The pseudo-βI-turn

Synthesis and conformational analysis of three cyclic hexapeptides cyclo(-Gly¹-Pro²-Phe³-Val⁴-Xra⁵-Phe⁶), Xaa= Phe (I), D-Phe (II) and D-Pro (III), were carried out to examine the influence of proline on the formation of reverse turns and the dynamics of hydrophobic peptide regions. Assignment of all ¹H and ¹³C resonances was achieved by homo- and heteronuclear 2D-NMR techniques (TOCSY, ROESY, HMQC, HMQC-TOCSY and HMBCS-270). The conformational analysis is based on interproton distances derived from ROESY spectra and homo- and heteronuclear coupling constants (E.COSY, HETLOC and HMBCS-270). For structural refinements restrained molecular dynamics (MD) simulations in vacuo and in DMSO were performed. Each peptide exhibits two conformations in DMSO solution due to cis-trans isomerism about the Gly-Pro peptide bond. Surprisingly the cis-Gly-Pro segment in the minor isomers is not involved in a βVI-turn, but forms a turn structure with cis-Gly-Pro in the i and i+ 1 positions. Although no stabilizing hydrogen bond is found in this turn, the φ and ψ-angles closely correspond to a βI-turn [Pro²:φ(i+ 1) -60°, ψ(i+ 1) -30° Phe³: φ(i+ 2) -100°, ψ(i+ 2) -50°]. Hence we call this structural element a pseudo-βI-turn. As expected, in the dominating all-trans isomers proline occupies the i+ 1 position of a standard βI-turn. Therefore, cis-trans isomerization of the Gly¹-Pro² amide bond only induces a local conformational rearrangement, with minor structural changes in other parts of the molecule. However, the geometry of the other regions is affected by the chirality of the i+ 1 amino acid for both isomers (βI for Phe⁵, βII′ for D-Phe⁵ or D-Prp⁵).     

Comparative conformational studies on cyclic hexapeptides corresponding to message sequence His-Phe-Arg-Trp of α-Melanotropin by NMR

Solution conformation of cyclo(Gly¹-His²-Phe³-Arg⁴-Trp⁵-Gly⁶) and its d -Phe analog corresponding to the message sequence [Gly-α-MSH_5-10] of α-MSH has been studied by 1D and 2D proton magnetic resonance spectroscopy in dimethyl sulfoxide (DMSO)-d₆ solution and in a DMSO-d₆/H₂O cryoprotective mixture. The NMR data for both the analogs in solution at 300 K cannot be interpreted based on a single ordered conformation, as evidenced by the broadening of only -NH resonances as well as the temperature coefficients of the amide protons. An analysis of the nuclear Overhauser effect (NOE) cross-peaks in conjunction with temperature coefficient data indicates an equilibrium of multiple conformers with a substantial population of particular conformational states at least in the d -analog. The molecular dynamics simulations without and with NOE constraints also reveal numerous low-energy conformers with two γ-turns, a γ-turn and a β-turn, two β-turns, etc. for both the analogs. The observed NMR spectra can be rationalized by a dynamic equilibrium of conformers characterized by a γ-bend at Gly⁶, two γ-bends at Phe³ and Gly⁶ and a conformer with a single β-turn and a γ-bend for the l -Phe analog. On the other hand, a conformation with two fused β-turns around the two tetrads His²-d -Phe³-Arg⁴-Trp⁵ and Trp⁵-Gly⁶-Gly¹-His² dominates the equilibrium mixture for the d -Phe analog. For the d -Phe analog, the experimentally observed average conformation is corroborated by molecular dynamics simulations as well as by studies in cryoprotective solvent.     

