Peptide conformations. 33. Conformational analysis of cyclic enkephalin analogs of the type Tyr-cyclo-(-N ω -Xxx-Gly-Phe-Leu-)*

Four conformationally restricted cyclic enkephalin analogs of the type Tyr-cyclo(-Nω -Xxx-Gly-Phe-Leu-) with Xxx = l -Orn, d -Orn, l -Lys and d -Lys have been synthesized by conventional methods and the conformation of the Bocprotected, the deprotected as well as the des Tyr¹ analogs analysed by proton and nitrogen n.m.r. spectroscopy. The assignments, of the resonances were performed by two-dimensional homo- and heteronuclear correlated spectroscopy in the normal (COSY, SECSY) as well as a modified (for the detection of small couplings) version. NOE difference spectroscopy was used to distinguish the amino acid residues with aliphatic side chains. The n.m.r. parameters suggest a rather rigid conformation of the ornithine analogs with two internal NH protons whereas the lysine peptides appear to be more flexible. The structure of the Orn²-analogs can be described by a Gly³-CO HN-Leu⁵-γⁱ-turn and a hydrogen bond Orn²-CO HN -Orn². The d -lysine compound seems to include a β-turn (Gly³-CO HN-d -Lys²). The relation of the different conformational properties of the four cyclic peptides and their biological activities are briefly discussed.     

Design of peptides with αβ-dehydro residues: Synthesis,crystal structure and molecular conformation of N-Boc-L-Ile-ΔPhe-L-Trp-OCH3

The dehydro-peptide Boc-L-Ile-ΔPhe-L-Trp-OCH₃ was synthesized by the azlactone method in the solution phase. The peptide was crystallized from methanol in an orthorhombic space group P2₁2₁2₁ with a = 10.777(2), b= 11.224(2), c= 26.627(10) Å. The structure was determined by direct methods and refined to an R value of 0.069 for 3093 observed reflections [l≥ 2σ(l)].The peptide failed to adopt a folded conformation with backbone torsion angles: φ₁, = 90.8(8)°, ψ₁= -151.6(6)°, φ₂= 89.0(8)°, ψ₂= 15.9(9)°, φ₃= 165.7(7)°, ψ^T₃= -166.0(7)°. A general rule derived from earlier studies indicates that a three-peptide unit sequence with a ΔPhe at the (i+ 2) position adopts a β-turn II conformation. Because the branched β-carbon residues such as valine and isoleucine have strong conformational preferences, they combine with the ΔPhe residue differently to generate a unique set of conformations in such peptides. The presence of β-branched residues simultaneously at both (i+ 1) and (i+ 3) positions induces unfolded conformations in tetrapeptides, but a β-branched residue substituted only at (i+ 3) positron can not prevent the formation of a folded β-turn II conformation. On the other hand, the present structure shows that a β-branched residue substituted at the (i+ 1) position prevents the formation of a β-turn II conformation. These observations indicate that a β-branched residue at the (i+ 1) position prevents a folded conformation whereas it cannot generate the same degree of effect from the (i+ 3) position. This may be because of the trans disposition of the planar ΔPhe side-chain with respect to the C=O group in the residue. The molecules are packed in an anti-parallel manner to generate N₂-H₂…O₂ (-x,y-1/2, -z+ 3/2) and N^ε1₃-H^ε1₃…O₁(-x,y -1/2, -z+ 3/2) hydrogen bonds.     

