Interchain Ionic Interactions Research Articles

Highly flame-retardant vinyl ester resins with well-balanced comprehensive performance

Flame-retardant vinyl ester resins (VER) have long been studied, however, very few flame retardants can efficiently improve the flame retardancy of the resin without deteriorating their other performance. In this work, a kind of liquid phosphorus-containing 1-vinylimidazole salt (called VC) was synthesized in a facile way with a very high yield. Then, VC was used as the flame-retardant crosslinking agent to obtain flame-retardant VER. Curing behaviors, flame retardancy, thermal stability, thermomechanical properties, and mechanical performance of the resin were investigated. VC had good compatibility with VER and the mixture presented a relatively low viscosity. VC/VER showed a high curing activity and can be efficiently cured in moderate conditions. VC was incorporated into the resin mainly by ionic bonds, by which interchain ionic interactions were constructed in the resin. With only 15 wt% additions of VC, the cured VC15/VER easily passed the UL 94 V0 rating and achieved a limiting oxygen index value of 32.7%. In the cone calorimetric test, VC15/VER severally presented a reduction in the peak heat release rate, total heat release rate, and total smoke release rate of 67.8%, 17.1%, and 27.3% compared to VER. VC15/VER showed good thermal stability, had a glass transition temperature of 151 °C (that of VER was 137 °C), and achieved high mechanical performance that can be comparable to VER. Hence, this work proposes a facile strategy for producing flame-retardant thermosets with well-balanced comprehensive performance, which exhibits potential in coatings, composites, and laminates for versatile applications.

Assembly studies of six intestinal intermediate filament (IF) proteins B2, C1, C2, D1, D2, and E1 in the nematode C. elegans.

The dimerisation properties of six intestine-expressed intermediate filament (IF) proteins (B2, C1, C2, D1, D2, E1) were analysed in blot overlay assay on membranes containing all of the eleven recombinant C. elegans IF proteins (A1, A2, A3, A4, B1, B2, C1, C2, D1, D2, and E1). The interactions detected in the blot assays exclusively comprise intestine-expressed IF proteins and the protein A4, which is found in the dauer larva intestine. About 86% of these interactions are heterotypic, while the remaining interactions relate to C1, C2, and D2 homodimers. These multiple modes of interaction were also supported by calculations of the numbers of possible interchain ionic interactions derived from the individual rod sequences. The results predict that the six B2, C1, C2, D1, D2, and E1 IF proteins are able to form as many as eleven different heteropolymeric and three homopolymeric IFs in the C. elegans intestine. This simple model of the intestinal IF meshwork enables us to speculate that our previously reported triple RNAi worms arrested or decreased their growth because of feeding reduction due to morphological defects of the mechanically compromised intestine.

A rod domain sequence in segment 1B triggers dimerisation of the two small Branchiostoma IF proteins B2 and A3

Previously, we cloned two Branchiostoma IF proteins A3 and B2 and demonstrated that both can form heteropolymeric IF based on a coiled coil dimer consisting of one B2 and one A3 polypeptide. In this study we continued in the characterisation of the B2/A3 heterodimer by searching for the sequences that play an important role in the triggering of the B2/A3 heterodimer. Using a series of deletion and chimeric B2, A3 and B1 constructs and the overlay assay as a tool, we were able to identify a part of the B2 sequence (segment 1A, linker L1 and the N-terminal part of segment 1B) which retains the ability of the full length protein B2 to specifically recognize A3 in blot overlays. Moreover, inspection of this A3-competent B2 fragment identified a short sequence in segment 1B which shares with the currently known trigger-like motif of cortexillin and other coiled coil proteins potential to form multiple inter-chain ionic interactions. Thus, a common and essential feature of trigger sequences with different primary structures found so far in IF and other coiled coil proteins seems to be their ability to form multiple inter-chain ionic interactions which brings the chains close to one another and allows coiled coil formation to propagate accordingly.

Shape Memory Behavior of Zinc Oleate-Filled Elastomeric Ionomers

Shape memory elastomeric compounds were prepared from mixtures of the zinc salt of a sulfonated poly{ethylene-r-propylene-r-(5-ethylidene-2-norbornene)} ionomer (Zn-SEPDM) and zinc oleate (ZnOl). Physical cross-links in the ionomer due to interchain ionic interactions provided a “permanent” cross-linked network, and strong dipolar interactions between the ionomer and a dispersed phase of crystalline ZnOl provided a temporary network. ZnOl concentrations as high as 50 wt % were studied. A temporary shape was achieved and fixed by deforming the compound above the melting point (Tm) of the ZnOl (∼78 °C) and then cooling the material under stress to below Tm. The permanent shape was recovered by reheating the sample to above Tm without an applied stress. The materials were characterized by thermal analysis, X-ray diffraction, dynamic mechanical analysis (DMA), and tensile tests, and the shape memory behavior was measured by DMA and static stretching experiments. Although shape fixing efficiencies of up to 90%...