Turn induction by N-aminoproline Comparison of the Gly-Pro-Gly and Glyψ[CO-NH-N]Pro-Gly sequences

The folded structure induced by the N-aminoproline residue (the hydrazino analogue of proline, denoted hPro) in the Boc-Gly¹-hPro²-Gly³-NHiPr hydrazino tripeptide has been characterized in the solid state by X-ray diffraction, and compared to the usual βII-turn structure in the Boc-Gly¹-Pro²-Gly³⁶-NHiPr cognate tripeptide. It is stabilized by a bifurcated hydrogen bond in which (Gly³)NH interacts with both (Gly¹)CO and (hPro²)N^x. This conformation is retained in CH₂Cl₂ and CHC1₃ solutions, and allows an overall folded conformation of the hydrazino tripeptide in which (iPr)NH is hydrogen-bonded to (Boc)CO. The hPro α-hydrazino acid residue appears to promote a local folded structure, and might behave as a β-turn mimic. © Munksgaard 1994.     

HUMAN β-ENDORPHIN: SYNTHESIS AND BIOLOGICAL ACTIVITY OF ANALOGS WITH SUBSTITUTIONS IN POSITIONS 2, 9, 18, 27 AND 31

Four analogs of the opioid peptide human β-endorphin (β_h-EP) have been synthesized: [d -Lys⁹,Phe²⁷,Gly³¹]-β_h-EP, [d -Phe¹⁸,Phe²⁷,Gly³¹)-β_h-EP, [d -Thr²,d -Lys⁹,Phe²⁷,Gly³¹]-β_h-EP, and [d -Thr²,d -Phe¹⁸,Phe²⁷,Gly³¹]-β_h-EP. All are practically indistinguishable from β_h-EP in the guinea pig ileum assay. All show diminished analgesic potency in the mouse tail-flick assay.     

Testing for cis‘ proline with α-aminoisobutyric acid substitution

Substitution of Pro residues with AIB (α-aminoisobutyric acid) residues in peptides provides a means of evaluating the presence of cis' proline conformations both in solution and, using bioassay data, in a receptor complex. ¹ H n.m.r. has been used to probe the DMSO solution conformation of all seven of the possible AIB/Pro isomers of bradykinin. AIB substitution for Pro² and/or Pro³ appears to stabilize a type III β-turn involving the N-terminal residues, but not an incipient 3₁₀ helix suggested by model peptides. These substitutions are correlated with low biological potencies, suggesting that such conformational features may be incompatible with receptor complexation. Alternatively, AIB⁷ -bradykinin analogs exhibit a variety of long range shift perturbations relative to bradykinin. The data suggests that bradykinin can adopt several folded conformations, including β-turns involving both Ser⁶-Pro⁷-Phe⁸-Arg⁹ and Phe⁵-Ser⁶-Pro⁷-Phe⁸. The relatively high biological activities of the AIB⁷-BK suggest that the complexed form of the peptide is characterized by a cis' Pro⁷ conformation.     

Comparative conformational analysis and in vitro pharmacological evaluation of three cyclic hexapeptide NK-2 antagonists

The conformational analysis of three cyclic hexapeptides is presented. Cyclo-(-Gln⁶-Trp⁷-Phe⁸-Gly⁹-Leu¹⁰-d -Met¹¹-) (1) and cyclo-(-Gln⁶-Trp⁷-Phe⁸-Gly⁹-Leu¹⁰-Met¹¹-) (2) are NK-2 antagonists in the hamster trachea assay, whereas cyclo-(-Gln⁶-Trp⁷-Phe⁸-(R)-Gly⁹-[ANC-2]Leu¹⁰-Met¹¹-) (3), where Gly⁹[ANC-2]Leu¹⁰ represents (2S)-2-((3R)-3-amino-2-oxo-1-pyrrolidinyl)-4-methylpentanoyl, is inactive as agonist and antagonist in this assay. In DMSO, the NMR results cannot be interpreted as being consistent with a single conformation. However, the combined interpretation of results from NMR spectroscopy, restrained molecular dynamics simulations with application of proton–proton distance information from ROESY spectra, and pharmacological results leads to a reduced number of conformational domains for each peptide, which can be compared with each other and may be classified as responsible for their biological activity. Trying to match the conformational domains approximately with regular β- and γ-turns, we find a γⁿ-turn at the position of the methionine occuring in all peptides. For the active peptides 1 and 2 we arrive at an inverse γⁱ-turn at Phe⁸, and βI′- or βII-turns with Gly⁹ and Leu¹⁰ at the corner positions, these β-turns having a similar topology with respect to the linking peptide unit. Other conformational domains common to only 1 and 2 support their classification as responsible for the biological activity.     

