Peptides helical transitions

The structure and tilt angle of the molecules relative to the surface normal were determined by their FTIR spectra. In helical peptides, the transition moment of amide-I band lies nearly parallel to the helix axis and that of amide-II perpendicular. Since transition moments, which lie parallel to the gold surface, cannot be detected in grazing angle FTIR, the ratio between the intensities of the amide-I band (1,665 cm-1) and amide-II band (1,550 cm-1) indicates to what extend the molecules in the monolayer are oriented perpendicular to the gold surface. Based on the FTIR spectra it was possible to calculate the tilt angle, namely the angle between the molecular axis and the surface normal. The frequencies of amide-I and amide-II vibrations indicate that the monolayer is indeed in an a helix form (Fig. 2a). 

Huston and Marshall (87) used this approach to map the reaction coordinates of the a- to 3io-helical transition in model peptides. 

The details of these sampling procedures that allow one to focus on the aspect of the problem of interest are the subject of a review by Beveridge (133). Application of this approach to determining conformational transitions in model peptides (137,139,140)are exemplified in the work of Elber s group on helix-coil (85, 86,141), the Brooks group on tum-coil (142-146), and Huston and Marshall and Smythe et al. (147,148)on helical transitions in peptides. 

Since 1973, several authors have proved that there is a relationship between thermostability of collagen and the extent of hydroxylation of the proline residues31,34). Equilibrium measurements of the peptides al-CB 2 of rat tail and rat skin revealed a higher rm, for al-CB 2 (rat skin)157). The sequence of both peptides is identical except that in the peptide obtained from rat skin, the hydroxylation of the proline residues in position 3 has occurred to a higher extent than in the case of al-CB 2 (rat tail). Thus, a mere difference of 1.8 hydroxy residues per chain causes a ATm of 26 K. Obviously, there are different stabilizing interactions in the triple-helical state, that means al-CB 2 (rat skin) forms more exothermic bonds than al-CB 2 (rat tail) in the coil triple-helix transition. This leads to an additional gain of enthalpy which overcompensates the meanwhile occurring losses of entropy. 

Although the n-n and tz-tz electronic transitions of the urea chromophore have not been studied as extensively as amides, the contribution of the backbone is expected to dominate the far UV spectra of oligoureas in a fashion similar to that which is observed for peptides. The CD spectra recorded in MeOH of oligoureas 177 and 178 show an intense maximum near 204 nm (Figure 2.48). This is in contrast to helical y" -peptides that do not exhibit any characteristic CD signature. 

As a prelude to our binding studies, the secondary structure of aPNA itself was examined using CD spectroscopy [52]. The first aPNA to be studied was the tail-to-tail bl dimer, [Ac-Cys-Gly-Ser -Asp-Ala-Glu-Ser -Ala-Ala-Lys-Ser -Ala-Ala-Glu-Ser -Ala-Aib-Ala-Ser -Lys-Gly-NH2]2- The far-UV CD spectra of this aPNA in water at 30 °C showed the double minimum at 220 nm (n-n transition) and 206 nm (n-n transition) as well as the maximum at 193 nm (n-n transition), characteristic of a peptide a-hehx. Upon increasing the temperature, the intensity of the minimum at 200 nm decreased indicating a transition from a-helix to random stracture. An isodichroic point at 202 nm was suggestive of a temperature-depen-dent a-helix to random coil transition. The helical content of this T5(bl)-dimer at 20°C in water was estimated to be 26% [40]. 

As host defense peptides are membrane-active molecules, safety mechanisms must be employed to avoid deleterious contacts with host cells. These mechanisms may involve the limitation of peptide activation to specific environments or niche-specific amplification. That most ct-helical peptides remain unstructured in aqueous solution and undergo conformational transitions to an activated state within hydrophobic environments supports this postulate. It has also been postulated that the order of anionic phospholipids in microbial plasma membranes likely induces optimal periodicity of polar residues within host defense peptides at the membrane surface. ... 

Independently, Ruan etal. (1990) demonstrated that unnatural metal-ligating residues may likewise be utilized toward the stabilization of short a helices by transition metal ions (including Zn " ")—these investigators reported that an 11-mer is converted from the random coil conformation to about 80% a helix by the addition of Cd at 4°C. These results suggest that the engineering of zinc-binding sites in small peptides or large proteins may be a powerful approach toward the stabilization of protein secondary structure. 

As the last example of a helix-sheet transition, Pagel et al. (2006) demonstrate a coiled coil peptide adopting three types of secondary structures and the transition between random coil, a-helical, and /3-sheet hbers can be simply triggered by changing the pH or peptide concentration. 

Pioneering work in this area was aimed at using specific metal ligand interactions to induce and stabilize secondary structures. This has been achieved by Ghadiri et al. for a-helical structures through the formation of transition metal and Ru(II) inert complexes with two imidazoles of His or one thiol of Cys and one imidazole of His in i, i + 3 or i, i + 4 relationships.1[37,38 In almost all cases the helix content and stability increased upon metal complexation, especially with i, i + 4 peptides. This work resembles the stabilization of helical structures using metal complexation by EDTA-like side chains discussed in Section 9.4.6. 

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