Amphipathic control of the 310‐/α‐helix equilibrium in synthetic peptides

Abstract: A series of short, amphipathic peptides incorporating 80% C^α,C^α‐disubstituted glycines has been prepared to investigate amphipathicity as a helix‐stabilizing effect. The peptides were designed to adopt 3₁₀‐ or α‐helices based on amphipathic design of the primary sequence. Characterization by circular dichroism spectroscopy in various media (1 : 1 acetonitrile/water; 9 : 1 acetonitrile/water; 9 : 1 acetonitrile/TFE; 25 mm SDS micelles in water) indicates that the peptides selectively adopt their designed conformation in micellar environments. We speculate that steric effects from ith and ith + 3 residues interactions may destabilize the 3₁₀‐helix in peptides containing amino acids with large side‐chains, as with 1‐aminocyclohexane‐1‐carboxylic acid (Ac₆c). This problem may be overcome by alternating large and small amino acids in the ith and ith + 3 residues, which are staggered in the 3₁₀‐helix.     

Bioactive N‐terminal undecapeptides derived from parathyroid hormone: the role of α‐helicity*

Abstract: The N‐terminal 1–34 segment of parathyroid hormone (PTH) is fully active in vitro and in vivo and it can reproduce all biological responses in bone characteristic of the native intact PTH. Recent studies have demonstrated that N‐terminal fragments presenting the principal activating domain such as PTH(1–11) and PTH(1–14) with helicity‐enhancing substitutions yield potent analogues with PTH(1–34)‐like activity. To further investigate the role of α‐helicity on biological potency, we designed and synthesized by solid‐phase methodology the following hPTH(1–11) analogues substituted at positions 1 and/or 3 by the sterically hindered and helix‐promoting C^α‐tetrasubstituted α‐amino acids α‐amino isobutyric acid (Aib), 1‐aminocyclopentane‐1‐carboxylic acid (Ac₅c) and 1‐aminocyclohexane‐1‐carboxylic acid (Ac₆c): Ac₅c‐V‐Aib‐E‐I‐Q‐L‐M‐H‐Q‐R‐NH₂ ( I ); Aib‐V‐Ac₅c‐E‐I‐Q‐L‐M‐H‐Q‐R‐NH₂ ( II ); Ac₆c‐V‐Aib‐E‐I‐Q‐L‐M‐H‐Q‐R‐NH₂ ( III ); Aib‐V‐Ac₆c‐E‐I‐Q‐L‐M‐H‐Q‐R‐NH₂ ( IV ); Aib‐V‐Aib‐E‐I‐Q‐L‐M‐H‐Q‐R‐NH₂ ( V ); S‐V‐Aib‐E‐I‐Q‐L‐M‐H‐Q‐R‐NH₂ ( VI ), S‐V‐Ac₅c‐E‐I‐Q‐L‐M‐H‐Q‐R‐NH₂ ( VII ); Ac₅c‐V‐S‐E‐I‐Q‐L‐M‐H‐Q‐R‐NH₂ ( VIII ); Ac₆c‐V‐S‐E‐I‐Q‐L‐M‐H‐Q‐R‐NH₂ ( IX ); Ac₅c‐V‐Ac₅c‐E‐I‐Q‐L‐M‐H‐Q‐R‐NH₂ ( X ); Ac₆c‐V‐Ac₆c‐E‐I‐Q‐L‐M‐H‐Q‐R‐NH₂ ( XI ). All analogues were biologically evaluated and conformationally characterized in 2,2,2‐trifluoroethanol (TFE) solution by circular dichroism (CD). Analogues I – V , which cover the full range of biological activity observed in the present study, were further conformationally characterized in detail by nuclear magnetic resonance (NMR) and computer simulations studies. The results of ligand‐stimulated cAMP accumulation experiments indicated that analogues I and II are active, analogues III , VI and VII are very weakly active and analogues IV , V , VIII–XI are inactive. The most potent analogue, I exhibits biological activity 3500‐fold higher than that of the native PTH(1–11) and only 15‐fold weaker than that of the native sequence hPTH(1–34). Remarkably, the two most potent analogues, I and II , and the very weakly active analogues, VI and VII , exhibit similar helix contents. These results indicate that the presence of a stable N‐terminal helical sequence is an important but not sufficient condition for biological activity.     

