. In an artificial seawater solution, the SOM and the organic tube lining mediated CaCO3 mineralization in vitro. [Copyright &y& Elsevier]