Reduced peptide bond cyclic somatostatin based opioid octapeptides Synthesis, conformational properties and pharmacological characterization

Conformation-activity relationships of opioid peptides with selective activities at opioid receptors

The discovery of endogenous opioid peptides 25 years ago opened up a new chapter in efforts to understand the origins and control of pain, its relationships to other biological functions, including inflammatory and other immune responses, and the relationships of opioid peptides and their receptors to a variety of undesirable or toxic side effects often associated with the nonpeptide opiates such as morphine including addiction, constipation, a variety of neural toxicities, tolerance, and respiratory depression. For these investigations the need for potent and highly receptor selective agonists and antagonists has been crucial since they in principle allow one to distinguish unequivocally the roles of the different opioid receptors (mu, delta, and kappa) in the various biological and pathological roles of the opioid peptides and their receptors. Conformational and topographical constraint of the linear natural endogenous opioid peptides has played a major role in developing peptide ligands with high selectivity for mu, delta, and kappa receptors, and in understanding the conformational, topographical, and stereoelectronic structural requirements of the opioid peptides for their interactions with opioid receptors. In turn, this had led to insights into the three-dimensional pharmacophore for opioid receptors. In this article we review and discuss some of the developments that have led to potent, selective, and stable peptide and peptidomimetic ligands that are highly potent and selective, and that have delta agonist, mu antagonist, and kappa agonist biological activities (other authors in this issue will discuss the development of other types of activities and selectivities). These have led to ligands that provide unique insight into opioid pharmacophores and the critical roles opioid ligands and receptor scan play in pain, addiction, and other human maladies.

Observations on the origin of the non-linear van't Hoff behaviour of polypeptides in hydrophobic environments

In this paper we describe a general procedure to determine the thermodynamic parameters associated with the interaction of polypeptides or proteins with immobilised lipophilic compounds such as non-polar n-octyl groups. To this end, the binding behaviour of an all L-alpha-polypeptide, 1, and its retro-inverso-isomer, 2, has been investigated with an n-octylsilica and water-organic solvent mixture containing different percentages of acetonitrile or methanol over the temperature range of 278-338 K. The results confirm that non-linear van'ts Hoff plots occur with this pair of polypeptide isomers, depending on the solvent composition. These findings are consistent with the changes in the thermodynamic parameters, enthalpy of association, delta Hoassoc,i, entropy of association, delta Soassoc,i, and heat capacity, delta Cop,i, all having significant temperature dependencies. Theoretical relationship linking the changes in the delta Hoassoc,i, delta Soassoc,i and delta Cop,i values of these polypeptide-non-polar ligate systems, as a function of temperature, T, have been validated. Significant differences were observed in the magnitudes of these thermodynamic quantities when acetonitrile or methanol was employed as the organic solvent. The origin of these solvent-dependent effects can be attributed to the hydrogen-bonding propensity of the respective solvent. Involvement of enthalpy-entropy compensation effects associated with the interaction of these polypeptides with the hydrophobic ligates has also been documented. Analysis of empirical extra-thermodynamic relationships associated with molecular structural properties of these polypeptides, such as the slope term, S, derived from the plots of the logarithmic capacity factor, log k'i, of these polypeptides vs. the volume fraction of the organic solvent, [symbol: see text] as a function of temperature, T, has also revealed similar correlations in terms of the interactive behaviour of polypeptides 1 and 2 under these experimental conditions. These findings provide an extended thermodynamic and extra-thermodynamic framework to examine the solvational, conformational and other equilibrium processes that polypeptides (or proteins) can undergo in the presence of n-alkylsilicas or other classes of immobilised hydrophobic surfaces. The experimental approach utilised in this study with these topologically similar polypeptides thus represents a generic procedure to explore the behaviour of polypeptides or proteins in non-polar environments in terms of their molecular properties and the associated linear free energy relationships that determine their interactive behaviour.

Comparison of the solution conformations of a human immunodeficiency virus peptidomimetic and its retro-inverso isomer using 1H NMR spectroscopy

The solution conformations of the all L-alpha-peptide 1 and the corresponding retro-all D-alpha-peptide 2, two 20-metric peptides which generate antibodies that cross-react with the gp 120 envelop protein of human immunodeficiency virus-1 (HIV-1), have been investigated by high-field 1H NMR spectroscopy. Complete sequential and inter-residue interaction assignments were made from the 2D NMR spectra acquired at 500 MHz and 600 MHz in 40% deuterotrifluoroethanol (d3-TFE)/H2O at pH 2.3, and in 300 mM sodium dodecyl sulphate (SDS) in 100% D2O or 90% H2O/10% D2O at pH 2.6. Based on analysis of the nuclear Overhauser effect (NOE) and amide exchange data, peptide 1 and its retro-inverso isomer 2 in the polar solvent environment of 40% d3-TFE/H2O at pH 2.3 show very similar topological features. However, in the relatively non-polar 300 mM SDS micellar environment, peptides 1 and 2 exhibit differences in their solution structures in terms of the amide backbone and side-chain orientations. In particular, under the SDS micellar condition, peptide 1 maintains much of the secondary structure observed for this 20-mer peptide in 40% d3-TFE/H2O, pH 2.3, whereas peptide 2 adopts a more extended structure. These NMR results provide the first confirmation that the secondary structure of the all L-a-peptide 1 is maintained in both polar and non-polar environments, whereas the secondary structure and topology of the notionally equivalent retro-inverso isomer depends more on the solvent conditions. These results with the all L-a-peptide 1 and its retro-inverso isomer 2 provide important insight into the conformational influences of the C- and N-end group with L-alpha- and retro-D-alpha-isomer pairs in non-polar environments, and thus have general relevance to the design of bioactive retro-inverso peptidomimetic analogues related to immunogenic or hormonal peptides.

