Non-electrostatic components of short and strong hydrogen bonds induced by compression inside fullerenes

Endo- and exohedral complexes of superphane with cations

Quite recently it has been shown in a previous study that superphane, that is, [2.2.2.2.2.2](1,2,3,4,5,6)cyclophane, is a very convenient molecule in the study of endohedral complexes and especially in the study of the influence of the caged entity (i.e., guest) on the structure of the host molecule. This advantage results from the presence of two parallel benzene rings joined together by six quite flexible ethylene bridges (spacers). This article examines the energetic and structural properties of endo- and exohedral complexes of superphane with the following cations: H⁺ , Li⁺ , Na⁺ , K⁺ , Be²⁺ , Mg²⁺ , Ca²⁺ , B³⁺ , Al³⁺ , Ga³⁺ . The stability of endohedral complexes has been shown to be strongly dependent on the charge and radius of the caged cation. The inclusion of the cation inside the superphane molecule causes its 'swelling', which is manifested by an increase in the distance between the benzene rings and elongations of the ring and spacer C-C bonds. In the case of exohedral complexes, three forms are investigated: with the cation above the benzene ring, with the cation interacting with the superphane window in the equatorial position, and with the cation interacting with the center of the C-C spacer bond. The first of these forms has been shown to be preferred. The cation⋯acceptor distance depends on the cation radius. Among the cations investigated, H⁺ and Be²⁺ are particularly reactive and predisposed to induce significant structural changes in the superphane molecule, forming C-H bond or C-Be-C bridges, respectively.

The hydrogen-bond configuration modulates the energy transfer efficiency in helical protein nanotubes

Energy transport in proteins is critical to a variety of physical, chemical, and biological processes in living organisms. While strenuous efforts have been made to study vibrational energy transport in proteins, thermal transport processes across the most fundamental building blocks of proteins, i.e. helices, are not well understood. This work studies energy transport in a group of "isomer" helices. The π-helix is shown to have the highest thermal conductivity, 110% higher than that of the α-helix and 207% higher than that of the 310-helix. The H-bond connectivity is found to govern thermal transport mechanisms including the phonon spectral energy density, dispersion, mode-specific transport, group velocity, and relaxation time. The energy transport is strongly correlated with the H-bond strength which is also modulated by the H-bond connectivity. These fundamental insights provide a novel perspective for understanding energy transfer in proteins and guiding a rational molecule-level design of novel materials with configurable H-bonds.

Morphological and Semi-empirical Study of the Pluronic F68/Imogolite/Sudan III Intersurfaces Composite for the Controlled Temperature Release of Hydrophobic Drugs

Some PluronicF68 (F68) triblock copolymer properties demonstrate surprising applications in selective drug administration, such as the transportation of hydrophobic anti-inflammatories through epithelial barriers. Nuclear magnetic resonance (¹H-NMR) spectroscopy was carried out for micelle precursor dispersions and F68 films modified with a synthetic imogolite (IMO) biocompatible hydrogel. Theoretical calculations and morphological assessment for the process of morphogenesis of dendritic crystallization were performed by molecular docking and atomic force microscopy (AFM) of the Sudan III-IMO-F68 composite, which was more hydrophobic than Sudan III-F68 and carried out the prolonged release of the Sudan III "drug" captured by a water-octanol interface determined by standard absorbance. Surface fusions were measured and compared to the unmodified matrix. However, despite the superior properties of the composite, the critical micelle concentration (CMC) was practically unmodified because solitary IMO strands attached to Sudan III formed Sudan III-IMO. These strands unraveled in a stable manner by expanding like a "spiderweb" in hydrophilic interfaces according to NMR analysis of the hydrogen one H₁ polarization of Sudan III and F68 methyl, whose correlation relates hydrophobicity of Sudan III-IMO-F68 with dendrite properties from F68 concentrations. CMC and surface fusions equivalent to F68 surface properties, calculated by differential scanning calorimetry and dynamic Raman spectroscopy, were determined by AFM and high-resolution ellipsometry. Our results show highly specialized pharmacological applications since micelle surfaces expand, triggering maximum deliveries of "Drugs" from its interior to the physiological environment. The implanted sensor prototype determined equilibria reached Sudan III according to temperature (32-50 °C) and time it took to cross the membrane model 1-octanol (48 h). The findings suggest that the targested design of a F68-IMO-"Drug" would function as a microdevice for the prolonged release of hydrophobic drugs. In addition, the said microdevice could regenerate the damaged tissue in the central nervous system or other organs of the body. This is due to the fact that it could perform both tasks simultaneously, given the properties and characteristics acquired by the compatible material depending on the temperature of the physiological environment.

