Folded conformations of the δ-selective opioid dermenkephalin with head-to-tail interactions. A simulated annealing study through NMR restraints

Despite similar tripeptide N-termini, dermorphin (Tyr-D-Ala-Phe-Gly-Tyr-Pro-Ser-NH₂) and dermenkephalin (Tyr-D-Met-Phe-His-Leu-Met-Asp-NH₂), naturally occuring opioid peptides from frog skin, exhibit high affinity but contrasting selectivity for the μ- and δ-opioid receptors, respectively. Structure-activity relationship studies have shown that the N-terminal tripeptide, Tyr-D-Xaa-Phe (where Xaa is either Ala or Met), is necessary for binding with both the μ- and δ-receptors while the nature and/or the conformation of the C-terminus His-Leu-Met-Asp-NH₂ of dermenkephalin are responsible for addressing the peptide to the δ-receptor. In order to examine the conformational characteristics that are related to the selectivity of dermenkephalin towards the δ-receptor, 50 NOE restraints (10 between nonadjacent residues), and 7 dihedral angles, derived from a two-dimensional ¹H-NMR study of dermenkephalin in dimethyl sulfoxide, were used in simulated annealing and energy minimization procedures. Twenty-four resulting conformers (60% of the generated structures) with no severe distance restraint violation were pooled into seven groups and three related families. These 24 conformers show close proximity between the two methionine residues, S-shaped structures, mean planes of N-terminal and C-terminal moieties almost at right angles to each other, a C-terminus region above the plane of the N-terminal region and g^? as preferential orientation in the side chain of the. Aside these similarities, families of conformers differ by the preferential orientation in the side chain of Tyr (t or g^?) and proximity between Tyr and Asp, or Tyr and the C-terminus. In contrast to previous models, practically no β-turn structures exist for dermenkephalin, most of the NH hydrogen bonds participating to γ-turns. The possible relationship between the conformational characteristics of dermenkephalin and the δ-opioid receptor selectivity is discussed. © Munksgaard 1996.     

Structure-activity relationships of dermorphin analogues containing N-substituted amino acids in the 2-position of the peptide sequence

A series of dermorphin analogues containing an N-alkylated amino-acid residue Xaa in the 2-position of the peptide sequence was synthesized (Xaa =N-methylalanine, proline, pipecolic acid, N-methylphenylalanine, 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid [Tic]). These peptides have the potential of assuming a cis Tyr^l-Xaa² peptide bond. Their in vitro opioid activity profiles were determined in μ and δ-receptor-representative binding assays and bioassays. Aside from [D-Pro²]dermorphin, all analogues showed high affinity for μ and/or δ-opioid receptors. Whereas most compounds were found to be full μ-agonists in the guinea pig ileum (GPI) assay, [Tic²]dermorphin (compound 7) was a partial μ-agonist. Replacement of Gly⁴in 7 with Phe resulted in an analogue (8) with weak μ-antagonist activity. Furthermore, analogues 7 and 8 both were potent § antagonists (K_c= 3–40 nM) against the §-agonists Leuenkephalin, DPDPE and deltorphin I in the mouse vas deferens (MVD) assay. Compound 3, containing l -Pro in the 2-position, turned out to be one of the most μ receptor-selective linear dermorphin analogues reported to date. Low-temperature HPLC experiments using micropellicular octadecyl silica as stationary phase revealed conformational heterogeneity of the dermorphin analogues which was ascribed to cis-trans isomerization around the Tyr^l-Xaa²-and Tyr⁵-Pro⁶ peptide bonds. In the case of analogue 7 four separate peaks corresponding to the four possible isomers were apparent at -5°C. Since opioid peptide analogues with a non-N-akylated l -amino acid residue in the 2-position are nearly inactive and cannot assume a cis peptide bond at the 1–2 position, these results support the hypothesis that the bioactive conformation of opioid peptides containing an N-alkylated l -amino acid residue in position 2 is characterized by a cis Tyr^l-Xaa² peptide bond.     

Opioid peptides Synthesis and binding properties of dermorphin related heptapeptides

C-Terminal amino acid residues of dermorphin (H-Tyr-D-Ala-Phe-Gly-Tyr-Pro-Ser-NH₂) were replaced by N^α-methyl- or D-amino acids in order to examine the effect on opioid activity. In binding studies based on displacement of μ, Δ, and κ opioid receptor selective radiolabels from guinea pig brain membranes, the 13 new analogues showed, like dermorphin, a negligible affinity for the κ binding site. The introduction of N^α-methyl- or D-amino acid residues at position 5, 6, or 7 of dermorphin, when matched with C-terminal amide function modifications, generally produced analogues with reversed μ/δ specificity.     