(αMe)Aun: a highly lipophilic,chiral, Cα‐tetrasubstituted α‐amino acid. Incorporation into model peptides and preferred conformation

Abstract: Using a chemo‐enzymatic approach we prepared the highly lipophilic, chiral, C^α‐methylated α‐amino acid (αMe)Aun. Two series of terminally protected model peptides containing either d ‐(αMe)Aun in combination with Aib or l ‐(αMe)Aun in combination with Gly were synthesized using solution methods and fully characterized. A detailed solution conformational analysis, based on FT‐IR absorption, ¹H NMR and CD techniques, allowed us to determine the preferred conformation of this amino acid and the relationship between chirality at its α‐carbon atom and screw sense of the helix that is formed. The results obtained strongly support the view that d ‐(αMe)Aun favors the formation of the left‐handed 3₁₀‐helical conformation.     

Chemical synthesis and receptor binding of catfish somatostatin: a disulfide‐bridged β‐d‐Galp‐(1→3)‐α‐d‐GalpNAc O‐glycopeptide*

Abstract: The glycopeptide hormone catfish somatostatin (somatostatin‐22) has the amino acid sequence H‐Asp‐Asn‐Thr‐Val‐Thr‐Ser‐Lys‐Pro‐Leu‐Asn‐Cys‐Met‐Asn‐Tyr‐Phe‐Trp‐Lys‐Ser‐Arg‐Thr‐Ala‐Cys‐OH; it includes a cyclic disulfide connecting the two Cys residues, and the major naturally occurring glycoform contains d ‐GalNAc and d ‐Gal O‐glycosidically linked to Thr⁵. The linear sequence was assembled smoothly starting with an Fmoc‐Cys(Trt)‐PAC‐PEG‐PS support, using stepwise Fmoc solid‐phase chemistry. In addition to the nonglycosylated peptide, two glycosylated forms of somatostatin‐22 were accessed by incorporating as building blocks, respectively, N^α‐Fmoc‐Thr(Ac₃‐α‐D‐GalNAc)‐OH and N^α‐Fmoc‐Thr(Ac₄‐β‐D‐Gal‐(1→3)‐Ac₂‐α‐D‐GalNAc)‐OH. Acidolytic deprotection/cleavage of these peptidyl‐resins with trifluoroacetic acid/scavenger cocktails gave the corresponding acetyl‐protected glycopeptides with free sulfhydryl functions. Deacetylation, by methanolysis in the presence of catalytic sodium methoxide, was followed by mild oxidation at pH 7, mediated by N^α‐dithiasuccinoyl (Dts)‐glycine, to provide the desired monomeric cyclic disulfides. The purified peptides were tested for binding affinities to a panel of cloned human somatostatin receptor subtypes; in several cases, presence of the disaccharide moiety resulted in 2‐fold tighter binding.     

Conformational preferences of oligopeptides rich in α-aminoisobutyric acid. III. Design,synthesis and hydrogen bonding in 310-helices

Two sterically constrained peptides {ⁱBoc-Aib-Aib-Aib-DkNap-Leu-Qx-Ala-Aib-Aib-F1, (Dk⁴Qx⁶[7/9]) and ⁱBoc-Aib-Aib-Aib-DkNap-Leu-Aib-Ala-Aib-Aib-Fl, (Dk⁴7/9)} containing α-aminoisobutyric acid (Aib) and Aib-class amino acids in conjunction with selected mono-α-alkyl amino acids were synthesized by an optimized TBTU/HOBt procedure. The use of Aib-class amino acids (e.g. DkNap and Qx), defined and discussed here, gives rise to the same overwhelmingly 3₁₀-helical backbone conformation as that provided by simpler Aib-rich peptides and homopeptides. The synthetic α,α-dialkylamino acids (DkNap, Qx) are aromatic homologues of the known alicyclic variants of Aib, the Ac₅c and Ac₆c amino acids. Two new organic solubilizing groups for peptides, ⁱBoc and 2-methoxyethylamine, are introduced. The ¹H nuclear magnetic resonance analyses of the Dk⁴s/p[7/9] and Dk⁴Qx⁶[7/9] peptides demonstrate the unambiguous 310s/b-helical hydrogen bonding pattern of these peptides, confirming the design objective of these sequence patterns containing greater than 50% Aib and Aib-class composition. © Munksgaard 1994.     

