Intermolecular Ionic Bonds Research Articles

APPLICATION OF ACACIA TANNINS IN ELASTIC LEATHER MATERIALS PRODUCTION TECHNOLOGY

An IR-spectroscopic study of the interaction of collagen with blackwood acacia (acacia melanoxylon) tannins and its use for tanning and filling semi-finished leather products in the production of elastic leathers for shoe uppers was conducted. The analysis of IR spectra of the interaction of the chrome tanning collagen products dermis with the acacia extract tannins can indicate the intermolecular ionic bonds formation and a hydrogen bonds dense network building-up with the amino groups of collagen and phenolic and carbonyl groups of the plant extract tannins. The following reagents were used for the semi-finished product tanning: sodium chloride TU 9192-069-00200009527-98, formic and sulfuric acids according to DSTU GOST 1706-78 and DSTU GOST 2184:2018, and sodium carbonate ISO 6353-2-83. Blackwood acacia extract together with the synthesis product of 2-naphtholsulfonic acid with dioxydiphenylsulfone as synthetic tanning agent BNS TU 17-06-165-89 were used to fill the tanned semi-finished product. The application of BNS TU 17-06-165-89 syntan in the process of finishing and filling of chrome tanned semi-finished leather product after catting it into thin slices with a thickness of 1.4 mm together with acacia extract tannins ensures an increase in its diffusion into the structure and provides a process duration reduction. The use of acacia extract tannins in the production technology of elastic hides makes it possible to obtain a filled-greased semi-finished product with increased physical and chemical properties: porosity, volumetric yield, air permeability by 21, 37, 86%, respectively, compared to the chrome tanning semi-finished product. The obtained results give the grounds for recommendation the use of acacia extract tannins in the wide range of elastic leathers production technologies.

Analysis of Protein-Protein Interactions for Intermolecular Bond Prediction.

Protein-protein interactions often involve a complex system of intermolecular interactions between residues and atoms at the binding site. A comprehensive exploration of these interactions can help reveal key residues involved in protein-protein recognition that are not obvious using other protein analysis techniques. This paper presents and extends DiffBond, a novel method for identifying and classifying intermolecular bonds while applying standard definitions of bonds in chemical literature to explain protein interactions. DiffBond predicted intermolecular bonds from four protein complexes: Barnase-Barstar, Rap1a-raf, SMAD2-SMAD4, and a subset of complexes formed from three-finger toxins and nAChRs. Based on validation through manual literature search and through comparison of two protein complexes from the SKEMPI dataset, DiffBond was able to identify intermolecular ionic bonds and hydrogen bonds with high precision and recall, and identify salt bridges with high precision. DiffBond predictions on bond existence were also strongly correlated with observations of Gibbs free energy change and electrostatic complementarity in mutational experiments. DiffBond can be a powerful tool for predicting and characterizing influential residues in protein-protein interactions, and its predictions can support research in mutational experiments and drug design.

Structure basis of the improved sweetness and thermostability of a unique double-sites single-chain sweet-tasting protein monellin (MNEI) mutant

The sweet protein monellin has an intensely sweet potency but limited stability. We have identified a double-sites mutant (E2N/E23A) of the single-chain monellin (MNEI) with both improved sweetness (about 3-fold) and thermostability (10 °C). However, the structural basis of its superior properties remains elusive until now. Herein we report its crystal structure at a resolution 1.90 Å. Similar to the wild-type, E2N/E23A adopts a wedge-shaped structure consisting of a five-strand β-sheet partially “wrapped” around an α-helix. However, distinguishing parts were present in the loops region, including a remarkable conformation shift from β-strand to loop around residue R39. Molecular docking revealed the persistence of conserved protein-receptor interface and formation of new intermolecular ionic bonds in the E2N/E23A-receptor complex involving the taste-active residue R39 of the sweet protein, which could account for its significant improvement of sweetness. On the other hand, a rearrangement of intramolecular interaction network including the C-H … π bond between A23 and F89 that led to enhanced hydrophobicity in the protein core, could be correlated with its improved thermostability. Furthermore, two new sweeter mutants of MNEI were created. These findings highlight the critical roles of key sweetness determinant residue R39 and hydrophobicity at the protein core for the sweetness and thermostability of the protein, respectively, which thus provide a deeper insight for understanding the structure-function relationship of the sweet protein as well as guidance for rational design of this unique biomacromolecule.

