3i0-helix

The third most frequent backbone conformation at disulfide ends is a tight turn. Sometimes it is a succession of turns, or bit of 3i0-helix. 

Monte Carlo and molecular dynamics calculations predict that, in aqueous solutions, the a-helix is substantially more stable than the 310-helix for (Ala), 79 and even for a decamer of Aib. 80 Thus, it seems very unlikely that Ala-rich peptides in water have any significant level of molecules that are wholly or largely in the 310-helix. However, isolated 310-helix hydrogen bonds or perhaps short stretches of 310-helix near the termini are possible. High-resolution NMR of the peptides Ac-(A4K)A-NH2 and Ac-AMAAKAWAAKAAAARA-NH2 81 have led to estimates of about 50% 3i0-helix population at the termini and 25% in the interior. The CD spectrum for a mixed a-310-helix has not been determined. 

Cys residues spaced i, i + 3 apart have been covalently linked to obtain cyclic peptides which fold into a 310-helix. Balaram et al.[159l synthesized Boc-c[-Cys-Pro-Aib-Cys-]-NHMe, wherein a disulfide bond links the two Cys residues. Pro and Aib residues were used to restrict conformational flexibility. NMR and X-ray diffraction studies revealed a highly folded, compact formation of an incipient 3i0-helix, with the S—S bridge lying approximately parallel to the helix axis. 

Using a number of orientational constraints from NMR measurements, the orientation of the peptide in the membrane can be determined, assuming that the structure of the peptide is known. The peptides we have studied mostly have well defined structures, such as a-helices or rigid cyclic conformations. The validity of the proposed peptide structure as it interacts with the membrane can also be tested using the fit of the data [45, 49], For example, the data can be fitted to different helical models, such as a- and 3i0-helix a good... 

Despite many years of experimental and theoretical studies devoted to it, the protein folding problem remains essentially unsolved because there are too many conformations that can occur in both the unfolded and the folded structure to be searched. The problem of protein folding is further compounded by transient disulfide bonds, solvent, and environmental effects in general that may play an important role in stabilizing particular folded states such as, e.g., the a- or the 3i0-helix. 

In the solid state, long Aib-rich peptides (more than eight residues) form a mixed 3i0/a-helix, depending on the solvent of crystallization. 35 37] The enhanced electrostatic interaction and improved packing in a 310-helix makes it very easy to crystallize. These are but empirical generalizations and it is the balance of these factors, along with the steric requirements of the amino acid side chains, which ultimately determines the type of helix that will be formed. 

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