(αMe)Nva: stereoselective syntheses and preferred conformations of selected model peptides

Abstract: Using different stereoselective chemical and chemoenzymatic approaches we synthesized the chiral, C^α‐methylated α‐amino acid l ‐(αMe)Nva with a short, linear side‐chain. A set of terminally protected model peptides to the pentamer level containing either (αMe)Nva or Nva in combination with Ala and/or Aib was prepared using solution methods and characterized fully. Two (αMe)Nva peptides were also synthesized using side‐chain hydrogenation of the corresponding C^α‐methyl, C^α‐allylglycine (Mag) peptides. A detailed solution and crystal‐state conformational analysis based on FT‐IR absorption, ¹H NMR and X‐ray diffraction techniques allowed us to define that: (i) (αMe)Nva is an effective β‐turn and 3₁₀‐helix former; and (ii) the relationship between (αMe)Nva chirality and the screw sense of the turn/helix formed is that typical of protein amino acids, i.e. l ‐(αMe)Nva induces the preferential formation of right‐handed folded structures. In more general terms, this study reinforced previous conclusions that peptides based on α‐amino acids with a C^α‐methyl substituent and a C^α‐linear alkyl substituent are characterized by a strong tendency to fold into turn and helical structures.     

Improved solid‐phase synthesis of α,α‐dialkylated amino acid‐rich peptides with antimicrobial activity*

Abstract: A homologous series of nonapeptides and their acetylated versions were successfully prepared using solid‐phase synthetic techniques. Each nonapeptide was rich in α,α‐dialkylated amino acids [one 4‐aminopiperidine‐4‐carboxylic acid (Api) and six α‐aminoisobutyric acid (Aib) residues] and also included lysines or lysine analogs (two residues). The incorporation of the protected dipeptide 9‐fluorenylmethyloxycarbonyl (Fmoc)‐Aib‐Aib‐OH improved the purity and overall yields of these de novo designed peptides. The helix preference of each nonapeptide was investigated in six different solvent environments, and each peptide's antimicrobial activity and cytotoxicity were studied. The 3₁₀‐helical, amphipathic design of these peptides was born out most prominently in the N‐terminally acetylated peptides. Most of the peptides exhibited modest activity against Escherichia coli and no activity against Staphylococcus aureus. The nonacetylated peptides (concentrations ≤100 μm ) and the acetylated peptides (concentrations ≤200 μm ) did not exhibit any significant cytotoxicity with normal (nonactivated) murine macrophages.     

Facile synthesis of Nα‐protected‐l‐α,γ‐diaminobutyric acids mediated by polymer‐supported hypervalent iodine reagent in water

Abstract: Hofmann rearrangement of N^α‐Boc‐l ‐Gln‐OH mediated by a polymer‐supported hypervalent iodine reagent poly[(4‐diacetoxyiodo)styrene] (PSDIB) in water afforded N^α‐Boc‐l ‐α,γ‐diaminobutyric acid (Boc‐Dab‐OH, 1 ) in 87% yield. N^α‐Z‐derivative (Z‐Dab‐OH, 2 ) was prepared with PSDIB in 83% yield. Since the reaction of N^α‐Fmoc‐Gln‐OH by this procedure did not proceed because of the insolubility of Fmoc‐Gln‐OH in aqueous media, we synthesized Fmoc‐Dab(Boc)‐OH ( 5 ) from 2 in 54% yield. Polymyxin B heptapeptide (PMBH) which contains four Dab residues was successfully synthesized in a solution‐phase synthesis.     

