Solvent-induced β-hairpin to helix conformational transition in a designed peptide

An octapeptide containing a central Aib-Gly- segment capable of adopting β-turn conformations compatible with both hairpin (βII' or βI′) and helical (βI) structures has been designed. The effect of solvent on the conformation of the peptide Boc-Leu-Val-Val-Aib-Gly-Leu-Val-Val-OMe (VIII; Boc: t-butyloxycarbonyl; OMe: methyl ester) has been investigated by NMR and CD spectroscopy. Peptide VIII adopts a well-defined β-hairpin conformation in solvents capable of hydrogen bonding like (CD3)2SO and CD3OH. In solvents that have a lower tendency to interact with backbone peptide groups, like CDCl3 and CD3CN, helical conformations predominate. Nuclear Overhauser effects between the backbone protons and solvent shielding of NH groups involved in cross-strand hydrogen bonding, backbone chemical shifts, and vicinal coupling constants provide further support for the conformational assignments in different solvents. Truncated peptides Boc-Val-Val-Aib-Gly-Leu-Val-Val-OMe (VII), Boc-Val-Val-Aib-Gly-Leu-Val-OMe (VI), and Boc-Val-Aib-Gly-Leu-OMe (IV) were studied in CDCl3 and (CD3)2SO by 500 MHz 1H-NMR spectroscopy. Peptides IV and VI show no evidence for hairpin conformation in both the solvents. The three truncated peptides show a well-defined helical conformation in CDCl3. In (CD3)2SO, peptide VII adopts a β-hairpin conformation. The results establish that peptides may be designed, which are poised to undergo a dramatic conformational transition

Pharmacological activities of some synthetic peptides related to dermorphin

Objectives: To investigate the relationship between the structure of demorphins (DM) and their pharmacological properties, six analogues of Hyp6DM and Pro6DM were synthesised and their biological activities were studied. Methods: The peptides were synthesised by the solid phase method using 9-fluorenylmethoxycarbonyl amino acid trichlorophenyl esters as coupling agents and Merrifield resin as solid support. The opioid agonist activity was studied using co-axially, electrically stimulated contraction of isolated guinea pig ileum (GPI, in vitro). Their analgesic activity was assessed in mice using Eddy's hot plate method and tail-flick method. The antidiarrhoeal activity was determined by the charcoal meal test in mice. Results: In the GPI assay, the synthetic analogues possess agonistic activity that are less pronounced than morphine. Peptides I and II (substitution of ser of position 7 and Gly at position 4 in Hyp6DM series respectively) possessed considerable analgesic activity but are almost inactive in the GPI assay. Peptide III (Pro6, Sar7DM) possess only analgesic activity. In GPI assay, peptide IV was inactive. Peptide V and VI had equipotent analgesic and antidiarrhoeal activity. Conclusion: Peptides with various structures can possess specificities that may prove useful in biological applications. Among them Sar4, Hyp6, Tyr7DM, Hyp6, Pro7DM, Pro6, Sar DM and Phg3, ProDM exhibited a high degree of selectivity in their activities

Solid phase synthesis of O-glycoopioid peptides related to dermorphin

Glycosylation of Fmoc-hydroxyamino acids with β-D-glucose pentaacetate has been carried out in the presence of several Lewis acids and BF3.Et2O has been found to be the most suitable one. Thus, 2,3,4,6-tetra-O-acetyl-β-D-glycopyranosyl derivatives of Fmoc-Ser/Thr/Tyr are prepared in a single step in reasonably good yields and high purity. The O-glycosylated derivatives are then converted to their corresponding trichlorophenyl esters for use in the solid phase synthesis of five glycoopioid peptides related to dermorphin. The effect of glycosylation on biological activity of dermorphin has been studied. Among the peptides synthesized, Tyr(β-D-Glc)5dermorphin and Ser(β-D-Glc)5, Tyr7)dermorphin exhibit considerable analgesic activity of about 80-90% compared to morphine and antidiarrhoeal activity of about 50% compared to dermorphin

