Cyclic Peptides VII. The Synthesis and Characterization of Cyclic Peptides with Repeating Pro-Gly Sequences

Rotaxanes of cyclic peptides

The synthesis of rotaxanes derived from the synthetic peptide macrocycles cyclo(l-ProGly)4 and cyclo(l-ProGly)5 and diammonium threads is described. [2]Rotaxanes are formed in good yields (56-63%), despite the disruption of internal amide-amide hydrogen bonding in the macrocycles.

All-L-Leu-Pro-Leu-Pro: a challenging cyclization

In this paper, we report the difficult synthesis of cyclo(Leu-Pro-Leu-Pro). While the cyclization of Leu-Pro-Leu-D-Pro did not cause problems, the all-L-peptide afforded cyclodimer rather than cyclotetrapeptide (cyclomonomer). A first attempt using our reversible backbone substitution methodology failed. However, we were successful in obtaining the desired cyclo(Leu-Pro-Leu-Pro) by decreasing the concentration. The ratio of cyclomonomer to cyclodimer was raised to 1:1.1 using BOP and 1:0.6 using HATU under our high dilution condition. The structures of the cyclopeptides were confidently assigned by electrospray ionization mass spectrometry and NMR.

Conformational and ion binding properties of a cyclic octapeptide, cyclo (Ala-Leu-Pro-Gly)2

The conformation and ion-binding characteristics of a cyclic octapeptide, cyclo (Ala-Leu-Pro-Gly)2, in a liphophilic solvent, acetonitrile, have been studied using CD and nmr spectroscopy. The peptide binds preferentially to divalent cations such as calcium, magnesium, and barium. The conformations of the free cyclic peptide and its calcium complex are very similar with well-defined beta- and gamma-turns. The cyclic peptide readily forms equimolar and possibly 2:1 (peptide:cation) complexes with divalent cations.

Crystal and molecular structure of cyclo(L-prolyl-glycyl)3. A cyclic hexapeptide with a cis peptide bond

The crystal structure of cyclo(L-Pro-Gly)3 was solved using X-ray crystallographic techniques. The backbone of the peptide is asymmetric and is made up of five trans peptide units and one cis peptide. There is a hydrogen bonded water bridge that links the carbonyl oxygens, O1 and O4. The molecules exist as dimers in the crystal lattice. The two molecules of the dimer are related by crystallographic twofold symmetry and are linked by two N-H ... O hydrogen bonds. The crystals are trigonal, space group P3(2)12 with a = 11.379(3), c = 32.93(1) and z = 6. The structure was solved by multisolution methods and refined by least squares technique to an R of 0.083.

Cyclo(L-prolylglycyl)3 and its sodium, potassium, and calcium ion complexes: a Raman spectroscopic study

Raman spectra of the cyclic hexapeptide cyclo-(L-prolylglycyl)3 and its Na+, K+, and Ca2+ complexes are reported for the solid state and for samples in solution. Model compounds and N-deuteration were used to aid mode identification. Spectra of the uncomplexed ionophore in solution are consistent with previously proposed solution conformations and permit the identification of spectral lines characteristic of proline-containing peptide bonds in the trans and the cis conformations. Upon cation complexation the prolyl carbonyl stretch bands sharpen and upshift 20-30 cm-1 (to 1690-1700 cm-1). The glycyl carbonyl stretch band is unaffected by Na+ complexation, upshifted approximately 15 cm-1 by K+ complexation, and downshifted approximately 20 cm-1 (to 1619 cm-1) by Ca2+ complexation. Arguments supporting the involvement of prolyl carbonyl groups in cation complexation are noted. Spectra of the Na+ complex of the tetramer cyclo(L-prolylglycyl)4 suggest an asymmetric structure.