A topographically and conformationally constrained,spin‐labeled, α‐amino acid: crystallographic characterization in peptides*

Abstract: 2,2,6,6‐Tetramethylpiperidine‐1‐oxyl‐4‐amino‐4‐carboxylic acid (TOAC) is a topographically and conformationally restricted, nitroxide containing, C^α‐tetrasubstituted α‐amino acid. Here, we describe the molecular and crystal structures, as determined by X‐ray diffraction analyses, of a TOAC terminally protected derivative, the cyclic dipeptide c(TOAC)₂·1,1,1,3,3,3‐hexafluoropropan‐2‐ol (HFIP) solvate, and five TOAC‐containing, terminally protected, linear peptides ranging in length from tetra‐ to hepta‐peptides. Incipient and fully developed, regular or distorted 3₁₀‐helical structures are formed by the linear peptides. A detailed discussion on the average geometry and preferred conformation for the TOAC piperidine ring is also reported. The X‐ray diffraction structure of an intramolecularly cyclized side product resulting from a C‐activated TOAC residue has also been determined.     

New tools for the control of peptide conformation: the helicogenic Cα‐methyl,Cα‐cyclohexylglycine*

Abstract: The novel C^α‐tetrasubstituted α‐amino acid C^α‐methyl, C^α‐cyclohexylglycine was prepared by hydrogenation of its C^α‐methyl, C^α‐phenylglycine precursor. Terminally protected homodi‐, homotri‐, and homotetrapeptides from C^α‐methyl, C^α‐cyclohexylglycine and co‐oligopeptides to the pentamer level in combination with Gly or α‐aminoisobutyric acid residues were prepared by solution methods and fully characterized. The results of a conformational analysis, performed by use of Fourier transform infrared (FT‐IR) spectrophotomet absorption, ¹H NMR, and X‐ray diffraction techniques, support the contention that this C^α‐methylated, C^β‐trisubstituted aliphatic α‐amino acid is an effective β‐turn and 3₁₀‐helix inducer in tri‐ and longer peptides as its C^α‐methyl valine parent compound, but partially divergent from the corresponding aromatic C^α‐methyl, C^α‐diphenylmethylglycine residue, known to promote folded and fully extended structures to a significant extent in these oligomers.     

Structural versatility of peptides from Cα,α-disubstituted glycines: crystal-state conformational analysis of peptides from Cα-methylhomophenylalanine, (αMe)Hph

The molecular and crystal structures of the C^α-tetrasubstituted, δ-branched α-amino acid C^α-methyl-homophenylalanine, H-d -(αMe)Hph-OH, and three peptides (to the pentamer level), including the homotripeptide, have been determined by X-ray diffraction. The peptides are Z-l -(αMe)Hph-(l -Ala)₂-OMe, pBrBz-[d -(αMe)Hph]₃-OtBu and Ac-(Aib)₂-l -(αMe)Hph-(Aib)₂-OtBu. All the (αMe)Hph residues prefer φ,ψ torsion angles in the helical region of the conformational map. The two terminally blocked tripeptides adopt a β-bend conformation stabilized by a 1→4 C = O?H-N intramolecular H-bond. The terminally blocked pentapeptide is folded in a regular 3₁₀-helix. In general, the relationship between (αMe)Hph α-carbon chirality and helix handedness is the same as that exhibited by protein amino acids. A comparison is also made with the conclusions extracted from published work on peptides from other types of C^α-alkylated aromatic α-amino acids. © Munksgaard 1996.     

