Extended -Peptide Strands, Turns and Formation of Sheet Structures

Extended Peptide Strands, Turns and Formation of Sheet Structures 

As mentioned in the introductory part of Section 2.2.3, fully extended y9-peptide strands can be generated by populating antiperiplanar conformations around the C a)-C fi) bond (0j values close to 180° Fig. 2.27A). 

A //-peptidic parallel sheetiike conformation with characteristic intermolecular 14-membered H-bonded rings was first observed in the crystal structure of -peptide Boc-/ -HVal-/ -HAla-/l -HLeu-OMe 116 (Fig. 2.28A) [10]. However, the struc- 

To prevent insolubility resulting from uncontrolled aggregation of extended strands, two adjacent parallel or antiparallel yS-peptide strands can be connected with an appropriate turn segment to form a hairpin. The / -hairpin motif is a functionally important secondary structural element in proteins which has also been used extensively to form stable and soluble a-peptide y9-sheet arrangements in model systems (for reviews, see [1, 4, 5] and references therein). The need for stable turns that can bring the peptide strands into a defined orientation is thus a prerequisite for hairpin formation. For example, type F or II turns formed by D-Pro-Gly and Asn-Gly dipeptide sequences have been found to promote tight a-pep-tide hairpin folding in aqueous solution. Similarly, various connectors have been 

As a consequence of their different turn geometry a 10-membered turn closed by H-bonds between NH and C=0 +i and a 12-membered turn closed by Id-bonds between C=0 and NH +3, antiparallel hairpins formed by y9-peptides 121 and 122 display opposite sheet polarities (see Fig. 2.30A and B). Comparison of backbone torsion angles (X-ray and NMR) for selected y9-amino acids residues within extended strand segments of peptides 117-122 are shown in Tab. 2.7. The observed values are close to ideal values for y9-peptide pleated sheets =-120° (or 120°), 01 = 180°, (/ =120°(or-120°). 

---