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Novel helical, structures unprecedented in the chemistry of α-polypeptides, may be found in polypeptides containing β and γ amino acids. The structural characterization of C12 and C14 -helices in oligo β-peptides was originally achieved using conformationally constrained cyclic β-residues. This study explores the conformational characteristics of proteinogenic β3 residues in homooligomeric sequences and addresses the issue of inducing a transition between C14 and C12 helices by the introduction of a guest α-residue. Folded C14 -helical structures are demonstrated for the nonapeptide Boc-[β3 (R)Val]9 -OMe by NMR methods in CDCl3 -DMSO mixtures, while the peptide was found to be aggregated in CDCl3 . The insertion of a guest Aib residue into an oligo-β-valine sequence in the octapeptide model Boc-[(β3 (R)Val)3 -Aib-(β3 (R)Val]4 -OMe results in well dispersed NH region in the NMR spectrum indicating folded structures in CDCl3 . Structure calculations for both the peptides using NOE distance constraints support a C14 helical structure in the homooligomer which transform into a C12 helix on introduction of the guest Aib residue.

The conformation of thymosin β9 in solution of 40% (v/v) 1,1,1,3,3,3-hexafluoro-2-propanol-d2 in water has been investigated by two-dimensional 1H-nmr spectroscopy. Under this condition thymosin β9 adopts an ordered structure. The determination of the conformation of the peptide was based on a set of 304 approximate interproton distance constraints derived from nuclear Overhauser enhancement measurements. The conformation of thymosin β9 includes two helical regions from residues 4 to 27 and 32 to 41. The two helices are separated by a poorly defined loop region between amino acids 28 and 31; the N-terminus of thymosin B9 shows random-coil structure only. © 1997 John Wiley & Sons, Inc. Biopoly 41: 623–634, 1997

By generating classes of random structures for trypsin inhibitor and carp myogen, each consistent with a given set of experimental or theoretical information, we have assessed the relative utility of various experiments and theories in deducing the conformation of macromolecules. We compare the calculated structures with known x-ray coordinates and compute for each class an average error. Small errors mean that the experimental or theoretical constraints limit the structures to the vicinity of the crystal structure, whereas large errors show that the constraints permit a wide variety of tertiary conformations. We find the following points to hold true: (1) Qualitative information on all the distances, as might be obtained from the correct prediction of interresidue contacts, effectively determines the structure (error approximately 1 A). (2) Quantitative information on a limited number of distances, as might be obtained from nmr or crosslinking experiments, significantly restricts the range of possible structures only when the number of distances given is comparable to the number of residues (error approximately 3 A). (3) Quantitative information on the distances of each residue to the center of mass of the molecule, as might in part be obtained from solvent accessibility and solution x-ray studies, is not particularly restrictive by itself (error approximately 5 A). (4) Complete qualitative local distance information, as might be obtained from secondary prediction and CD/ORD studies, is clearly consistent with a wide variety of tertiary structures (error approximately 7 A).

The assignments of several 1H lines in the spectrum of the lac repressor DNA‐binding domain (or “headpiece”) are documented. These new assignments complement those obtained in a previous study [E.R.P. Zuiderweg, R. Kaptein, and K. Wüthrich (1983) Eur. J. Biochem. 137 , 279] and were made from pure absorption phase two‐dimensional nuclear Overhauser enhancement (2D NOE) spectra and 1H 2D double‐quantum spectra of this protein. Based on the complete set of resonance assignments, a large number of interresidue NOEs are identified; these are documented and their relation with distance constraints is discussed. The identification of these NOEs is a prerequiste for the determination of the spatial structure of this protein, for which no crystallographic data are available.