Coiled coils thermal stability

This effect has also been observed in polyphosphazenes containing alkyl- or phenyl-carborane as pendent groups.12 A typical synthetic route to poly(phenyl-carboranyl-di-trifluoroethoxy-phosphazene) having pendent phenyl-carborane groups is shown in scheme 4. A substantial improvement in the thermal stability of the polymer was observed. This is attributed to a retardation of the ring-chain de-polymerization mechanism due to steric hindrance effects of the carborane units, inhibiting helical coil formation. 

The final class of polymers containing carboranyl units to be mentioned here is the polyphosphazenes. These polymers comprise a backbone of alternating phosphorous and nitrogen atoms with a high degree of torsional mobility that accounts for their low glass-transition temperatures (-60°C to -80°C). The introduction of phenyl-carboranyl units into a polyphosphazene polymer results in a substantial improvement in their overall thermal stability. This is believed to be due to the steric hindrance offered by the phenyl-carborane functionality that inhibits coil formation, thereby retarding the preferred thermodynamic pathway of cyclic compound formation (see scheme 12). 

It was found that the steam supply to the reactor was often superheated (just prior to shutdown to 330°C) [10], Although this degree of superheat would not grossly increase the temperature of the inner reactor wall in contact with the liquid (or the bulk liquid temperature) [11], it seems probable that any reaction material splashed onto and dried out at the top of the coil-heated wall would have become heated to a much higher temperature. Further detailed work on the thermal stability of the mixture showed that a previously unsuspected very slow exothermic decomposition existed, beginning at 180°C and proceeding at an appreciable rate only above 200°C, so that the exotherm was insufficient to heat the contents of the reactor from the last recorded temperature of 158°C to the decomposition temperature of 230°C in 7.5 h [12,13,14], It was concluded that an alternative (effectively an external) source of heat was necessary to account for the observed effect, and the residual superheat from the steam at 330°C seems to have been that source. 

The a-helical coiled coil-based screening system already provided a wide variety of information about the interactions of fluorinated amino acids within hydrophobic and hydrophilic protein environments. Investigations on the thermal stability as well as the replicase activity have both emphasized the orthogonal properties of fluorinated aliphatic amino acid side chains. The term orthogonal in this context has been chosen by us to demonstrate that they are in fact hydrophobic... 

In terms of physiological conditions, DNA exists almost totally in the double-helical conformation. Certain oligonucleotides also exhibit the ability to form triple-helixes with the DNA double strands. The stability of DNA secondary structures strongly depends on the environmental conditions, such as ionic strength, pH, temperature and solvents. Temperature is the most widely used environmental variable for the quantitative characterization of the helix-coil transitions. A commonly accepted characteristic of the thermal stability of DNA secondary structures is melting temperature, Tm. The binding of cationic species, such as multivalent... 

Overall, the effects of polycations on the helix-coil transition of DNA are similar to those exhibited by the small cations both of them induce thermal stabilization of DNA duplex and decrease the cooperativity of the helix-coil transition In both cases the reasons for the stabilization effects are the screening of the negatively charged phosphate groups of the DNA, which reduces their electrostatic repulsion (Schildkraut, 1965). However, the effective concentration of a simple salt at which the same stabilization effect is achieved is ca. 103 times higher than the effective concentrations of a polycation. Consequently, polycations provide for a much more efficient screening of the DNA phosphate groups than the small cations. 

Comprehensive thermodynamic analyses of the simulations demonstrate that changes in the unfolded state cause the thermal stabilization [34]. This observation is in accord with the idea that the unfolded state is not just a random coil but, rather, retains some residual structures, and it was observed that the conjugated glycans interfered with the formation of these structures in the unfolded state. This interference destabilized the unfolded state, shifted the thermodynamic equilibrium toward the folded state, and resulted in an overall thermodynamic stabilization. 

Liu et al. have described the solution structure of a mutant of the homodimer protein transcription factor 1, TFl. The dimeric core, consisting of the N-terminal helices and beta sheets, is more tightly packed than wire type, and this might be responsible for its increased thermal stability. Also, Gordon-Smith et al reported the dimeric structure of a C-terminal fragment of Bovine IFl, which inhibits the hydrol) ic action of the FIFO ATP synthase in mitochondria under anaerobic conditions. Most imusually, the molecule forms an anti-parallel coiled-coil in which three histidine residues occupy key positions at the dimer interface. ... 

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