Sequence Comparisons of Intermediate Filament Chains: Evidence of a Unique Functional/Structural Role for Coiled-Coil Segment 1A and Linker L1

A comprehensive analysis of the sequences of all types of intermediate filament chains has been undertaken with a particular emphasis on those of segment 1A and linker L1. This has been done to assess whether structural characteristics can be recognized in the sequences that would be consistent with the role of each region in the recently proposed “swinging head” hypothesis. The analyses show that linker L1 is the most flexible rod domain region, that it is the most elongated structure (on a per residue basis), and that it is the most variable region as regards sequence and length. Segment 1A has one of the two most highly conserved regions of sequence in the rod domain (the other being at the end of segment 2B), with seven particular residues conserved across all chain types. It also contains one of the very few potential interchain ionic interactions that could be conserved across all chain types. However, the aggregation of chains in segment 1A is specified less precisely overall by interchain ionic interactions than are the other coiled-coil segments. The apolar residue contents in positions a and d of the heptad substructure are the highest of any coiled-coil segment in the intermediate filament family. Segment 1A also displays an amino acid composition atypical of not only coiled-coil segments 1B and 2B, but indeed of two-stranded coiled coils in general. Nonetheless, molecular modeling based on the crystal structure of the monomeric 1A fragment from human vimentin shows that coiled-coil formation is plausible. The most extensive regions of apolar/aromatic residues lie at the C-terminal end of segment 2B in the helix termination motif and in segment 1A in and close to the helix initiation motif. The predicted stability of the individual α-helices in segment 1A is greater than in those comprising segments 1B and 2B, though potential intrachain ionic interactions are either lacking or are minimal in number. Analysis of the 1A sequence and those regions immediately N- and C-terminal to it has shown that the capping residues are near optimal close to the previously predicted ends, thus adding to the likely stability of the α-helical structure. However, a second terminating sequence is predicted in 1A (about 10 residues back from the C-terminus). This allows the possibility of some unwinding of the α-helical structure of 1A immediately adjacent to linker L1 when the head domains no longer stabilize the coiled-coil structure. All of these data are consistent with the concept of a flexible hinge at L1 and with the ability of the two α-helical coiled-coil strands to separate under appropriate conditions and partly unwind at their C-terminal ends to allow the head domains a greater degree of mobility, thus facilitating function.

A single amino acid can switch the oligomerization state of the alpha-helical coiled-coil domain of cartilage matrix protein.

We have studied the oligomerization of an alpha-helical coiled-coil using as an example a peptide corresponding to the C-terminal domain of cartilage matrix protein. By replacing one arginine residue, which forms an interchain ionic interaction with a glutamic acid residue, with glutamine, we found that this peptide assembles into a homotetramer at neutral pH in contrast to the native molecule which forms homotrimers. At acidic and basic pH, however, we again observed the trimer conformation. Another arginine, which is probably involved in an intrachain salt bridge, has no effect on the assembly. Our data demonstrate that besides the specific distribution of hydrophobic residues, interchain ionic interactions can be crucial in modulating the association behavior of alpha-helical coiled-coil domains.

Protein destabilization by electrostatic repulsions in the two-stranded alpha-helical coiled-coil/leucine zipper.