Partial [αMe]Aun scan of [l‐Leu11‐OMe]‐trichogin GA IV,a membrane active synthetic precursor of the natural lipopeptaibol

Abstract: We synthesized using solution‐phase methods three analogs of [l ‐Leu¹¹‐OMe] trichogin GA IV, a membrane active synthetic precursor of the lipopeptaibol antibiotic in which the N‐terminal n‐octanoyl group and each of the three Aib residues in positions 1, 4 and 8 are replaced by an acetyl group and the lipophilic C^α,α‐disubstituted glycine l ‐(αMe)Aun, respectively [partial (αMe)Aun scan]. FT‐IR absorption and CD analyses unequivocally show that the main three‐dimensional structural features of [l ‐Leu¹¹‐OMe] trichogin GA IV are preserved in the analogs. Also, [l ‐Leu¹¹‐OMe] trichogin GA IV and the three N^α‐acetylated l ‐(αMe)Aun analogs exhibit strictly comparable membrane‐modifying properties. Taken together, these results strongly favor the conclusion that a shift of the long hydrocarbon moiety from the N^α‐blocking group to the side‐chain of the 1, 4 or 8 residue does not have any significant effect on the conformational properties or the membrane activity of [l ‐Leu¹¹‐OMe] trichogin GA IV and, by extension, of the natural lipopeptaibol.     

Chiral recognition in dipeptides containing 1-aminocyclopropane carboxylic acid or α-aminoisobutyric acid: NMR studies in solution

Protected dipeptides containing 1-aminocyclopropane carboxylic acid (Ac₃c) or α-aminoisobutyric acid (Aib) residues at the C-terminus and Phe, Val or Ala residues at the N-terminus displayed different proton NMR spectra for the pure enantiomers and the racemic mixtures in deuterochloroform (CDCl₃) solution. An unequal mixture of enantiomers showed two sets of resonances (NMR nonequivalence), one corresponding to major and the other to minor enantiomer. The NMR nonequivalence was originated by the presence of the C-terminal Ac₃c or Aib residues, which have been known for their unique spatial preferences in avoiding an extended (C₅) conformation. When a C₅ conformation favoring residue such as glycine was incorporated in place of Ac₃c or Aib, negligible NMR nonequivalence was observed. The magnitude of the NMR nonequivalence depended on the side chain as well as on the protecting groups at N-terminus α-amino acid. For the same peptide, the magnitude of nonequivalence increased with increasing solution concentration and/or with decreasing the solution temperature. The NMR nonequivalence disappeared in polar solvent-like deuterated dimethylsulfoxide (DMSO-d₆). A preference for hetero-chiral recognition leading to dimeric association under fast exchange conditions had been invoked to explain the observed phenomenon. The dipeptides thus prepared could well serve as ‘model peptides’ for the evaluation of any preparative methods.     

Conformational properties of hybrid peptides containing α‐ and ω‐amino acids*

Abstract: This review briefly surveys the conformational properties of guest ω‐amino acid residues when incorporated into host α‐peptide sequences. The results presented focus primarily on the use of β‐ and γ‐residues in αω sequences. The insertion of additional methylene groups into peptide backbones enhances the range of accessible conformations, introducing additional torsional variables. A nomenclature system, which permits ready comparisons between α‐peptides and hybrid sequences, is defined. Crystal structure determination of hybrid peptides, which adopt helical and β‐hairpin conformations permits the characterization of backbone conformational parameters for β‐ and γ‐residues inserted into regular α‐polypeptide structures. Substituted β‐ and γ‐residues are more limited in the range of accessible conformation than their unsubstituted counterparts. The achiral β,β‐disubstituted γ‐amino acid, gabapentin, is an example of a stereochemically constrained residue in which the torsion angles about the C^β–C^γ (θ₁) and C^α–C^β (θ₂) bonds are restricted to the gauche conformation. Hybrid sequences permit the design of novel hydrogen bonded rings in peptide structures.     

Entrapping unusual folding characteristics of the β‐Ala residues in a model peptide: Boc‐β‐Ala‐Aib‐β‐Ala‐NHCH3

Abstract: Crystal structure analysis of a model peptide: Boc‐β‐Ala‐Aib‐β‐Ala‐NHCH₃ (β‐Ala: 3‐amino propionic acid; Aib: α‐aminoisobutyric acid) revealed distinct conformational preferences for folded [φ≈136°, µ ≈ ?62°, ψ ≈100°] and semifolded [φ ≈ 83°, µ ≈ ?177°, ψ ≈ ?117°] structures of the N‐ and C‐terminus β‐Ala residues, respectively. The overall folded conformation is stabilized by unusual N_i···H‐N_i+1 and nonconventional C–H···O intramolecular hydrogen bonding interactions.     