Conformational analyses by 1H NMR and computer simulations of cyclic hexapeptides related to somatostatin containing acidic and basic peptoid residues

We report the conformational analysis by ¹H NMR in DMSO and computer simulations involving distance geometry and molecular dynamics simulations at 300K of peptoid analogs of the cyclic hexapeptide c-[Phe¹¹-Pro⁶-Phe⁷-D-Trp⁸-Lys⁹-Thr¹⁰]. The analogs c-[Phe¹¹-Nasp⁶-Phe⁷-D-Trp⁸-Lys⁹-Thr¹⁰] ( 1 ), c-[Phe¹¹-Ndab⁶-Phe⁷-D-Trp⁸-Lys⁹-Thr¹⁰] ( 2 ) and c-[Phe¹¹-Nlys⁶-Phe⁷-D-Trp⁸-Lys⁹-Thr¹⁰] ( 3 ) where Nasp denotes N-(2-carboxyethyl) glycine, Ndab N-(2-aminoethyl) glycine and Nlys N-(4-aminobutyl) glycine are subject to conformational studies. The results of free and restrained molecular dynamics simulations at 300K are reported and give insight into the conformational behaviour of these analogs. The compounds show two sets of nuclear magnetic resonance signals corresponding to the cis and trans orientations of the peptide bond between residues 11 and 6. The backbone conformation of the cis isomers that we believe are the bioactive isomers of the three compounds are very similar to each other while there are larger variations amongst the trans isomers. The binding data to the isolated receptors show that the introduction of the Nlys residue in analog 3 leads to an enhancement of binding potency to the hsst5 receptor compared with analog 2 while maintaining identical binding potency to the hsst2 receptor. The Nasp⁶ analog 1 binds weakly to the hsst2 and is essentially inactive towards the other receptors. Comparison of the conformations and binding activities of these three analogs indicates that the Nlys residue extends sufficiently far to allow binding to a negatively charged binding domain on the hsst5 receptor. According to this model, the Ndab analog 2 cannot extend far enough to allow for binding to the receptor pocket. The loss of activity observed for the Nasp⁶ compound 1 indicates that the presence of a negatively charged residue in position 6 is unfavorable for binding to the hsst receptors.     

CONFORMATIONAL CHARACTERIZATION OF CYCLOPENTAPEPTIDE,LVAL-LPRO-GLY-LVAL-GLY: A REPEATING ANALOGUE OF ELASTIN

The cyclopentapeptide, ·L·Val¹-L· Pro²-Gly³-L· Val⁴-Gly⁵, was synthesized and its conformational characterization was carried out using n.m.r. and theoretical energy calculations. The n.m.r. studies indicated the existence of a cis Val¹-Pro² peptide bond in water and a very strong intramolecular H-bond between the Val¹ NH and Gly³ C=O groups. This H-bond forms a β-turn (type II) placing Val⁴ Gly⁵ residues within the turn. Two minimum energy conformations were derived, one of which agrees very well with the solution conformation.     

Synthesis and biological activities of linear and cyclic enkephalin analogues containing a Ψ (E,CH=CH) or Ψ (CH2CH2) isosteric replacement

The peptide CO–NH function was replaced by a trans carbon-carbon double bond or by a CH₂–CH₂ isostere in enkephalin analogues of DADLE, DCDCE–NH₂ or DPDPE. In DADLE the 2-3 and the 3-4 peptide bond was modified, whereas in the cyclic analogues the Gly³-Phe⁴ bond was replaced by the isosteres GlyΨ (E,CH=CH)Phe [5-amino-2-(phenylmethyl)-3(E)-pentenoic acid] or GlyΨ(CH₂CH₂)Phe [5-amino-2-(phenylmethyl)pentanoic acid]. In general, the modification results in a drop in potency which is the largest for the flexible CH₂–CH₂ replacement. The Gly³Ψ (E,CH=CH)Phe⁴ DCDCE–NH₂ analogue retains considerable potency. These results confirm the importance of the peptide function at the 2-3 and 3-4 position in enkephalin analogues for biological potency.     