Novel μ-selective Met-enkephalinamide analogs with antagonist activity.

Four novel μ-selective peptide antagonists have been synthesized and examined for receptor binding, analgesic agonist and antagonist activity and energy conformational properties. These peptides were designed by analogy to results of molecular modeling of 3-phenyl piperidines which led to incorporating four modified tyrosine residues, m-Tyr, β-methyl-m-Tyr, N-phenethyl-m-Tyr and α, β-dimethyl-m-Tyr into D-Ala²-Met⁵-enkephalinamide. Peptides were synthesized by stepwise solution synthesis using an active ester coupling procedure. Receptor binding assays were performed on rat brain homogenates and data were analyzed by a modified version of the program LIGAND. Analgesic agonist and antagonist activity was evaluated by the mouse tail-flick test. Energy-optimized conformations were obtained using a program called Molecule-AIMS. The results demonstrate that relative ratios of in vivo agonist and antagonist potencies in D-Ala²-Met⁵-enkephalinamides can be modulated by chemical modification of the tyrosine residue. A shift in the phenolic-OH position from para to meta significantly enhances relative antagonist versus agonist activity; addition of a β-CH₃ group to the m-Tyr enhances μ-selectivity and leads to nearly equal agonist/antagonist activity. Energy conformational studies indicate that all analogs with high μ-receptor affinity examined have a common energy accessible B'_II 2–3 turn conformation similar to that previously identified for high μ-affinity binding in peptides, lending further support to this candidate conformer. This conformer also has tyrosine side-chain angles which allowed total overlap with the amine and phenolic groups of a known structure of 3-(m-OH phenyl)-piperidine. This structural similarity together with the observation of mixed agonist antagonist activity in both types of opioids confirms the rationale upon which design of these peptides was based.     

A theoretical conformational study of substituted o-anisamides as models of a class of dopamine antagonists

The quantum mechanical PCILO method has been used to investigate the conformational behaviour of N-(2-aminoethyl)- and N-(2-dimethylaminoethyl)-o-anisamide, two model molecules of substituted benzamides. The molecules are shown to have only limited conformational freedom due to the presence of two intramolecular hydrogen bonds acting as conformational locks. The molecules in their preferred conformation are characterized by a distance between the centre of the aromatic ring and the nitrogen atom of almost 6 Å, i.e. almost 1 Å longer than in the fully extended dopamine conformers. Some implications at the receptor level of this topographical dissimilarity are discussed.     

Conformational comparison of µ‐selective endomorphin‐2 with its C‐terminal free acid in DMSO solution,by 1H NMR spectroscopy and molecular modeling calculation

Abstract: In order to make clear the structural role of the C‐terminal amide group of endomorphin‐2 (EM2, H‐Tyr‐Pro‐Phe‐Phe‐NH₂), an endogenous µ‐receptor ligand, in the biological function, the solution conformations of endomorphin‐2 and its C‐terminal free acid (EM2OH, H‐Tyr‐Pro‐Phe‐Phe‐OH), studied using two‐dimensional ¹H NMR measurements and molecular modeling calculations, were compared. Both peptides were in equilibrium between the cis and trans isomers around the Tyr‐Pro ω bond in a population ratio of ≈ 1 : 2. The lack of significant temperature and concentration dependence of NH protons suggested that the NMR spectra reflected the conformational features of the respective molecules themselves. Fifty possible 3D structures for the each isomer were generated by the dynamical simulated annealing method under the proton?proton distance constraints derived from the ROE cross‐peaks. These energy‐minimized conformers, which were all in the φ torsion angles estimated from J_NHCαH coupling constants within ± 30°, were then classified in groups one or two according to the folding backbone structures. All trans and cis EM2 conformers adopt an open conformation in which their extended backbone structures are twisted at the Pro²–Phe³ moiety. In contrast, the trans and cis conformers of EM2OH show conformational variation between the ‘bow’‐shaped extended and folded backbone structures, although the cis conformers of its zwitterionic form are refined into the folded structure of the close disposition of C‐ and N‐terminal groups. These results indicate clearly that the substitution of carboxyl group for C‐terminal amide group makes the peptide flexible. The conformational requirement for µ‐receptor activation has been discussed based on the active form proposed for endomorphin‐1 and by comparing conformational features of EM2 and EM2OH.     