Peptides from chiral Cα,α-disubstituted glycines

The molecular and crystal structures of one derivative and three model peptides (to the pentapeptide level) of the chiral C^α,α-disubstituted glycine Cα-methyl, Cα-isopropylglycine [(αMe)Val] have been determined by X-ray diffraction. The derivative is mClAc-l -(α Me)Val-OH, and the peptides are Z-l -(αMe)Val-(l -Ala)₂-OMe monohydrate, Z-Aib-L-(αMe)Val-(Aib)₂-OtBu, and Ac-(Aib)₂-l -(αMe)Val-(Aib)₂OtBu acetonitrile solvate. The tripeptide adopts a type-I β-turn conformation stabilized by a 1 ← 4N-H . O=C intramolecular H-bond. The tetra- and pentapeptides are folded in regular right-handed 3₁₀-helices. All four L-(αMe)Val residues prefer φ, Ψ angles in the right-handed helical region of the conformational map. The results indicate that: (i) the (αMe)Val residue is a strong type-I/III β-turn and helix former, and (ii) the relationship between (αMe)Val chirality and helix screw sense is the same as that of Cα-monosubstituted protein amino-acids. The implications for the use of the (αMe)Val residue in designing conformationally constrained analogues of bioactive peptides are briefly discussed.     

Solid‐phase synthesis of peptides containing the spin‐labeled 2,2,6,6‐tetramethylpiperidine‐1‐oxyl‐4‐amino‐4‐carboxylic acid (TOAC)

Abstract: 2,2,6,6‐Tetramethylpiperidine‐1‐oxyl‐4‐amino‐4‐carboxylic acid (TOAC) is a nitroxide spin‐labeled, achiral C^α‐tetrasubstituted amino acid recently shown to be not only an effective β‐turn and 3₁₀/α‐helix promoter in peptides, but also an excellent rigid electron paramagnetic resonance probe and fluorescence quencher. Here, we demonstrate that TOAC can be effectively incorporated into internal positions of peptide sequences using Fmoc chemistry and solid‐phase synthesis in an automated apparatus.     

Channel‐forming,self‐assembling,bishelical amphiphilic peptides: design,synthesis and crystal structure of Py(Aibn)2, n = 2, 3, 4

Abstract: The design, synthesis, characterization and self‐assembling properties of a new class of amphiphilic peptides, constructed from a bifunctional polar core attached to totally hydrophobic arms, are presented. The first series of this class, represented by the general structure Py(Aib_n)₂ (Py = 2,6‐pyridine dicarbonyl unit; Aib = α, α′‐dimethyl glycine; n = 1–4), is prepared in a single step by the condensation of commercially available 2,6‐pyridine dicarbonyl dichloride with the methyl ester of homo oligoAib peptide (Aib_n‐OMe) in the presence of triethyl amine. ¹H NMR VT and ROESY studies indicated the presence of a common structural feature of 2‐fold symmetry and an NH…N hydrogen bond for all the members. Whereas the Aib3 segment in Py(Aib3)₂ showed only the onset of a 3₁₀‐helical structure, the presence of a well‐formed 3₁₀‐helix in both Aib4 arms of Py(Aib4)₂ was evident in the ¹H NMR of the bispeptide. X‐ray crystallographic studies have shown that in the solid state, whereas Py(Aib2)₂ molecules organize into a sheet‐like structure and Py(Aib3)₂ molecules form a double‐stranded string assembly, the tetra Aib bispeptide, Py(Aib4)₂, is organized to form a tetrameric assembly which in turn extends into a continuous channel‐like structure. The channel is totally hydrophobic in the interior and can selectively encapsulate lipophilic ester (CH₃COOR, R = C₂H₅, C₅H₁₁) molecules, as shown by the crystal structures of the encapsulating channel. The crystal structure parameters are: 1b , Py(Aib2)₂, C₂₅H₃₇N₅O₈, sp. gr. P2₁2₁2₁, a = 9.170(1) Å, b = 16.215(2) Å, c = 20.091(3) Å, R = 4.80; 1c , Py(Aib3)₂, C₃₃H₅₁N₇O₁₀·H₂O, sp. gr. P, a = 11.040(1) Å, b = 12.367(1) Å, c = 16.959(1) Å, α = 102.41°, β = 97.29°, γ = 110.83°, R₁ = 6.94; 1 da, Py(Aib4)₂?et ac, C₄₁H₆₅N₉O₁₂?1.5H₂O·C₄H₈O₂, sp. gr. P, a = 16.064(4) Å, b = 16.156 Å, c = 21.655(5) Å, α = 90.14(1)°, β = 101.38(2)°, γ = 97.07(1)°, Z = 4, R₁ = 9.03; 1db, Py(Aib4)₂?amylac,C₄₁H₆₅N₉O₁₂?H₂O ·C₇H₁₄O₂, P2₁/c, a = 16.890(1) Å, b = 17.523(1) Å, c = 20.411(1) Å, β = 98.18 °, Z = 4, R = 11.1 (with disorder).     