Combined use of NMR,distance geometry and MD calculations for the conformational analysis of opioid peptides of the type [D(L)-Cys2, D(L)-Cys5]enkephalin

The solution structures of a series of conformationally restricted pentapeptides with a sequence H-Tyr¹-Cys²-Gly³ Phe⁴-Cys⁵-OH cyclic (2-5) disulfide, where the cysteines possess either the D or L configuration, were examined by a combined approach including NMR measurements as well as MD calculations. It turned out that at least one low energy conformer of H-Tyr¹-Cys²-Gly³-Phe⁴-Cys⁵-OH cyclic (2-5) disulfide (DCDCE), as well as one conformer out of the group of calculated conformers for H-Tyr¹-D-Cys²-Gly³-Phe⁴-Cys⁵-OH cyclic (2-5) disulfide (DCLCE), satisfies the NMR data obtained in this study, whereas for the derivative H-Tyr^l-Cys²-Gly³-Phe⁴-Cys⁵-OH cyclic (2-5) disulfide, which contains solely L-Cys (LCLCE), there is no single structure compatible with the NMR data. © Munksgaard 1996.     

Structure of a paralytic peptide from an insect,Manduca sexta

Abstract: Paralytic peptide 1 (PP1) from a moth, Manduca sexta, is a 23-residue peptide (Glu-Asn-Phe-Ala-Gly-Gly-Cys-Ala-Thr-Gly-Tyr-Leu-Arg-Thr-Ala-Asp-Gly-Arg-Cys-Lys-Pro-Thr-Phe) that was first found to have paralytic activity when injected into M. sexta larvae. Recent studies demonstrated that PP1 also stimulated the spreading and aggregation of a blood cell type called plasmatocytes and inhibited bleeding from wounds. We determined the solution structure of PP1 by two-dimensional¹H NMR spectroscopy to begin to understand structural–functional relationships of this peptide. PP1 has an ordered structure, which is composed of a short antiparallel β-sheet at residues Tyr¹¹-Thr¹⁴ and Arg¹⁸-Pro²¹, three β turns at residues Phe³-Gly⁶, Ala⁸-Tyr¹¹ and Thr¹⁴-Gly¹⁷, and a half turn at the carboxyl-terminus (residues Lys²⁰-Phe²³). The well-defined secondary and tertiary structure was stabilized by hydrogen bonding and side-chain hydrophobic interactions. In comparison with two related insect peptides, whose structures have been solved recently, the amino-terminal region of PP1 is substantially more ordered. The short antiparallel β-sheet of PP1 has a folding pattern similar to the carboxyl-terminal subdomain of epidermal growth factor (EGF). Therefore, PP1 may interact with EGF receptor-like molecules to trigger its different biological activities.     

STUDIES ON THE CONFORMATION AND INTERACTIONS OF ELASTIN: NUCLEAR MAGNETIC RESONANCE OF THE POLYHEXAPEPTIDE

Synthesis, proton magnetic resonance and carbon-13 magnetic resonance characterizations, including complete assignments, are reported for the polyhexapeptide of elastin, HCO-Val-(Ala₁-Pro₂-Gly₃-Val₄-Gly₅-Val₆)₁₈-OMe. Temperature dependence of peptide NH chemical shifts and solvent dependence of peptide C-O chemical shifts have been determined in several solvents and have been interpreted in terms of four hydrogen bonded rings for each repeat of the polyhexapeptide. The more stable hydrogen bonded ring is a β-turn involving Ala₁ C-O…HN·Val₄ More dynamic hydrogen bonds are an 11-atom hydrogen bonded ring Gly₃ NH · O-C Gly₅, a 7-atom hydrogen bonded ring (a γ-turn) Gly₃ C-O … HN · Gly₅ and a 23-atom hydrogen bonded ring Val₆₁NH … O-C Val_6(l+l). This set of hydrogen bonds results in a right-handed β-spiral structure with slightly more than two repeats (approximately 2.2) per turn of spiral. The β-spiral structure is briefly discussed relative to data on the elastic fiber.     