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

The conformational space of the potent μ-selective opioids [D-Ala²,MePhe⁴,Gly-o1⁵]enkephalin (DAGO) and [D-Ala²,MePhe⁴,Gly-o1⁵]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 β₁₁-type turn around Gly³-MePhe⁴. For the trans isomer, a Tyr¹-MePhe⁴ phenyl ring separation between 8.5 and 12.5 Å 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 μ-opioid receptor selectivity and activity of these peptides.     

Dermorphin tetra- and heptapeptide analogues containing a [3,4] amide bond replacement by a carbon-carbon double and single bond

Seven dermorphin hepta- and tetrapeptide analogues containing [3,4] amide bond replacement by a carbon-carbon double and single bond were prepared. ¹H NMR studies of the pseudoheptapeptide in DMSO indicate the presence of extended conformations with stacking of the side chains in the N-terminal part and an inverse γ-turn around Ser⁷ in the conformational equilibrium. The binding data show that the affinity of the analogues for the μ-receptor is only slightly diminished in the d -Ala² series and is more affected in the d -Arg² series. Since the Gly⁴NH is not present in these compounds we conclude that this NH is not required to stabilize the bioactive conformation nor is it directly involved in binding to the receptor.     

Crystal-state structural analysis of two γ-lactam-restricted analogs of Pro-Leu-Gly-NH2

The crystal structures of two analogs of Pro-Leu-Gly-NH, (1), containing a γ-lactam conformational constraint in place of the -Leu-Gly- sequences, are described. The highly biologically active (S,R)-diastere-omer 2a is semi-extended at the C-terminus, with the N-terminal Pro residue in the unusual “C₅” conformation [ψ₁=– 0.8(15)°] stabilized by a (peptide)N-H…N(amino) intramolecular H-bond [the N(3)…N(4) separation is 2.687(11)Å]. Conversely, the N,N′-isopropylidene aminal trihydrate of the (S,S)-diastereomer 2b, compound 3, adopts a β-bend conformation at the C-terminus, as already reported for 1. However, the backbone torsion angles [φ= 57.4(4), ψ₂=– 129.9(3)°; ψ₃= - 92.3(4), ψ₃= 6.4(5)°] lie close to the values expected for the corner residues of an ideal type-II β-bend. A weak intramolecular 4 → 1 H-bond is seen between the Gly carboxyamide anti-NH and Pro C=O groups. In the newly formed 2,2,3,4-tetraalkyl-5-oxo-imidazolidin-1-yl moiety the ψ₁ torsion angle is 12.9(4)° and the intramolecular N(3)…N(4) separation is 2.321(4)Å.     

Peptide inhibitors of E. collagenolyticum bacterial collagenase -effect of N-methylation

Peptide inhibitors of E. collagenolyticum bacterial collagenase, HS-CH₂-CH₂-CO-Pro-Yaa (Yaa = Ala, Leu, Nle), have been N-methylated at the Yaa position. The N-methylation slightly increases the inhibitory potency of the modified peptides as compared to the parent compounds. The conformational effects of the N-methylation have been investigated by both ¹H 2D-NMR and molecular mechanics energy minimization. Three low-energy conformers have been predicted for the unmethylated parent compounds (Yaa = Ala, Leu, Nle). They are characterized by the Ψ value of the central proline residue: Ψ_Pro= 150^c(trans' conformation), Ψ_pro= 70° (C₇ conformation) and Ψ_Pro= -50°(cis' conformation). The N-methylation has been found to strongly increase the energy of the C₇ conformer and to a less extent the energy of the cis' conformer. This leaves the trans' conformation as the only low-energy conformer. The ROESY experiments have established that both the N-methyl peptides and the parent compounds adopt the same preferred backbone conformation in water solution, i.e. the trans conformation. Based on these results, the activities of the N-methyl peptides are discussed and a possible conformation of the inhibitor in the bound state is proposed.     

