Helix breaking

Helix-Forming and Helix-Breaking Behavior of the Amino Acids... 

Signal sequences PhoE n.a. 21 SDS Helix-break-helix 118... 

Chapter D is concerned with intrinsic viscosity and translational friction coefficient. Published data for the molecular-weight dependence of these quantities of polypeptides in helieogenic solvents and helix-breaking solvents are summarized, and the variations of these quantities during the helix-coil transition are described. 

In most experimental studies, the environmental conditions of a given polypeptide sample are varied by changing either the temperature of the system or the composition of a solvent mixture consisting of a helix-supporting solvent and a helix-breaking solvent. A curve of helical fraction fN versus temperature at fixed solvent composition is called the thermally induced or thermal transition curve, while a curve of fN versus solvent composition at fixed temperature is called the solvent-induced transition curve. The former is classified into two types normal and inverse (or reverse), depending on whether fN decreases or increases with the rise in temperature. 

Figure 7a and b compares the transition curves of PBLA and PBLG in mixtures of EDC and DCA (24,25). It is seen that the helix of PBLA is far less stable than that of PBLG against the helix-breaking action of DCA. Note that... 

Except for the system PCBL-m-cresol (23), all the reported inverse transitions were observed in binary mixtures which consisted of a helicogenic solvent and a helix-breaking acid such as DCA and trifluoroacedc acid (TFA). Figure 9 shows, with the data for PBLG in mixtures of EDC and DCA (24), features of the... 

It is probably the intermolecular hydrogen bonding between helix-breaking solvent and peptide residues that is mainly responsible for the occurrence of... 

All these results are in qualitative conformity to the anticipation that the helix-breaking DCA would be selectively adsorbed on PBLG. However, the data of Strazielle et al. for a mixture of EDC (0.24) and DCA (0.76) were quite... 

This chapter summarizes important data for intrinsic viscosity and translational friction coefficient of polypeptides. The first half of the chapter discusses the data obtained in helicogenic solvents and in helix-breaking solvents. It is actually a supplement to the review article by Benoit et al. (61), in which such data published by 1967 were surveyed critically. The second half of the chapter is concerned with the helix-coil transition region. The context here is largely descriptive because of the lack of relevant theory. 

The viscosity-molecular weight relations of polypeptides in helix-breaking solvents such as DCA and TFA are, as would be expected, very similar to those of randomly coiled polymers this can be seen from the v values summarized in Table 4. 

Table 4. Values of v, the exponent in the Houwink-Mark-Sakurada relation, and characteristic ratio 0/(Mj ) for polypeptides in helix-breaking solvents...

Fig. 26. Stockmayer-Fixman plots for PBLA in three helix-breaking solvents, constructed from the data by Hayashi et al. (22)...

Circular dichroism (CD) studies on a-elastin (Tamburro et al., 1977), K-elastin, bovine, and human tropoelastin (Debelle et al., 1995 Vrhovski et al., 1997) have demonstrated a conformational transition to increased a-helical content with increasing temperature. The a-helical content predicted for tropoelastin is probably confined to the crosslinking domains, as the rest of the molecule is rich in helix breaking proline residues (Muiznieks et al., 2003). 

Besides this statistical mechanical approach to the question of helix stability, the problem has also been addressed by conformational energy calculations. First, the helix-breaking tendencies of such residues as serine and aspartic acid can be accounted for by the tendency toward formation of side chain-backbone hydrogen bonds in nonhelical conformations163 (Figures 20 and 21). Second, the free energies of the helical and statistical coil forms in water have... 

The Chou-Fasman rules in brief (Fasman, 1985), are A cluster of FOUR a-helix-promoting amino-acid residues (Glu+, Ala, Leu, His+, Met, Gin, Trp, Val, or Phe) in a run of SIX amino acids will initiate an a-helix to form, until sets of a-helix-breaking amino acids (those with s-values less than 1.00 in this table) are encountered. Proline cannot occur in an a-helix in the inner part of a polypeptide chain, or in a helix in the C-terminal end of a polypeptide chain, but can occur within the last three residues in the TV-terminal end of an a-helix. A cluster of THREE (3-s true tare-forming amino-acid residues out of a run of FIVE amino adds will initiate the formation of a (3-sheet, which will end when a set of four (3-sheet-breaking amino acids is reached. ... 

---