Conformational studies of diastereomeric cyclic enkephalins by 1H-NMR and computer simulations

Conformational studies of cyclic enkephalin analogues with L- or D-proline in position 3

The conformation of a series of cyclic enkephalin analogues of a general formula X(1)-cyclo[Y(2)-Z(3)-Nal(4)-Leu(5)] (Nal: beta-(2-naphthyl)alanine), where X = Tyr, Phe, or Phe(NO(2)), Y = D-Dab or L-Dab (Dab: 2,4-diaminobutyric acid), and Z = D-Pro or L-Pro, was studied by means of NMR spectroscopy and theoretical conformational analysis with the Empirical Conformational Energy Program for Peptides and Proteins force field plus solvation. The NMR measurements were performed in dimethyl sulfoxide solution. The nuclear Overhauser effect intensities and coupling constants were used to compute the statistical weights of the conformations of the ensemble generated in global conformational searches. The purpose of this study was to determine whether introducing the D- or L-proline residue in position 3 can produce peptides with both rigid backbone and significant separation of the pharmacophore groups in position 1 and 4 (as required for high affinity for the mu-type opioid receptors). It was found that the analogues with D-Dab in position 2 and D-Pro in position 3 possess a stable type II' beta-turn at positions 3 and 4, which rigidifies the cyclic backbone; this finding was confirmed by independent measurements of the temperature coefficients of the amide protons, which indicated very significant screening of the Leu(5) amide proton from the solvent. However, these analogues were found to possess a short interchromophore distance. The analogues containing both Dab and Pro in the L-configuration are characterized by a larger interchromophore distance; however, they do not possess a stable beta-turn and have therefore a higher conformational flexibility. The modifications proposed in this work are therefore not likely to lead to enkephalin analogues with a high affinity for the mu-receptors.

Influence of solvent and configuration of residues at positions 2 and 3 on distance and mobility of pharmacophore groups at positions 1 and 4 in cyclic enkephalin analogues

The analgesic activity of opioid peptides is mainly connected with their affinity and selectivity for the mu-receptors. The biological activity of cyclic opioid analogues depends on mutual orientation and conformational freedom of aromatic pharmacophore groups at positions 1 and 4. The distance and distance distributions between chromophores at positions 1 [Phe(p-NO(2)), p-nitrophenylalanine] and 4 [Nal, beta-(2-naphthyl)alanine], which constitute an energy donor-acceptor pair, were calculated based on measured fluorescence intensity decays of a donor (Nal). The influence of the solvent and configuration of the residues at position 2 and 3 on donor-acceptor distance distribution and mobility of pharmacophore groups at position 1 and 4 in cyclic enkephalin analogues are discussed.

Comparative conformational analyses of mu-selective dermorphin and delta-selective deltorphin-II in aqueous solution by 1H-NMR spectroscopy

Two-dimensional 1H-NMR methods have been used to obtain complete proton resonance assignments and possible solution conformations of dermorphin (H-Tyr-D-Ala-Phe-Gly-Tyr-Pro-Ser-NH2) and deltorphin-II (H-Tyr-D-Ala-Phe-Glu-Val-Val-Gly-NH2), naturally occurring mu- and delta-selective opioids, respectively, in order to examine the conformational characteristics that are closely related to the selectivities towards mu/delta-opioid receptors. With the use of the proton-proton distances derived from ROESY measurements in aqueous solution, 50 possible 3D structures are generated by means of distance geometry calculations. The conformers which satisfy the distance constraints and the torsion angles estimated from JNHC alpha H vicinal coupling constants within the allowable range are then subjected to molecular dynamics simulations for 10 ps after equilibration. Although dermorphin and deltorphin-II are both in equilibrium among many flexible conformers, some conformational differences are observed between these peptides: many conformers of dermorphin show a structure rounded at the N-terminal Tyr-D-Ala-Phe-Gly-Tyr and C-terminal Gly-Tyr-Pro-Ser-NH2 moieties, which are almost at right angles to each other, while those of deltorphin-II are characterized by a 'hook'-shaped backbone structure in which the nearly extended conformation of the Val-Val-Gly-NH2 sequence is located under the folded conformation of the N-terminal Tyr-D-Ala-Phe-Glu sequence. The possible relationship between these conformational characteristics and the mu/delta-opioid receptor selectivities is discussed.