Crystal structure and conformation of glycyl-glycyl-sarcosine

Crystals of the tripeptide, glycyl-glycyl-sarcosine (C₇H₁₃N₃O₄) from aqueous methanol are orthorhombic, space group Pbcn with cell parameters at 294 K of a = 8.279(1), b = 9.229(4), c = 24.447(5) Å, V = 1868.0 ų, M.W. = 203.2, and Z = 8. The crystal structure was solved and refined using CAD-4 data (1171 reflections ≥ 3σ) to a final R-value of 0.053. The first peptide linkage is trans and planar whereas the second peptide link between Gly and sarcosine is cis and appreciably non-planar (w = 7.4°). The peptide backbone has an extended conformation at the N-terminal part but adopts a polyglycine-II type of conformation at the C-terminal part. The backbone torsion angles are: Ψ₁, =? 173.9, w₁=? 177.8, (φ, Ψ₂) = (-178.8, -170.8), w₂= 7.4, (φ₃, Ψ₃) = (-81.6, 165.6°).     

Evidence for a C-terminal turn in PBAN An NMR and distance geometry study

The solution conformation of the pheromone biosynthesis activating neuropeptide (PBAN) of the moth Helicoverpa zea has been determined using homonuclear two-dimensional nuclear magnetic resonance techniques and distance geometry-restrained energy minimization. The insect peptide hormone showed a random distribution of conformers in aqueous solution, whereas in a less polar medium of trifluoroethanol and water, a reordering process was observed. In particular, the C-terminal region (Phe-Ser-Pro-Arg-Leu-NH₂) adopts a type I′β-turn conformation, residues 20-27 are in a helix conformation, and residues 1-19 exhibit a high degree of flexibility. Direct observation of the C-terminal β-turn configuration of PBAN is consistent with a previous report that showed a rigid, cyclic analog of the C-terminal pentapeptide of PBAN retained pheromonotropic activity [Nachman, R.J., Kuniyoshi, H., Roberts, V.A., Holman, G.M. & Suzuki, A. (1993). Biochem. Biophys. Res. Commun. 661-666], Furthermore, our results show no interaction between the C-terminal turn and the rest of the polypeptide chain, thus providing further evidence that the C-terminal turn is indeed the important conformation recognized by the PBAN receptor. © Munksgaard 1996.     

Preferred conformation of the benzyloxycarbonyl-amino group in peptides*

Structural parameters, derived from X-ray crystallographic data, have been compiled for 35 derivatives of amino acids, peptides, and related compounds, which contain the N-terminal benzyloxycarbonyl (Z) group. The geometry of the urethane moiety of this end group is closely similar to that of the tert-butoxycarbonyl (Boc) group, except for a relaxation of some bond angles because the Z group is sterically less crowded than the Boc group. For the same reason, the Z group has greater conformational flexibility. As a result, packing forces in the crystal may cause greater deformations of bond angles, resulting in larger variations of observed bond lengths and bond angles than in Boc-peptide crystals. The aromatic rings of the Z end groups tend to stack in crystals. Conformational energy calculations indicate that most conformations of Z-amino acid-N' -methylamides and of corresponding Boc derivatives have similar dihedral angles and relative energies, i.e. the nature of the N-terminal end group has little effect on the conformational preferences of the residue next to it. In particular, the computed fraction of molecules with a cis urethane (C-N) bond is similar for the two derivatives: 0.51 and 0.42 in Boc-Pro-NHCH₃ and Z-Pro-NHCH₃, respectively, and 0.02 in the two Ala derivatives. There exist several computed conformations of Z-Ala-NHCH₃ and Z-Pro-NHCH₃ in which the phenyl ring and the C-terminal methylamide group are close to each other. Because of favorable nonbonded interactions, such conformations are of low energy.     

Structural requirements for dermorphin opioid receptor binding

Structural features influencing binding activity of dermorphin to opioid receptors have been investigated in the rat brain through the synthesis and evaluation of binding affinity of a series of synthetic dermorphin analogs. Tritiated dermorphin was used as primary ligand. The single population of high affinity dermorphin binding sites present in the rat brain is clearly of an opioid nature since bound radiolabeled dermorphin was fully displaced with high affinity either by morphine or naloxone. Displacement of tritiated dermorphin by all alkaloid opiates or dermorphin related peptides tested was monophasic, consistent with simple competitive inhibition at a single population of binding sites. Dermorphin (Tyr-d -Ala-Phe-Gly-Tyr-Pro-Ser-NH₂) was the most potent competitor in all experiments. The d -configuration of the amino acid residue in position 2 was found to be of crucial importance for binding. Replacement of d -Ala² with l -Ala led to a deleterious effect, this analog being 1/5000th as potent as dermorphin in displacing bound tritiated dermorphin from its receptor. Shorter dermorphin homologs, dermorphin-(1-4)-NH₂ and dermorphin-(1-3)-NH₂, were found to be 20 and 40-fold less potent, respectively, than dermorphin. The C-terminal carboxamide function is of significant importance for manifestation of the full intrinsic binding potency of dermorphin. Deamidated dermorphin had 1/5th the potency of the parent peptide. This suggests that while the whole dermorphin sequence is required for the expression of the full intrinsic binding activity of the molecule, the N-terminal tripeptide is a key structure as it contains the features which allow receptor recognition.     