Ionic Interactions of the C-Terminal Domain of Apolipoprotein A-I are Responsible for Oligomerization

Human apoA-I is an anti-atherogenic protein circulating in plasma. The protein is a single polypeptide of 243 amino acid residues, and is the major protein component of high density lipoprotein. The three-dimensional structure of the protein remains unknown, however, secondary structure analysis revealed that apoA-I is predominately α-helical in nature. The N-terminal (NT) domain is made of a bundle of amphipathic α-helices, while the structure of the C-terminal (CT) domain is less defined. Lipid-free apoA-I exists as a heterogeneous population of oligomers. The CT domain is responsible for oligomerization, and several critical lysine residues that may be responsible for oligomerization have been identified. Site-directed mutagenesis was used to design a series of mutant proteins in which lysine residues of the CT domain were changed into glutamine (K195, K206, K208, K226, K238, and K239). The proteins were over-expressed in Escherichia coli and purified with Ni-affinity and gel filtration chromatography. Circular dichroism analysis showed that the helical content and midpoint of guanidine denaturation was not affected by the amino acid changes. Crosslinking with dimethylsuberimidate and size-exclusion FPLC showed that the extent of oligomerization of apoA-I decreased incrementally with each lysine substitution. The lysines of CT-apoA-I may form ionic bonds with oppositely charged residues in neighboring apoA-I. E234 and E235 are in close proximity to K238 and K239, and may form intermolecular ionic bonds upon self-association. To test this, E234 and E235 were mutated to glutamine. These mutant proteins retained their secondary structure and stability properties were not changed. However, the amount of cross-linking was significantly reduced. This result indicates that both lysine and glutamate residues in CT-apoA-I are critical residues responsible for self-association. Therefore, the glutamine variants will be excellent candidates to obtain the high-resolution structure of the protein.

Robust Surface Nano-Architecture by Alkali–Carboxylate Ionic Bonding

Ionic bonding in supramolecular surface networks is a promising strategy to self-assemble nanostructures from organic building blocks with atomic precision. However, sufficient thermal stability of such systems has not been achieved at metal surfaces, likely due to partial screening of the ionic interactions. We demonstrate excellent stability of a self-assembled ionic network on a metal surface at elevated temperatures. The structure is characterized directly by atomic resolution scanning tunneling microscopy (STM) experiments conducted at 165 °C showing intact domains. This robust nanometer-scale structure is achieved by the on-surface reaction of a simple and inexpensive compound, sodium chloride, with a model system for carboxylate interactions, terephthalic acid (TPA). Rather than distinct layers of TPA and NaCl, angle resolved X-ray photoelectron spectroscopy experiments indicate a replacement reaction on the Cu(100) surface to form Na-carboxylate ionic bonds. Chemical shifts in core level electron states confirm a direct interaction and a +1 charge state of the Na. High-temperature STM imaging shows virtually no fluctuation of Na-TPA island boundaries, revealing a level of thermal stability that has not been previously achieved in noncovalent organic-based nanostructures at surfaces. Comparable strength of intermolecular ionic bonds and intramolecular covalent bonds has been achieved in this surface system. The formation of these highly ordered structures and their excellent thermal stability is dependent on the interplay of adsorbate-substrate and ionic interactions and opens new possibilities for ionic self-assemblies at surfaces with specific chemical function. Robust ionic surface structures have potential uses in technologies requiring high thermal stability and precise ordering through self-assembly.

Dominant impact of the &agr;-L-guluronic acid chain on regulation of the mass transfer character of calcium alginate membranes

The dominant impact of the &agr;-L-guluronic acid chain on regulation of the mass transfer character of calcium alginate membranes was investigated. The polymer frameworks of the membranes were successfully regulated by altering the mass fraction of homopolymeric blocks of &agr;-L-guluronic acid (FGG) in the entire molecular chain of alginate. Sodium alginate is well-known as a hydrophilic polysaccharide. Its molecular chain is composed of &agr;-L-guluronic acid and &bgr;-D-mannuronic acid. The polymeric structure of calcium alginate is mainly constructed by intermolecular ionic bonds with homopolymeric blocks of the &agr;-L-guluronic acid junction zone. The entire alginate polymer chain then forms into a swollen gel and a membrane. The water fraction of the swollen membrane and the mechanical strength changed with FGG. The volumetric water fraction based on the swollen state of the membrane was evaluated from the gravitational water content (HM). The change in the mass transfer mechanism of the membrane was evident in the effective diffusion coefficient, especially for the smaller molecules (e.g., urea). Good correlation between the volumetric water fraction and the effective diffusion coefficient strongly suggested that the mass transfer channels in the alginate membrane were dominantly regulated by the mass fraction of homopolymeric blocks of &agr;-L-guluronic acid (FGG). The relationship between the effective diffusion coefficient and the volumetric water fraction agreed with Yasuda’s free volume theory. The tortuosity was markedly increased and the effective d iffusion coefficient was reduced. In conclusion, the homopolymeric blocks of &agr;-L-guluronic acid have a dominant impact on the mass transfer character in the calcium alginate membrane.

