X-Pro peptides: synthesis and solution conformation of benzyloxycarbonyl-(Aib-Pro)<SUB>4</SUB>methyl ester. Evidence for a novel helical structure

The synthesis of the octapeptide, benzyloxycarbonyl-(&#945;-aminoisobutyryl-L-prolyl)4-methyl ester [Z-(Aib-Pro)4-OMe] and an analysis of its solution conformation is reported. The octapeptide is shown to possess three strong intramolecular hydrogen bonds on the basis of studies of the solvent and temperature dependence of NH chemical shifts and rates of hydrogen-deuterium exchange. 13C studies are consistent with a structure involving only trans Aib-Pro bonds, while ir experiments support a hydrogen-bonded conformation. The Aib 3, 5, and 7 NH groups are shown to participate in hydrogen bonding. A 310 helical conformation compatible with the spectroscopic data is suggested. The proposed conformation consists of three type III &#946;-turns with Aib and Pro at the corners and stabilized by 4&#8594; 1 intramolecular hydrogen bonds

An incipient 3<SUB>10</SUB> helix in Piv-Pro-Pro-Ala-NHMe as a model for peptide folding

The molecular mechanism of helix nucleation in peptides and proteins is not yet understood and the question of whether sharp turns in the polypeptide backbone serve as nuclei for protein folding has evoked controversy. A recent study of the conformation of a tetrapeptide containing the stereochemically constrained residue &#945;-aminoisobutyric acid, both in solution and the solid state, yielded a structure consisting of two consecutive &#946;-turns, leading to an incipient 3<SUB>10</SUB> helical conformation. This led us to speculate that specific tri- and tetra-peptide sequences may indeed provide a helical twist to the amino-terminal segment of helical regions in proteins and provide a nucleation site for further propagation. The transformation from a 3<SUB>10 </SUB>helical structure to an &#945;-helix should be facile and requires only small changes in the &#966; and &#968; conformational angles and a rearrangement of the hydrogen bonding pattern. If such a mechanism is involved then it should be possible to isolate an incipient 3<SUB>10</SUB> helical conformation in a tripeptide amide or tetrapeptide sequence, based purely on the driving force derived from short-range interactions. We have synthesised and studied the model peptide pivaloyl-Pro-Pro-Ala-NHMe (compound I) and provide here spectroscopic evidence for a 3<SUB>10 </SUB>helical conformation in compound I

Rotational isomerism about the C<SUB>&#945;</SUB>-CO bond in proline derivatives

The 270 MHz <SUP>1</SUP>H n.m.r. spectrum of benzyloxycarbonyl-Pro-N-methylamide in CDCl<SUB>3</SUB> is exchange broadened at 293&#176; K. Spectral lines due to two species are frozen out at 253&#176; K and a dynamically averaged spectrum is obtained at 323&#176; K. A selective broadening of the C<SUP>&#946;</SUP> and C<SUP>&#947;</SUP> resonances in the <SUP>13</SUP>C n.m.r. spectrum is observed at 253&#176; K, with a splitting of the C<SUP>&#946;</SUP> and C<SUP>&#947;</SUP> resonances into a pair of lines of unequal intensity. A similar broadening of C<SUP>&#946;</SUP> and C<SUP>&#947;</SUP> peaks is also detected in pivaloyl-Pro-N-methylamide where cis-trans interconversion about the imide bond is precluded by the bulky t-butyl group. The rate process is thus attributed to rotation about the C<SUP>&#945;</SUP>-CO bond (&#968;) and a barrier (&#916;G<SUP>#</SUP>) of 14kcal mol<SUP>-1</SUP> is estimated. <SUP>13</SUP>C n.m.r. data for pivaloyl-Pro-N-methylamide in a number of solvents is presented and the differences in the C<SUP>&#946;</SUP> and C<SUP>&#947;</SUP> chemical shifts are interpreted in terms of rotational isomerism about the C<SUP>&#945;</SUP>-CO bond