Allosteric communication pathways and thermal rectification in PDZ-2 protein: a computational study

Allosteric communication in proteins is a fundamental and yet unresolved problem of structural biochemistry. Previous findings, from computational biology ( Ota, N.; Agard, D. A. J. Mol. Biol. 2005 , 351 , 345 - 354 ), have proposed that heat diffuses in a protein through cognate protein allosteric pathways. This work studied heat diffusion in the well-known PDZ-2 protein, and confirmed that this protein has two cognate allosteric pathways and that heat flows preferentially through these. Also, a new property was also observed for protein structures: heat diffuses asymmetrically through the structures. The underling structure of this asymmetrical heat flow was a normal length hydrogen bond (∼2.85 Å) that acted as a thermal rectifier. In contrast, thermal rectification was compromised in short hydrogen bonds (∼2.60 Å), giving rise to symmetrical thermal diffusion. Asymmetrical heat diffusion was due, on a higher scale, to the local, structural organization of residues that, in turn, was also mediated by hydrogen bonds. This asymmetrical/symmetrical energy flow may be relevant for allosteric signal communication directionality in proteins and for the control of heat flow in materials science.

Are short, low-barrier hydrogen bonds unusually strong?

In a symmetric hydrogen bond (H-bond), the hydrogen atom is perfectly centered between the two donor atoms. The energy diagram for hydrogen motion is thus a single-well potential, rather than the double-well potential of a more typical H-bond, in which the hydrogen is covalently bonded to one atom and H-bonded to the other. Examples of symmetric H-bonds are often found in crystal structures, and they exhibit the distinctive feature of unusually short length: for example, the O-O distance in symmetric OHO H-bonds is found to be less than 2.5 Å. In comparison, the O-O distance in a typical asymmetric H-bond, such as ROH···OR(2), ranges from about 2.7 to 3.0 Å. In this Account, we briefly review and update our use of the method of isotopic perturbation to search for a symmetric, centered, or single-well-potential H-bond in solution. Such low-barrier H-bonds are thought to be unusually strong, owing perhaps to the resonance stabilization of two identical resonance forms [A-H···B ↔ A···H-B]. This presumptive bond strength has been invoked to explain some enzyme-catalyzed reactions. Yet in solution, a wide variety of OHO, OHN, and NHN H-bonds have all been found to be asymmetric, in double-well potentials. Examples include the monoanion of (±)-2,3-di-tert-butylsuccinic acid and a protonated tetramethylnaphthalenediamine, even though these two ions are often considered prototypes of species with strong H-bonds. In fact, all of the purported examples of strong, symmetric H-bonds have been found to exist in solution as pairs of asymmetric tautomers, in contrast to their symmetry in some crystals. The asymmetry can be attributed to the disorder of the local solvation environment, which leads to an equilibrium among solvatomers (that is, isomers that differ in solvation). If the disorder of the local environment is sufficient to break symmetry, then symmetry itself is not sufficient to stabilize the H-bond, and symmetric H-bonds do not have an enhanced stability or an unusual strength. Nor are short H-bonds unusually strong. We discuss previous evidence for "short, strong, low-barrier" H-bonds and show it to be based on ambiguous comparisons. The role of such H-bonds in enzyme-catalyzed reactions is then ascribed not to any unusual strength of the H-bond itself but to relief of "strain."