Multiconformational NMR analysis of sandostatin (octreotide): equilibrium between beta-sheet and partially helical structures

This paper reports a detailed conformational analysis by 1H NMR (DMSO-d6, 300 K) and molecular modeling of the octapeptide D-Phe1-Cys2-Phe3-D-Trp4-Lys5-Thr6-Cys7+ ++-Thr8-ol (disulfide bridged) known as sandostatin (or SMS 201-995 or octreotide) with both somatostatin-like and opioid-like bioactivities. This is the initial report on sandostatin showing that attempts to explain all NMR data using a single average conformation reveal several important inconsistencies including severe violations of mutually exclusive backbone-to-backbone NOEs. The inconsistencies are solved by assuming an equilibrium between antiparallel beta-sheet structures and conformations in which the C-terminal residues form a 3(10) helix-like fold (helical ensemble). This conformational equilibrium is consistent with previous X-ray diffraction investigations which show that sandostatin can adopt both the beta-sheet and the 3(10) helix-like secondary structure folds. In addition, indications of a conformational equilibrium between beta-sheet and helical structures are also found in solvent systems different from DMSO-d6 and for other highly bioactive analogs of sandostatin. In these cases a proper multiconformational NMR refinement is important in order to avoid conformational averaging artifacts. Finally, using the known models for somatostatin-like and opioid-like bioactivities of sandostatin analogs, the present investigation shows the potentials of the proposed structures for the design of novel sandostatin-based conformationally restricted peptidomimetics. These analogs are expected to refine the pharmacophore models for sandostatin bioactivities.

Folded structures in protonated reduced dipeptides

Reduced dipeptides with the general formula RCO-Xaa-rXbb-N+HR'R" (rXbb, reduced analogue of residue Xbb: NH-C alpha HR1-CrH2) are shown to adopt a folded conformation in solution and in the solid state. The protonated reduced amide bond is an active proton donor capable of interacting with a peptide carbonyl to give a strong hydrogen bond topologically equivalent to the i+2 or i+3-->i interaction. The resulting conformation is similar to the y- or beta-turn structure found in peptides and proteins.

Conformational restriction of Tyr and Phe side chains in opioid peptides: information about preferred and bioactive side-chain topology

The side chain of Tyr and Phe was fixed into the gauche(-) or gauche(+) conformation by using the Tic Htc structures, and into the trans conformation by using an aminobenzazepine-type (Aba) structure. When incorporated into dermorphin or deltorphin II, the Tic and Htc analogues all showed a large decrease in both mu and delta affinities and activities. Fixation of Phe(3) in the trans rotamer resulted in a large increase in delta affinity in the dermorphin analogue, whereas in the [Aba(3)-Gly(4)] deltorphin II analogue, good delta affinity is maintained despite the removal of the Glu side chain. Whereas several authors propose a gauche(-) preferred conformation for the Phe(3) side chain, these results suggest a trans conformation at the delta receptor. The use of these conformationally constrained residues for evaluating the preferred solution conformation in the flexible N-terminal tripeptide Tyr-D-Ala-Phe is illustrated. The (1)H-nmr parameters--chemical shift, temperature dependence, and nuclear Overhauser effects to the D-Ala(2) methyl protons in the different analogues--provide direct evidence to confirm the proposed sandwich conformation in the native peptides.

Influence of sample pH on the conformational backbone dynamics of a pseudotripeptide (H-Tyr-Tic psi [CH2-NH]Phe-OH) incorporating a reduced peptide bond: an NMR investigation