Synthesis,Biological Activity,and NMR‐Based Structural Studies of Deltorphin I Analogs Modified in Message Domain with a New α,α‐Disubstituted Glycines

This article describes new deltorphin I analogs in which phenylalanine residues were replaced by the corresponding (R) or (S)‐α‐benzyl‐β‐azidoalanine, α‐benzyl‐β‐(1‐pyrrolidinyl)alanine, α‐benzyl‐β‐(1‐piperidinyl)alanine, and α‐benzyl‐β‐(4‐morpholinyl)‐alanine residues. The potency and selectivity of the new analogs were evaluated by a competitive receptor binding assay in the rat brain using [³H]DAMGO (a μ ligand) and [³H]DELT (a δ ligand). The affinity of analogs containing (R) or (S)‐α‐benzyl‐β‐azidoalanine in position 3 to δ‐receptors strongly depended on the chirality of the α,α‐disubstituted residue. The conformational behavior of peptides modified with (R) or (S)‐α‐benzyl‐β‐(1‐piperidinyl)Ala, which displays the opposite selectivity, was analyzed by ¹H and ¹³C NMR. The μ‐selective Tyr‐d ‐Ala‐(R)‐α‐benzyl‐β‐(1‐piperidinyl)Ala‐Asp‐Val‐Val‐Gly‐NH₂ lacks the helical conformation observed in the δ‐selective Tyr‐d ‐Ala‐(S)‐α‐benzyl‐β‐(1‐piperidinyl)Ala‐Asp‐Val‐Val‐Gly‐NH₂. Our results support the proposal that differences between δ‐ and μ‐selective opioid peptides are attributable to the presence or absence of a spatial overlap between the N‐terminal message domain and the C‐terminal address domain.     

A combined extended and helical backbone for Boc‐(Ala‐Leu‐Ac7c‐)2‐OMe*

Abstract: The structure of the peptide Boc‐Ala‐Leu‐Ac₇c‐Ala‐Leu‐Ac₇c‐OMe (Ac₇c,1‐aminocycloheptane‐1‐carboxylic acid) is described in crystals. The presence of two Ac₇c residues was expected to stabilize a 3₁₀‐helical fold. Contrary to expectation the structural analysis revealed an unfolded amino terminus, with Ala(1) adopting an extended β‐conformation (φ = ?93°,ψ = 112°). Residues 2–5 form a 3₁₀‐helix, stabilized by three successive intramolecular hydrogen bonds. Notably, two NH groups Ala(1) and Ac₇c(3) do not form any hydrogen bonds in the crystal. Peptide assembly appears to be dominated by packing of the cycloheptane rings that stack against one another within the molecule and also throughout the crystal in columns.