The Charge Structure of Helix 1 in the Prion Protein Regulates Conversion to Pathogenic PrPSc

The prion diseases are transmissible neurodegenerative disorders linked to a pathogenic conformer (PrP(Sc)) of the normal prion protein (PrP(C)). Accumulation of PrP(Sc) occurs via a poorly defined process in which PrP(Sc) complexes with and converts endogenous PrP(C) to nascent PrP(Sc). Recent experiments have focused on the highly charged first alpha helix (H1) of PrP. It has been proposed that two putative asparagine-to-arginine intrahelical salt bridges stabilize H1 in PrP(C) yet form intermolecular ionic bonds with adjacent PrP molecules during conversion of PrP(C) to PrP(Sc) (M. P. Morrissey and E. I. Shakhnovich, Proc. Natl. Acad. Sci. USA 96:11293-11298, 1999). Subsequent work (J. O. Speare et al., J. Biol. Chem. 278:12522-12529, 2003 using a cell-free assay of PrP(Sc) conversion suggested that rather than promoting conversion, the salt bridges stabilize PrP(C) against it. However, the role of individual H1 charges in PrP(Sc) generation has not yet been investigated. To approach this question, we systematically reversed or neutralized each charged residue in H1 and tested the effect on conversion to PrP(Sc) in scrapie-infected murine neuroblastoma (ScN2a) cells. We find that replacements of charged H1 residues with like charges permit conversion, while charge reversals hinder it. Neutralization of charges in the N-terminal (amino acids 143 to 146) but not the C-terminal (amino acids 147 to 151) half of H1 permits conversion, while complete reversal of charge orientation of the putative salt bridges produces a nonconvertible PrP. Circular dichroism spectroscopy studies and confocal microscopy immunofluorescence localization studies indicated that charge substitutions did not alter the secondary structure or cell surface expression of PrP(C). These data support the necessity of specific charge orientations in H1 for a productive PrP(Sc)-PrP(C) complex.

Novel polymer blends based on poly(ether-urethane) ionomer and ion-containing styrene copolymer

The structure-property relationships of thermoplastic polymer blends based on poly(ether-urethane) ionomer (PEUI) and ion-containing styrene-acrylic acid copolymer (S-co-AA(K)) have been investigated by using DMTA, DSC and TGA, as well as tensile tests. Convergence of the glass transition temperature (Tg) values of the PEUI and the S-co-AA(K) components in the blends studied, as compared to the individual polymers, was found and explained by improving compatibility of the components due to increasing effective density of physical networks formed by ion-dipole and ion-ion interactions of ionic groups of the components. Character of E'=f(T) and E''=f(T) dependencies confirms the increase of the effective density of physical networks in the compositions studied compared to individual PEUI and S-co-AA(K). Improvement of end-use properties, i.e. thermal stability and tensile properties has been found for the PEUI/S-co-AA(K) compositions with lower content of S-co-AA(K) (i.e. <10 mass%) and explained by formation of additional network of intermolecular ionic bonds between the functional groups of PEUI and S-co-AA(K).

Einflu� niedermolekularer ionischer Verbindungen auf die L�slichkeit und die rheologischen Eigenschaften von Weizenkleber

Interactions between gluten proteins and ion of low molecular weight are of great importance for the theological properties. Dependent on type and quantity of the ions present the gluten is weakened or strengthened. Low concentrations of NaCl (3.6% calculated on dry gluten) weakens the gluten by combining with charged amino acid side chains under separation of intermolecular ionic bonds, while high concentration (>5%) have an opposite effect and strengthen H-bridges and hydrophobic bonds by net structureforming influences on the water. In the presence of bipolar ions, such as amino acids, the gluten becomes very strong. There is evidence, that this effect is caused by intermolecular ionic bonds. Because of the slightly positive net charge of gluten in the neutral range, dicarboxylic acids show a similar effect, while diamines weaken the gluten. Phospholipids at low concentrations also seem to build up ion bridges between protein chains, but at high concentrations a weakening of gluten is observed, probably caused by lipid double layers, separating the protein chains. p]Interactions between gluten proteins and