Conformational analysis of mu-selective [D-Ala2,MePhe4]enkephalins

The conformational space of the potent mu-selective opioids [D-Ala2,MePhe4,Gly-ol5]enkephalin (DAGO) and [D-Ala2,MePhe4,Met(O)-ol5]enkephalin (FK 33-824) has been analyzed by 1H-NMR spectroscopy and theoretical calculations involving systematic conformational searching and energy minimizations. A cis-trans equilibrium of the Gly3-MePhe4 amide bond is induced by the N-methyl group, and the more energetically favoured trans isomer is proposed as the biologically relevant form. A compact interaction between the side chains of Tyr1 and D-Ala2 was demonstrated by NOE and ROE effects in both peptides in D2O and DMSO-d6, further supported by shielding of the D-Ala2 methyl protons in both solvents. Analysis of coupling constants, NOE and ROE data indicated significant restriction of the conformational freedom of the MePhe4 side-chain for both peptides in the two solvents. The NMR results and theoretical calculations point towards folded low energy conformations characterized by a beta II-type turn around Gly3-MePhe4. For the trans isomer, a Tyr1-MePhe4 phenyl ring separation between 8.5 and 12.5 A was accompanied by proximity between the D-Ala2 side chain and the C-terminal in low energy conformations. The results are in good agreement with available data on related active enkephalins. The conformational effects induced by simultaneous incorporation of D-Ala2 and MePhe4 in enkephalins is discussed in the light of the enhanced mu-opioid receptor selectivity and activity of these peptides.

Conformation of deltorphin-II in membrane environment studied by two-dimensional NMR spectroscopy and molecular dynamics calculations

Two-dimensional homonuclear Hartmann-Hahn spectroscopy and NOESY (nuclear Overhauser effect and exchange spectroscopy) 1H-NMR techniques have been used to obtain complete proton resonance assignments and to perform a conformational investigation of deltorphin-II (Tyr-D-Ala-Phe-Glu-Val-Val-Gly-NH2), a naturally occurring delta-selective opioid peptide, in the membrane-mimetic micelles of perdeuterated dodecylphosphocholine. This was done in order to examine conformational characteristics that would be closely related to the selectivity towards the delta-opioid receptor. With the use of the proton-proton distances derived from NOESY measurements, 50 possible three-dimensional structures were generated by means of distance-geometry calculations, and 25 of them were subjected to the molecular-dynamics simulations of 10 ps, which were energetically constrained for the NOE interproton distances. Most of the possible conformers simulated showed a common feature such that the conformation of deltorphin-II is characterized by the S-shaped back-bone structure in which the turn conformation of the Val-Val-Gly-NH2 sequence is located under the helically folded conformation of the N-terminal Tyr-D-Ala-Phe-Glu sequence. The possible relationship between this conformational characteristic and the delta-opioid-receptor selectivity has been discussed.

14-membered cyclic opioids related to dermorphin and their partially retro-inverso modified analogues. I. Synthesis and biological activity

As a continuation of our program to study structure-activity relationships of opiate peptides, we report the syntheses and biological activities of a series of 14-membered cyclic dermorphin analogues closely related to enkephalin analogue Tyr-c[D-A2bu-Gly-Phe-Leu] incorporating a phenylalanine at the third position in place of glycine. In addition to two parent dermorphin analogues Tyr-c[D-A2bu-Phe-Phe-(L and D)-Leu], four stereoisomeric retro-inverso modified analogues Tyr-c[D-A2bu-Phe-gPhe-(S and R)-mLeu] with a reversed amide bond between residues four and five, and Tyr-c[D-Glu-Phe-gPhe-(L and D)-rLeu] with two reversed amide bonds between residues four and five, and between residue five and the side chain of residue two have been synthesized. The results from the guinea pig ileum (GPI) and mouse vas deferens (MVD) assays show that all analogues are superactive at either one or both opiate receptors and in general display higher activities as compared to the corresponding enkephalin analogues with a glycine at the third position. Results from the in vitro biological assays and conformational analysis using 1H-NMR spectroscopy (adjoining paper) will provide useful information to understand the role of the Phe3 aromatic side chain in dermorphin, and that of the Phe4 aromatic side chain in enkephalin, on opiate activity since these cyclic dermorphin analogues contain two Phe residues at both the third and fourth positions.