Synthesis and opioid activity of partial retro-inverso analogs of dermorphin

We studied the effect of partial retro-inverso modification of selected peptide bonds of dermorphin (H-Tyr-d -Ala-Phe-Gly-Tyr-Pro-Ser-NH₂. The modifications concern two consecutive peptide bonds (Phe³-Cly⁴-Tyr⁵, I) or a single one (Gly⁴-Tyr⁵-, II or Phe³-Gly⁴, III). All pseudoheptapeptides showed low opioid activity in the in vitro and in vivo tests. Compound III has a biological potency comparable to that of morphine but only 2–5% of original dermorphin when tested in guinea pig ileum preparation and in mice tail-flick assay after intra-cerebro or subcutaneous administration.     

Studies of peptide antibiotics. XLIV. Syntheses and biological activities of gramicidin S analogs containing δ-hydroxy-l-norvaline or glycine

The antibiotic gramicidin S (GS) has the structure of cyclo (-l -Val¹-L-Orn²-l -Leu³-d -Phe⁴-l -Pro⁵-L-Val¹′-l -Orn²′-l -Leu³′-d -Phe⁴′-l -Pro⁵′-) and is basic in character. Five GS analogs including [Gly^1,1′]-GS and the neutral [l -Hnv^2,2′]-GS (Hnv represents δ-hydroxynorvaline) were synthesized by the solid-phase method to evaluate the role of l -Val^1,1′ and l -Orn^2,2′ residues in GS. The hybrid analogs ([Gly¹]-GS and [l -Hnv²]-GS) and [d -Tyr^4,4′]-GS showed high antibacterial activities, whereas [Gly^1,1′]-GS and [l -Hnv^2,2′]-GS possessed no activity. Inhibitory effects by these analogs for the adsorption of ¹⁴C-labeled GS on cells of bacteria sensitive to GS were determined. The structure-activity relationship of GS is discussed on the basis of the results on these GS analogs.     

NON-ELASTOMERIC POLYPEPTIDE MODELS OF ELASTIN

The synthesis of two polyhexapeptides is reported. The polyhexapeptides are H-(Val⁶-Ala¹-Pro²-Gly³-θ⁴-Gly⁵)_n -Val-OMe where θ= Vol or Lys with a mole ratio of 3.5:1 and H-(Val⁶-Ala¹-Pro²-Gly³-Lys⁴-Gly⁵)_n-Val-OMe. The first polymer was utilized with a previously synthesized polyhexapeptide, H-(Val⁶-Ala¹ -Pro²-Gly³-θ-Gly⁵)_n-Val-OMe where θ was either Val⁴ or Glu⁴ at a mole ratio of 3.5:1, to obtain an intermolecular cross-linked matrix by means of primary amide bond formation between the γ-carboxyls of the Glu residues of one copolymer and the α-amino groups of the Lys residues of the other copolymer. The cross-linking reaction was run during a temperature-elicited phase separation with flow orientation of the polymers. An insoluble, non-elastomeric, cross-linked, polyhexapeptide matrix was obtained. The nature of the insoluble polyhexapeptide matrix was well-demonstrated by the polymer, H-(Val⁶-Ala¹-Pro²-Gly³-θ⁴-Gly⁵)_n-Val-OMe where θ⁴ is Vol or Lys, which could be formed into cellophane-like, non-elastomeric sheets which would tear and which could be shown by microscopy to have sharp edges. This very different property of the polyhexapeptide of tropoelastin as compared to the elastomeric polypentapeptide of tropoelastin is discussed in terms of a different structural role. The purity of key intermediate hexamers and of the polyhexapeptides is demonstrated by carbon-13 magnetic resonance in addition to the usual analytical methods.     