The destabilizing effect of electrostatic repulsions on protein stability has been studied by using synthetic two-stranded alpha-helical coiled-coils as a model system. The native coiled-coil consists of two identical 35-residue polypeptide chains with a heptad repeat QgVaGbAcLdQeKf and a Cys residue at position 2 to allow formation of an interchain disulfide bridge. This peptide, designed to contain no intrahelical or interhelical electrostatic interactions, forms a stable coiled-coil structure at 20 degrees C in benign medium (50 mM KCl, 25 mM PO4, pH 7) with a [urea]1/2 value of 6.1 M. Four mutant coiled-coils were designed to contain one or two Glu substitutions for Gln per polypeptide chain. The resulting coiled-coils contained potential i to i' + 5 Glu-Glu interchain repulsions (denoted as peptide E2(15,20)), i to i' + 2 Glu-Glu interchain repulsions (denoted E2(20,22)), or no interchain ionic interactions (denoted E2(13,22) and E1(20)). The stabilities of the coiled-coils were determined by measuring the ellipticities at 222 nm as a function of urea or guanidine hydrochloride concentration at 20 degrees C in the presence and absence of an interchain disulfide bridge. At pH 7, in the presence of urea, the stabilities of E2(13,22) and E2(20,22) were identical suggesting that the potential i to i' + 2 interchain Glu-Glu repulsion in the E2(20,22) coiled-coil does not occur. In contrast, the mutant E2(15,20) is substantially less stable than E2(13,22) or E2(15,20) by 0.9 kcal/mol due to the presence of two i to i' + 5 interchain Glu-Glu repulsions, which destabilize the coiled-coil by 0.45 kcal/mol each. At pH 3 the coiled-coils were found to increase in stability as the number of Glu substitutions were increased. This, combined with reversed-phase HPLC results at pH 7 and pH 2, supports the conclusion that the protonated Glu side chains present at low pH are significantly more hydrophobic than Gln side chains which are in turn more hydrophobic than the ionized Glu side chains present at neutral pH. The protonated Glu residues increase the hydrophobicity of the coiled-coil interface leading to higher coiled-coil stability. The guanidine hydrochloride results at pH 7 show similar stabilities between the native and mutant coiled-coils indicating that guanidine hydrochloride masks electrostatic repulsions due to its ionic nature and that Glu and Gln in the e and g positions of the heptad repeat have very similar effects on coiled-coil stability in the presence of GdnHCl.

NuMA/centrophilin: sequence analysis of the coiled-coil rod domain

Nuclear mitotic apparatus protein (NuMA), also known as centrophilin, has been shown in previous work to contain a centrally located sequence of length 1485 residues that has both a heptad substructure and a high propensity for alpha-helix formation. Further analysis of this sequence here has revealed that NuMA will form a two-stranded coiled-coil structure with multiple (18) points at which the conformation is interrupted either by proline-containing segments or by discontinuities in the phasing of the heptad substructure. It has also been shown that the two chains will be parallel (rather than antiparallel), that they will lie in axial register, and that this arrangement will be stabilized by a large number of interchain ionic interactions. Interestingly the coiled-coil rod domain is also shown to lack any significant long-range periodicity in the linear distribution of either its acidic or its basic residues. Hence there is no direct evidence from the sequence data that NuMA molecules will aggregate to form closely packed filaments within nuclear space.

The Role of Interhelical Ionic Interactions in Controlling Protein Folding and Stability: De Novo Designed Synthetic Two-stranded α-Helical Coiled-Coils

The role of interchain ionic interactions in controlling protein folding and stability has been studied by using de novo designed synthetic two-stranded α-helical coiled-coils. The model coiled-coil (denoted as EK) consists of two identical 35-residue polypeptide chains with a heptad repeat KgLaGbAcLdEeKf and a Cys residue at position 2 and an Ala residue at position 16 in each chain. The Lys residues at positions "g" in one chain and Glu residues at positions "e" in the other chain are expected to form interchain ion-pairs in the coiled-coil structure. This peptide forms a stable coiled-coil structure in benign medium (50 mM KCl, 25 mM PO4, pH 7) with a [urea]1/2 value of 3·5 M. In contrast, two peptide analogs EE (EgLaGbAcLdEeKf) and KK (KgLaGbAcLdKeEf), which differ from EK in that EE contains only negatively charged Glu residues and KK contains only positively charged Lys residues at both positions e and g, each show a random coil structure in benign buffer. However, peptide EE or KK can form a stable coiled-coil structure if the interchain ionic repulsions are effectively suppressed either by changing pH or by using high salt concentrations. An equimolar mixture of these two peptides displays 100α α-helical content under the same conditions. These results demonstrate that although the interhelical ionic attractions are not essential for coiled-coil formation, a large number of these weak interactions can play an important role in the assembly of helices. Though interhelical ionic repulsions destabilize the homo-stranded coiled-coil, electrostatic attractions may stabilize the hetero-stranded coiled-coil. In addition, this study also suggests that the folding process for these synthetic model coiled-coils does not involve a single-stranded α-helix as a significantly populated folding intermediate.