14-membered cyclic opioids related to dermorphin and their partially retro-inverso modified analogues. II. Preferred conformations in solution as studied by 1H-NMR spectroscopy

The 1H-NMR studies were extensively carried out to elucidate preferred conformations of a series of 14-membered cyclic dermorphin analogues containing two phenylalanines at both the third and fourth positions, e.g., Tyr-c[D-A2bu-Phe-Phe-(L and D)-Leu], Tyr-c[D-A2bu-Phe-gPhe-(S and R)-mLeu], and Tyr-c[D-Glu-Phe-gPhe-(L and D)-rLeu]. The temperature coefficients of the amide proton chemical shifts, vicinal 1H-1H coupling constants for the NH-CH groupings, and nuclear Overhauser effects provided information regarding the preferred conformations of the backbones. The conformational preferences and flexibility of the side chains were also estimated from the vicinal 1H-1H coupling constants around the C-C beta and C beta-C bonds in the articulated side chains. A comparison of the results obtained was made with the results previously obtained for the corresponding enkephalin analogues containing a glycine at the third position. It was found that the replacement of the glycine with the phenylalanine at the third position increases the conformational flexibility of the molecules with an L-, or S-, residue at the fifth position but reduces the flexibility of the molecules with D-, or R-, residue at the same position. The rotating frame nuclear Overhauser experiments gave direct evidence for compact conformations, with the Tyr side chain folding back over the 14-membered ring in Tyr-c[D-Glu-Phe-gPhe-rLeu], which displays relatively high selectivity for the delta-receptor over the mu-receptor. This observation is in agreement with our model proposed for the cyclic enkephalin analogues: folded forms with close aromatic ring placement are required for the activity at the delta-receptor.

Conformational studies of stereoisomeric 14-membered cyclic enkephalin analogues containing 1-naphthylalanine at the fourth position: chirality effect of leucine at the fifth position on biological activity and receptor selectivity

In order to study structure-activity relationships of enkephalin-related analogues, we report the biological activity and conformational analysis of four 14-membered cyclic enkephalin analogues with beta-(1-naphthyl) alanine in place of phenylalanine at the fourth position, Tyr-c[D-A2bu-Gly-(L and D)-beta Nal(1)-(L and D)-Leu]. The L-beta Nal(1)-containing analogues display higher activity at both the mu and delta receptors than the corresponding analogues with the L-Phe residue. In contrast to the linear enkephalins, the cyclic analogues with the D-beta Nal(1) residue are also active at the mu receptor since the relative spatial arrangement of functional groups required for biological activity is achieved by the constrained nature of the cyclic molecules. A comparison of the findings from the conformational analysis and biological assays establishes that relatively extended structures, in which the two aromatic side chains are oriented in opposite directions with a approximately 14 A separation, is required for activity at the mu receptor. On the other hand, folded conformations with nearly parallel orientations and a close proximity (less than 10A) of the aromatic rings of the Tyr and beta Nal(1) residues are required for activity at the delta receptor. It should be noted that the overall structures and thus the biological profiles of the 14-membered cyclic enkephalin analogues are strongly dependent on the conformation of the second residue. The folded conformations with parallel orientation of the two aromatic side chains of Tyr-c[D-A2bu-Gly-L-beta Nal(1)-D-Leu] is stabilized by an interaction between the Tyr phenolic OH proton and beta Nal(1) C*O groups. This analogue, which shows the highest activity at both the mu and delta receptors among the four stereoisomers studied, displays an increase of the fraction of the side-chain chi 1 = t conformer for the beta Nal(1) residue. It is concluded that the incorporation of the D-Leu residue at the fifth position increases the relative fraction of the folded conformations with parallel orientation of the aromatic side chains, and hence enhances activity at the delta receptor as compared to the corresponding L-Leu containing analogue.