Synthesis of peptides mediated by KOBt

Coupling of Fmoc-amino acid chlorides can be mediated by the potassium salt of 1-hydroxybenzotriazole (KOBt), the reaction being carried out in an organic medium. The use of a base like NaHCO3/Na2CO3 or DIEA/NMM/pyridine is not necessary. Coupling is fast and racemization free; the work-up, isolation of the product and scale-up are easy. The pentapeptide sequence of Fmoc-Leuenkephalin was thus synthesized in the solution phase on a 5 mmol scale without isolation of any intermediate. Acylation of C-protected N- methylamino acid esters by Fmoc-N-methylamino acid chlorides by this procedure is also feasible, as demonstrated by the synthesis of cyclosporin A fragments 4-7 and 8-11. The peptides obtained in high yields were crystalline solids, unlike earlier reports in which they were obtained mostly as oily or foamy intermediates. They showed spectral properties identical with those of the authentic compounds

Design of folded peptides

The construction of complex protein folds relies on the precise conversion of a linear polypeptide chain into a compact 3-dimensional structure. The interplay of forces that link sequence and folding is intricate and yet to be firmly elucidated. Examination of protein 3-dimensional structures suggests that complex tertiary folds and quaternary associations can be deconstructed into a limited number of secondary structural elements, such as strands, helices, and turns, which are assembled using loosely structured loops. The stability of a specific fold is determined by tertiary interactions between residues which are distant in sequence. De novo design of existing or novel protein folds demands a thorough understanding of the rules that underlie protein structure and stability

Solid-phase synthesis of oxytocin using iodotrichlorosilane as Boc deprotecting agent

Deprotection of the tert-butoxycarbonyl group during solid-phase synthesis of peptides can be conveniently and efficiently carried out using a neutral reagent, silicon tetrachloride/sodium iodide (iodotrichlorosilane). This simple and rapid method has been advantageously employed during the solid-phase synthesis of the pituitary hormone, oxytocin

Molecular Carpentry: Piecing Together Helices and Hairpins in Designed Peptides

The design of a peptide that contains two distinct elements of secondary structure, helix and beta-hairpin, is described. Two designed 17-residue peptides: Boc-Val-Ala-Leu-Aib-Val-Ala-Leu-Gly-Gly-Leu-Phe-Val-d-Pro-Gly-Leu-Phe-Val-OMe (I) and Boc-Leu-Aib-Val-Ala-Leu-Aib-Val-Gly-Gly-Leu-Val-Val-d-Pro-Gly-Leu-Val-Val-OMe (II) have been conformationally characterized by NMR spectroscopy. Peptides I and II contain a seven-residue helical module at the N terminus and a eight-residue beta-hairpin module at the C terminus, which are connected by a conformationally flexible Gly-Gly segment. The choice of the secondarystructure modules is based upon prior crystallographic and spectroscopic analysis of the individual modules. Analysis of 500 MHz 1H NMR data, recorded as solutions in methanol, suggests that the observed pattern of chemical shifts, 3JHN¡CaH values, temperature coefficients of the NH chemical shifts, and backbone inter-residue nuclear Overhauser effects favor helical structures for residues 1 - 7 and beta-hairpin structures for residues 10 - 17. The spectroscopic data are compatible with termination of the helical segment by formation of a Schellman motif; this restricts Gly(8) to a lefthanded alpha-helical conformation. Gly(9) is the only residue with multiple conformational possibilities in f,y space. Possible orientations of the two secondary-structure modules are considered. This study validates the use of stereochemically rigid peptide modules as prefabricated elements in the construction of synthetic protein mimics

Synthetic peptides related to δ-receptor agonist dermorphin gene associated peptide

Five peptides related to the δ-receptor agonist, dermorphin gene associated peptide (DGAP), having structural modifications at the positions 2, 6 and 7 were synthesised by the solid phase method using 9-fluorenylmethoxycarbonyl amino acid trichlorophenyl esters as coupling agents and p-alkoxybenzyl alcohol resin as the solid support. The biological activities of these peptides were determined in vitro using guinea pig ileum and mouse vas deferens assays and in vivo employing the analgesic and antidiarrhoeal assays. The δ-receptor selectivity was determined by the ratio of IC50 values in the MVD assay to that of the GPI assay and all the synthetic peptides were found to possess δ-receptor selectivity; Nva6DGAP being the most selective in the present series. Eth6DGAP exhibits a higher selectivity with regard to analgesic to antidiarrhoeal activity in the in vivo assay