Synthesis and receptor binding analysis of dermorphin hepta-, hexa- and pentapeptide analogues

Sixteen dermorphin analogues were synthesized and characterized for μ- and δ-opioid receptor binding properties using [³H]DAGO and [³H]DPDPE, respectively. The analogues included the following: substitutions at position 4 and/or the C-terminal residue; deletions of Gly⁴ or Pro⁶-Ser⁷; inclusion of Z or an acetyl group on the β-amino group of Lys⁷; and the presence of either a C-terminal amide or free acid group. Two peptides, [Lys7-OH]- and [Lys⁷-NH₂]dermorphin, had μ-affinities (K_iμ= 0.15–0.13 nm ) and μ-selectivities (K_iδ/K_iμ= 1158–1482) higher than dermorphin (K_iμ= 0.28 nm ; K_iδ/K_iμ= 295) and best fitted a one-site binding model similar to dermorphin. Significantly better (P <0.0001) fits to a two-site binding model vs. a one-site model were observed with four dermorphin analogues: [Lys(Z)⁷-OH]heptapeptide, [des-Gly⁴(Tyr⁴,Pro⁵,Asn⁶-OH)]hexapeptide and two pentapeptides, [Tyr⁵-NH₂] and [Trp⁴,Asn⁵-OH]. Our data revealed a complex binding pattern for dermorphin analogues to brain μ-receptors in which Hill coefficients less than 0.85 generally suggest heterogeneity of μ-receptors; however, only detailed analyses of the data derived from the non-linear regression fits for one- or two-components gave evidence for the possible existence of two separate [³H]DAGO binding sites. Eight of our dermorphin analogues had significantly better fits for a two-site model (P <0.05), but only four seemed to have two distinct Kⁱ, values (P <0.0001).     

Distinct conformational preferences of three cyclic β-casomorphin-5 analogs determined using NMR spectroscopy and theoretical analysis

The conformational properties of three cyclic β-casomorphin analogs based on the general formula H-Tyr-c[-D-Orn-2-Nal-D-Pro-Xaa-] (2-Nal = 2-naphthylalanine; Xaa = D-Ala, Sar or NMe-Ala) in DMSO solution were investigated using NMR spectroscopy in conjunction with molecular modeling techniques. The D-Ala⁵- and Sar⁵-analogs (compounds 1 and 2, respectively) are potent mixed μ-agonist/§-antagonists with high μ- and §-opioid receptor affinities, whereas the NMe-Ala⁵-analog (compound 3) is a potent μ-agonist and a weak partial §-agonist. Distinct conformational differences emerged for the three compounds studied. Flexibility in the bare ring structures was found to increase in the order 3<2<1. The increased structural rigidity of 3 may be responsible for its low §-receptor affinity as compared to the two other analogs. A low fractional population of conformers containing two cis peptide bonds was found for compound 2 but not for analog 1 or 3. Initial evidence for this observation was obtained from NMR differential line-broadening experiments and later confirmed by molecular mechanics simulations. Comparison of the temperature dependence of amide proton chemical shifts acquired for the three cyclic analogs indicate a large degree of intramolecular hydrogen bonding for 1 but not for the other two peptides. © Munksgaard 1996.     