Conformation-based design of two cyclic physalaemin analogues

Two new cyclic analogues of physalaemin were designed on the basis of the conformation found in DMSO solution. Glp-Ala-cyclo(-Asp-Pro-Asn-Lys-)-Phe-Tyr-Gly-Leu-Met-NH2 (1) was synthesized by cyclization of physalaemin. In 2 the Asp residue was replaced by Glu. The linear analogue of 2 was synthesized by the solid phase method and subsequently cyclized. Two-dimensional nmr methods were employed to assign the proton and carbon resonances. Proton-proton distances were extracted from rotating frame nuclear Overhauser effect spectra and used as restraints in the molecular dynamics calculations. Analogue 1 was found to have a similar conformation as physalaemin, whereas 2 did not form intramolecular hydrogen bonds. The pharmacological evaluation revealed that both peptides have similar potencies as physalaemin in the dog carotid artery (NK-1 receptor). Therefore, the charged side chains of physalaemin appear not essential for NK-1 activation. However, the other tachykinin receptors show good sensitivity to the cyclic peptides. It is concluded that the replacement of a salt bridge by an amide bond connecting the side chains of natural residues might provide useful information about the biological significance of some charged side chains of neurokinins.

A conformational comparison of two stereoisomeric cyclic dermorphin analogues employing NMR and computer simulations

In a continuation of our program to study the structure-activity relationship of peptide opiates, we report the conformational analysis of two cyclic tetrapeptides related to dermorphin--Tyr-c[D-Orn-Phe-Asp]-NH2 and Tyr-c[D-Asp-Phe-Orn]-NH2. These analogues have similar binding properties marked by a high selectivity for the mu-opioid receptors because of a drastic decrease in the affinity for the delta-opioid receptor. The conformational preferences of these analogues of dermorphin determined from proton nmr, molecular dynamics, and energy minimizations are quite similar. The constraint of the 13-membered ring formed from cyclization is quite evident from the conformational analysis. The constrained ring system acts as a template maintaining the relative orientation of the exocyclic tyrosine and side chain of phenylalanine. Two intramolecular hydrogen bonds measured for the D-Orn analogue in DMSO were disrupted upon the addition of water. For the D-Asp analogue, two intramolecular hydrogen bonds were found stable in DMSO and water. The global conformations of the two peptides determined from nuclear Overhauser effects did not change with the solvent titration. The difference in the hydrogen bonding within the 13-membered ring may account for the slight differences observed in the efficacy of the analogues at the mu-opioid receptors.

Enkephalin analogues containing beta-naphthylalanine at the fourth position

To examine the importance of the aromatic side chains of enkephalin on opiate activity, we report the synthesis and conformational analysis of a series of analogues related to enkephalin with beta-naphthylalanine in place of phenylalanine at the fourth position. Three linear analogues (Tyr-D-Ala-Gly-(L and D)-beta Nal(1)-Leu-NH2 and Tyr-D-Ala-Gly-beta Nal(2)-Leu-NH2) were initially synthesized to examine the effect of the substitution on biological activity. The increased activity of these peptides at the mu-opiate receptor, compared to native Leu-enkephalin, prompted us to examine the more conformational constrained analogues, Tyr-c[D-A2bu-Gly-(L and D)-beta Nal(1)-Leu], incorporating a alpha, gamma-diaminobutyric acid at the second position and cyclization to the carboxylic end of the leucine. These two cyclic analogues provide insight into the necessity for the L chirality of the aromatic residue at position 4. The Tyr-c[D-A2bu-Gly-L-beta Nal(1)-Leu] analogue is highly potent and displays a slight preference for the mu receptor. The conformational analysis indicates that despite the high flexibility of the tyrosine side chain, the aromatic rings of the tyrosine and naphthylalanine are relatively distant from each other. The presence of two intramolecular hydrogen bonds help maintain the conformation of the 14-membered backbone ring that keeps the side chains directed away from each other. These findings are in agreement with our model of an extended structure required for mu selectivity and a folded form with close aromatic ring placement for delta selectivity.

Role of the conformational element in peptide-receptor interactions. Studies with cyclic opioid peptide analogs

Biological activity profiles of three different families of cyclic opioid peptide analogs are presented. It is illustrated that conformational constraints introduced through peptide cyclizations can have drastic effects on receptor affinity, selectivity and 'efficacy' ('intrinsic activity'). Conformational studies of cyclic opioid peptides by various physico-chemical techniques have been initiated and have already produced insight into the conformational requirements of the various opioid receptor types. On the basis of the results obtained, conformational restriction of opioid peptides may represent a first promising step towards the goal of developing peptide mimetics.