In the present paper we investigate the influence of sample pH on the conformational and dynamical properties of the pseudotripeptide H-Tyr-Tic psi [CH2-NH]Phe-OH (TIP[psi]; Tic: 1,2,3,4,-tetrahydroisoquinoline-3-carboxylic acid) using various one- and two-dimensional nmr techniques in conjunction with molecular modeling. Studies were conducted at three different pH levels corresponding to the zwitterionic peptide containing a formal positive charge (pH 3.1), the deprotonated molecule (pH 9.1), and a situation at neutral pH (pH 7.2) involving both protonated and deprotonated states of the reduced peptide bond. Analysis of the one-dimensional1H-nmr spectra reveals that in solution TIP[psi] is in slow dynamic exchange between conformations containing cis and trans configurations of the Tyr-Tic bond. An nmr pH dependence study of the cis:trans ratio indicated that the exchange process was governed by the protonation state of the reduced bone amine. From the nmr data, reduced peptide bond pK alpha values of 6.5 and 7.5 were determined for the cis and trans conformers, respectively. It was concluded that conformations containing a trans Tyr-Tic bond are stabilized at low pH by an intramolecular hydrogen bond between the Tyr carbonyl and the reduced peptide bond protonated amine. This observation was corroborated by molecular mechanics investigations that revealed low energy trans structures compatible with nmr structural data, and furthermore, were consistently characterized by the existence of a strong N+ H ... O = C interaction closing a seven-membered cycle. The dynamics of cis-trans isomerization about the Tyr-Tic peptide bond were probed by nmr exchange experiments. The selective presaturation of exchanging resonances carried out at several temperatures between 50 and 70 degrees C allowed the determination of isomerization rate constants as well as thermodynamic activation parameters. delta G not equal to values were in close agreement with the cis-->trans energy barrier found in X-Pro peptide fragments (approximately 83 kJ/mol). A large entropic barrier determined for the trans-->cis conversion of TIP[psi] (5.7 JK-1 mol-1 at pH 3.1;6.5 J K-1 mol-1 at pH 9.1) is discussed in terms of decreased solvent molecular ordering around the conformers possessing a trans Tyr-Tic bond. Evidence that the neutral form of the reduced peptide bond gains rigidity upon protonation was obtained from relaxation measurements in the rotating frame. T1 rho measurements of several protons in the vicinity of the reduced peptide bond were made as a function of spin-lock field.(ABSTRACT TRUNCATED AT 400 WORDS)

Cross-reactivity of antibodies to retro-inverso peptidomimetics with the parent protein histone H3 and chromatin core particle. Specificity and kinetic rate-constant measurements

A series of monoclonal antibodies has been generated against an hexapeptide of sequence IRGERA corresponding to the C-terminal residues 130-135 of histone H3 and three analogues of this model peptide. The analogues correspond to the D-enatiomer, containing only D-residues, and two retro-peptides containing NH-CO bonds instead of natural amide peptide bonds. The chirality of each residue was maintained in the retro-peptide and inverted in the retro-inverso-peptide. Monoclonal antibodies were generated from mice immunized with the analogues coupled to neutral small unilamellar liposomes containing monophosphoryl lipid A as adjuvant. The reactivity of antibodies with the four analogues and with the parent protein H3 was studied in enzyme-linked immunosorbent assay and in a biosensor system. The equilibrium affinity constant (Ka) toward the retro-inverso-peptide of two out of three antibodies of IgG1 isotype induced against the L-hexapeptide was 7-75-fold higher than toward the homologous L-peptide. The range of Ka values of four antibodies of IgG1 and IgG2a isotypes generated against the retro-inverso-peptide was 0.6-1.9 x 10(9) M-1 for both the retro-inverso- and L-peptides. Furthermore, antibodies to the L- and retro-inverso-peptides cross-reacted strongly (in some cases better than with the homologous peptide) with the parent histone H3 and with chromatin subunits containing H3. The results are thus promising in respect to the potential use of retro-inverso-analogues, which are particularly stable, in the design of much more potent synthetic vaccines or to generate antibody probes.

Implications of replacing peptide bonds in the COOH-terminal B chain domain of insulin by the psi (CH2-NH) linker

To evaluate more thoroughly the importance of main-chain structure and flexibility in ligand interactions with the insulin receptor, we undertook to synthesize analogues with reduced peptide bonds in the COOH-terminal B chain domain of the hormone (a stable, but adjustable beta-strand region). By use of solid-phase, solution-phase and semisynthetic methods, analogues were prepared in which ArgB22 of des-octapeptide(B23-B30)-insulin was extended by the sequences Gly-Phe-psi (CH2-NH)-Phe-NH2, Gly-Gly-psi(CH2-NH)-Phe-Phe-NH2, Gly-Phe-psi (CH2-NH)-Phe-Phe-Thr-Pro-Ala-Thr-OH, and Gly-Phe-Phe-psi (CH2-NH)-Phe-Thr-Pro-Ala-Thr-OH, and were studied with respect to their abilities both to interact with the hepatocyte insulin receptor and to form soluble anion-stabilized hexamers in the presence of Co2+ and phenol. Additional analogues of des-pentapeptide(B26-B30)-insulin were also examined. Overall, our results show that, whereas all analogues retain considerable ability to form organized metal ion-coordinated complexes in solution, the reduction of peptide bonds both proximal and distal to the critical side chain of PheB25 results in analogues with severely diminished receptor binding potency. We conclude that the peptide carbonyls from both PheB24 and PheB25 are important for insulin-receptor interactions and that the structural organization of the region when insulin is bound to its receptor differs from that occurring during simple monomer-monomer and higher-order interactions of the hormone.