Solution conformations of the peptide backbone for DPDPE and its β-MePhe4-substituted analogs

The solution structures of DPDPE, a conformationally restricted pentapeptide with the sequence H-Tyr¹-d -Pen²-Gly³-Phe⁴-d -Pen⁵-OH, and its four β-MePhe⁴-substituted analogs were examined by a combined approach including the NMR measurements in DMSO and water as well as independent energy calculations. It was concluded that several low energy conformers of DPDPE backbone satisfy the NMR data obtained in this study as well as in previous studies by other authors. These possible solution conformers of DPDPE in both DMSO and water share virtually the same type of cyclic backbone structure, with the Gly³ residue in a conformation close to a γ-turn, and the Phe⁴ residue in a conformation close to α-helical torsion angles. They differ in the space arrangements of the flexible Tyr¹ moiety. The solution structures of the β-MePhe⁴-substituted analogs of DPDPE are interesting. For analogs with an S-configuration at the C^α atom in the Phe⁴ residue, the cyclic backbone conformations resemble those of DPDPE itself, whereas for analogs with an R-configuration at the C^α atom, the backbone conformation is somewhat different. This observation is in line with the high biological potencies and selectivities displayed by the former compounds but not by the latter ones. It was noted also that as far as the peptide backbone conformers are concerned, some of the possible DPDPE conformers in water are similar to the previously suggested model for the δ-receptor-bound conformation of DPDPE, becoming virtually identical to this conformation by rotating the side chains of the Tyr¹ and the Phe⁴ residues.     

Single diastereomeric desaminotyrosylalanyl tetra- and heptapeptides with opioid antagonistic activity

The N-terminal dipeptide Tyr-d -Ala of a p-selective agonist, dermorphin tetrapeptide (DT, H-Tyr-d -Ala-Phe-Gly-NH₂) and δ-selective agonist deltorphin C (DEL-C, H-Tyr-d -Ala-Phe-Asp-Val-Val-Gly-NH₂) was changed into an aminodiacyl moiety. The relevant synthetic step is a nucleophilic substitution of bromine from a chiral 2-bromopropanamide by the amino group of tyrosine, with overall retention of configuration. The resulting pseudo tetra- and heptapeptides I-VI were characterized for μ and δ-opioid receptor binding properties using [³H]DAGO and [³H]DPDPE, respectively, and in a bioassay using guinea pig ileum (GPI) and mouse vas deferens (MVD). As a result of chemical alteration of N-terminal dipeptide moiety, all synthesized analogs showed considerable reduction in opioid receptor affinity compared to μ and δ-prototypes (500-fold on the μ-site, analog I , and 125-fold on the δ-site, analog IV ). Interestingly, analogs I and IV showed moderate antagonist activity, respectively, on GPI and MVD, with pA₂ values of 6.05 and 6.82. Analog IV did not exhibit the δ-antagonist potency and δ-selectivity of TIPP peptides. © Munksgaard 1995.     

Conformational features responsible for the binding of cyclic analogues of enkephalin to opioid receptors III. Probable binding conformations of μ-agonists with phenylalanine in position 3

Theoretical conformational analysis was carried out for several tetrapeptide analogues of β-casomorphin and dermorphin containing a Phe residue in position 3. Sets of low-energy backbone structures of the μ-selective peptides [N-Me-Phe³, d -Pro⁴]-morphiceptin and Tyr-d -Orn-Phe-Asp-NH₂ were obtained. These sets of structures were compared for geometrical similarity between themselves and with the low-energy conformations found for the δ-selective peptide Tyr-d -Cys-Phe-d -Pen-OH and nonactive peptide Tyr-Orn-Phe-Asp-NH₂. Two pairs of geometrically similar conformations of μ-selective peptides, sharing no similarity with the conformations of peptides showing low affinity to the μ-receptor, were selected as two alternative models of probable μ-receptor-bound backbone conformations. Both models share geometrical similarity with the low-energy structures of the linear μ-selective peptide Tyr-d -Ala-Phe-Phe-NH₂. Putative binding conformations of Tyr¹ and Phe³ side chains are also discussed.     

Harnessing D-amino acids for peptide motif designs

In examining the use of d -amino acids in designing specific peptide folding motifs, the tetrapeptide Boc-d -Glu-Ala-Gly-Lys-NHMe 1 and its analog 2 featuring l -Glu were synthesized for a comparison of their solution conformations by NMR spectroscopy. The temperature coefficients of amide proton resonances, NOE data, side-chain CH₂ anisotropies and salt titration results suggest a weak type _II reverse-turn conformation for peptide 2 , and a tandem type II ’ turn-3₁₀-helix conformation of appreciable conformational stability for peptide 1 in apolar solvents. The latter is of potential interest as the N-terminal helix cap that could support the design of longer 3₁₀ helices. Possible origins of appreciable difference in the conformational stabilities of the diastereomers are discussed.