Ionic Interactions in the Coiled-coil Domain of Laminin Determine the Specificity of Chain Assembly

Laminins are a family of large (800 to 900 kDa) multidomain glycoproteins specifically found in basement membranes. They consist of one heavy A chain and two light chains B1 and B2, and several tissue-specific laminin isoforms exist. Chain assembly is mediated through the formation of a rod-like triple-stranded α-helical coiled-coil domain about 75 nm long. The interacting edges of the chains are mostly formed by hydrophobic residues in positions a and d of an (abcdefg) n heptad sequence repeat and by a distinct pattern of charged residues in positions e and g. Here, we have analyzed the sequences of known laminin chain in an effort to relate them to interaction potential. Initially, those sequences localized in the long arm were arranged in an optimum heptad-repeating scheme. The interacting edges between chains were then analyzed for interchain hydrophobic and ionic interactions. The short heptad blocks were allowed to shift axially with respect to each other to maximize the number of interactions. The number of hydrophobic interactions was very high and similar for all chain combinations, but especially so for homodimers. As these were not observed experimentally, it seems that hydrophobic interactions probably represent only a prerequisite for coiled-coil formation. The number of ionic interactions, however, directly resembles the interaction potential observed in in vitro experiments. In particular, the number of interchain ionic interactions is high for parallel heterodimer configurations of A and B chains, but low for homodimer arrangements. When the laminin isoform chains, rat s-laminin (B1s) and human merosin (Am), are included in the analysis, they show rather low but high ionic interaction potentials between them and distinct mouse laminin chains are predicted. For mouse laminin the analysis was extended to a full three-stranded coiled-coil structure. The highest number of interchain ionic interactions occurs for an anti-clockwise chain arrangement of A → B1 → B2 when viewed from the N terminus. None of the laminin chains appears to be designed for the formation of homodimers, although such conformation are frequently found in other α-fibrous proteins.

Block copolymer ionomers: 1. Synthesis and physical properties of sulphonated poly(styrene-ethylene/butylene-styrene)

The styrene end-blocks of a poly(styrene-(ethylene- co-butylene)-styrene) (SEBS) triblock copolymer were sulphonated up to a concentration of 18 mol%. Sodium and zinc salts of the sulphonated polymers exhibited behaviour indicative of strong ionic interchain interactions, similar to what is observed with other ionomers. Dilute solution viscosities were enhanced upon sulphonation, the T g of the styrene blocks was increased and the ionomers exhibited a rubbery plateau region above the styrene block T g that was absent in the base resin. Whereas SEBS was insoluble in toluene and methanol mixed solvents, the sulphonated polymers were soluble. Moreover, the addition of the alcohol cosolvent significantly lowered the solution viscosity by solvating the interchain ionic interactions. The sulphonated polymers were thermal-oxidatively more stable and more hygroscopic than the unmodified polymers.

Structural features in the heptad substructure and longer range repeats of two-stranded α-fibrous proteins

Considerable sequence data have been collected from the intermediate filament proteins and other α-fibrous proteins including myosin, tropomyosin, paramyosin, desmoplakin and M-protein. The data show that there is a clear preference for some amino acids to occur in specific positions within the heptad substructure that characterizes the sequences which form the coiled-coil rod domain in this class of proteins. The results also indicate that although there are major similarities between the various proteins there are also key differences. In all cases, however, significant regularities in the linear disposition of the acidic and the basic residues in the coiled-coil segments can be related to modes of chain and molecular aggregation. In particular a clear trend has been observed which relates the mode of molecular aggregation to the number of interchain ionic interactions per heptad pair.

Structure of rabbit skeletal myosin: Analysis of the amino acid sequences of two fragments from the rod region

The amino acid sequences of fragments from light meromyosin and heavy meromyosin subfragment-2 have been analysed and structural features noted. As with other α-fibrous protein sequences, there is a regular disposition of apolar residues in positions a and d of the heptapeptide-type repeat characteristic of the coiled-coil conformation. The common occurrence of acidic and basic residues in the e and g positions, respectively, give rise to a maximum number of interchain ionic interactions when the two parallel chains of myosin are in axial register. Although the quasi-repeating heptapeptides in the sequences both have two points of discontinuity (unlike that in most other α-fibrous proteins), secondary structure prediction methods indicate that the fragments will be 90 to 100% α-helical. Fast Fourier transform techniques have revealed a significant periodicity of about 27.4 ± 0.3 residues (~41 Å) in the linear disposition of the acidic residues and the basic residues in both of the fragments. This period is compatible with similarly directed myosin molecules in the thick filament being axially staggered with respect to one another by an odd multiple of 143 Å. Preliminary evidence is also presented to show that the sequence of the rod region of myosin may have a 28 residue gene duplication repeat.