The evaluation of type I and type II β-turn mixtures

Circular dichroism (CD) and¹ H-{¹H}NOE spectra were obtained for Piv-Pro-Ser-NHCH₃(1),[Piv-(CH₃)₃-C-CO], Boc-Pro-Ser-NHCH₃ (2) and Boc-Val-Ser-NHCH₃ (3), to determine the solution conformation of these p-turn models. In the crystal, 1 and 3 adopt an ideal type I β-turn, while 2 is characterized by a semifolded backbone geometry incorporating a cis Boc-Pro tert-amide bond. The predominance of a β-turn conformation in solution was suggested for models 1-3 on the basis of ¹H-{¹H}NOE data. In a nonpolar solvent the prevailing trans rotamer form (>80%) of 2 has a β-turn conformation according to heteronuclear NOE measurement. Positive ¹H-{¹H} NOEs were detected between the H^α(Pro)/NH(Ser), Hα(Ser)/NH(Ser) and NH(NHCH₃3)/HN(Ser) protons in the trans Boc-Pro rotamer form of 2 at -20° in CDCl₃. Similar positive homonuclear NOE enhancements were also observed on the appropriate proton signals in other models, such as Boc-Val-Ser-NHCH₃ (3). Boc-Val-D-Ser-NHCH₃ (4) and Boc-Pro-D-Ser-NHCH₃ (5), in various solvents. The ¹H- {¹H)NOE experiments carried out in CD₃CN clearly showed that besides the type I (or III) β-turn structure, one of the main conformations of models 1-5 is close to the type II β-turn backbone geometry in a nonpolar solvent. Unexpectedly, the conformational mixture of models 1-3 were characterized by class C (helix-like) CD spectra, although class C spectra are generally only correlated with the type I β-turn conformation. These acyclic models are the first carefully investigated examples of -L-L- triamide systems, containing a significant amount of a type II β-turn, as well as the type I p-turn and, however, yielding a class C circular dichroism spectra. The CD spectra recorded for 3 and 4 in acetonitrile were ‘calibrated’ using the ¹H-{¹H}NOE data. Such a “calibration”, as well as the semi-quantitative CD and NMR comprehensive analyses, demonstrated that class C, class B, as well as class C’ CD spectra may be obtained from the linear combination of the same two-component spectra, with different conformational weights. Therefore, it is suggested that the extraction of the conformational components of such models, simply on the basis of their CD spectra, must be made with caution.     

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.     

Motifs and conformational analysis of amino acid residues adjoining β-turns in proteins

Using a data set of 250 non-homologous high-resolution globular proteins, a systematic analysis of the conformations that precede and succeed (positions i and i+3) the various classical β-turn types has been carried out. The collective conformation of a specific β-turn type, including the flanking positions, termed motif, has been studied. In all the four turn types, the majority of examples are preceded and succeeded by extended conformation. Some of the other observations are: (1) In a type I β-turn, Gly at position i+ 3 has a higher favorability to occur with positive ø and does not prefer the major motif βα_R-α_R-β. (2) The left-handed alpha;-helical conformation (alpha;_L) is not preferred at both the flanking positions for type I'and II β-turns, (3) The β–β motif is favourable for all the turn types and the motif β–α_L very highly favourable for type I. © Munksgaard 1996.     

THE SOLUTION CONFORMATION OF THE FERRICHROMES.

Ferrichrome, ferricrocin and ferrichrysin are ferric cyclohexapeptides whose general primary composition is represented by Res³-Res²-Gly¹-Orn³-Orn²-Orn¹, where the Res^2,3 sites are occupied by glycyl or l -seryl residues and Orn^1,2,3 stands for δ-N-acetyl-δ-N-hydroxy-l -ornithyl. The 220 MHz proton magnetic resonance spectra of deferriferricrocin and deferriferrichrysin in aqueous and in deuterodimethyl sulfoxide solutions are reported and discussed in terms of the molecular conformations. In (CD₃)₂SO the chemical shift temperature dependences of the amide proton resonances suggest two-fold symmetric structures containing two intramolecular hydrogen bonds. Such hydrogen-bonding, and other features of the data, are consistent with antiparallel β-pleated sheet structures, as in the Schwyzer model for cyclohexapeptides. In terms of sites paired by the hydrogen bonds, however, the β-fold differs. In water a random conformation is suggested for these peptides.     

Peptides related to the active fragment of “proline rich polypeptide”, an immunoregulatory protein of the ovine colostrums

The preferred solution conformation of the PRP-hexapeptide (Tyr-Val-Pro-Leu-Phe-Pro) and of some of its structural analogues was investigated by NMR- spectroscopy, spectrofluorimetry and computer simulation technic. It was found that the preferred conformation is characterized by cis′-conformation of Pro³ and the γ-turn on the Leu⁴-residue: for Val² and Phe⁵ a β-structure seems to be privileged. In such a conformation Val² and Leu⁴ residues occupy exactly the same positions in space as residues i and i+ 3 in an α-helix. It suggests that the PRP-hexapeptide can interact with receptor protein inducing or stabilizing its helical conformation by “knobs into holes” packing.     