Backbone modifications in cyclic pep tides Conformational analysis of a cyclic pseudopentapeptide containing a thiomethylene ether amide bond replacement

NMR and X-ray crystallographic studies have shown that cyclic pentapeptides of the general structure cyclo(D-Xxx-Pro-Gly-Pro-Gly) possess β- and γ-turn intramolecular hydrogen bonds. As part of our continuing series surveying the compatibility of various amide bond replacements on peptide structure, we have synthesized cyclo(D-Phe-Proψ [CH₂S]Gly-Pro-Gly). The pseudopeptide was prepared by solid phase methods and cleaved from the resin by a new procedure involving phase transfer catalysis using K₂CO₃ and tetrabutylammonium hydrogen sulfate. Cyclization was carried out with the use of DPPA, HOBt, and DMAP to afford the product in 69% yield. The conformational behavior of the pseudopeptide was analyzed by ¹H and ¹³C (1D and 2D) NMR techniques. The backbone modification replaced the amide bond that is involved in a γ-turn intramolecular hydrogen bond in the all-amide structure. In CDCl₃, the pseudopeptide adopted the same all-trans conformation as its parent, although the remaining β-turn hydrogen bond was weaker according to Δδ/ΔT_NH measurements. In DMSO-d₆, the all-trans conformer and a second conformer were observed in a ratio of 55:45. These conformers, which slowly inter converted on the NMR time scale, could be separately assigned; peaks due to chemical exchange were readily distinguishable by the ROESY technique as reported earlier by others. ¹³C and ROESY experiments suggested the minor conformer contained one cis amide bond at the Gly¹-Pro² position. Thus, both the location and type of amide surrogate are important determinants affecting the compatibility of the replacement with a particular conformational feature.     

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.     

Synthesis and solution structure of an S-glycosylated cyclic hexapeptide

Synthesis and conformational analysis of the S-glycosylated cyclic hexapeptide cyclo(-d -Pro¹-Phe²-Cys³(tetra-O-acetyl-β-d -galactopyranosyl)-Trp⁴-Lys(Z)⁵-Phe⁶-) I was carried out to examine the influence of a saccharide residue in position i of a standard β-turn on the formation of reverse turns and on the biological activity. Synthesis was carried out in the liquid phase employing a galactosylated cysteine building block. The cyclization reagents DPPA/NaHCO₃ avoided high dilution conditions. Spectroscopic data were extracted from homo- and heteronuclear 2D-NMR techniques (TOCSY, NOESY, HMQC, HMQC-TOCSY, HMBCS-270). For structural refinement restrained molecular dynamics (MD) simulations in vacuo and with explicit DMSO as solvent were performed. Finally, simulations in DMSO without experimental restraints provided insight in stability and dynamics of the structural model. A comparison of the S-glycosylated Cys³ peptide with the analogous Thr³ peptide exhibits a similar overall conformation of the hexapeptide [βII’d -Pro-Phe and another β-turn about Trp⁴-Lys⁵(Z)]. However, the latter shows a distinct dynamic flip βI, βII in the glycopeptide, whereas the Thr-analogue only populates βI. This influence is attributed to a βI stabilizing effect of a hydrogen bridge of Thr-O, in position i to the NH of the amino acid in position i+ 2, which is lacking in the glycosylated compound.     

Stabilization of β-turn conformations in enkephalins

Stereochemical constraints have been introduced into the enkephalin backbone by substituting α-aminoisobutyryl (Aib) residues at positions 2 and 3, instead of Gly. ¹H n.m.r. studies of Tyr-Aib-Gly-Phe-Met-NH₂, Tyr-Aib-Aib-Phe-Met-NH₂ and Tyr-Gly-Aib-Phe-Met-NH₂ demonstrate the occurrence of folded, intramolecularly hydrogen bonded structures in organic solvents. Similar conformations are also favoured in the corresponding t-butyloxycarbonyl protected tetrapeptides, which lack the Tyr residue. A β-turn centred at positions 2 and 3 is proposed for the Aib²-Gly³analog. In the Gly²-Aib³analog, the β-turn has Aib³-Phe⁴as the corner residues. The Aib²-Aib³analog adopts a consecutive β-turn or 3₁₀ helical conformation. High in vivo biological activity is observed for the Aib²and Aib²-Aib³analogs, while the Aib³peptide is significantly less active.