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).     

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.     

CD-n.m.r. study of the solution conformation of bradykinin analogs containing α-aminoisobutyric acid

The conformation in aqueous solution of several α-aminoisobutyric acid (AIB)-containing analogs of bradykinin (BK) has been probed by complementary CD and ¹H n.m.r. measurements. The conclusion reached is that substitution of AIB for Pro² and/or Pro³ in BK stabilizes a degree of β-turn conformation in the N-terminal tetrapeptide moiety of the resulting analogs. Changing the solvent from water to DMSO or TFE further enhances the contribution of particular hydrogen bonded structures to the time-averaged conformation of these peptides. Bradykinin and [AIB⁷]-BK adopt similar hydrogen bonded conformations in TFE, apparently with a contribution from a β-turn involving their common Arg¹-Pro²-Pro³-Gly⁴ moiety. The contrasting biological activities of BK and its AIB-analogs are considered in terms of the conformational analogy between the AIB-residue and cis¹ Pro and the propensity for a β-turn at the N-terminus of the peptide.     

Stereochemistry of peptides containing 1-aminocycloheptane-1-carboxylic acid (Ac7c)

The crystal structures of four peptides incorporating l-aminocycloheptane-l-carboxylic acid (Ac₇c) are described. Boc-Aib-Ac₇c-NHMe and Boc-Pro-Ac₇c-Ala-OMe adopt β-turn conformations stabilized by an intramolecular 4 × 1 hydrogen bond, the former folding into a type-I/III β-turn and the latter into a type-II β-turn. In the dipeptide esters, Boc-Aib-Ac₇c-OMe and Boc-Pro-Ac₇c-OMe, the Ac₇c and Aib residues adopt helical conformations, while the Pro residue remains semi-extended in both the molecules of Boc-Pro-Ac₇c-OMe found in the asymmetric unit. The cycloheptane ring of Ac₇c residues adopts a twist-chair conformation in all the peptides studied. ¹H-NMR studies in CDCl₃ and (CD₃)₂SO and IR studies in CDCl₃, suggest that Boc-Aib-Ac₇c-NHMe and Boc-Pro-Ac₇c-Ala-OMe maintain the β-turn conformations in solution.     

Conformational investigation of α,β-dehydropeptides VII*. Conformation of Ac-Pro-ΔAla-NHCH3 and Ac-Pro-(E)-ΔAbu-NHCH3: comparison with (Z)-substituted α,β-dehydropeptides†

The crystal structure and solution conformation of Ac-Pro-ΔAla-NHCH₃ and the solution conformation of Ac-Pro-(E)-ΔAbu-NHCH₃ were investigated by X-ray diffraction method and NMR, FTIR and CD spectroscopies. Ac-Pro-ΔAla-NHCH, adopts an extended-coil conformation in the crystalline state, with all-trans peptide bonds and the ΔAla residue being in a C₅ form, φ₁=– 71.4(4), ψ₁=– 16.8(4), φ₂=– 178.4(3) and ψ₂= 172.4(3)°. In inert solvents the peptide also assumes the C₅ conformation, but a γ-turn on the Pro residue cannot be ruled out. In these solvents Ac-Pro-(E)- ΔAbu-NHCH₃ accommodates a βII-turn, but a minor conformer with a nearly planar disposition of the CO—NH and C=C bonds (φ₂～0°) is also present. Previous spectroscopic studies of the (Z)-substituted dehydropeptides Ac-Pro-(Z)- ΔAbu-NHCH, and Ac-Pro-ΔVal-NHCH₃ reveal that both peptides prefer a βII-turn in solution. Comparison of conformations in the family of four Ac-Pro-ΔXaa-NHCH₃ peptides let us formulate the following order of their tendency to adopt a β-turn in solution: (Z)- ΔAbu > (E)- δAbu > ΔVal; ΔAla does not. None of the folded structures formed by the four compounds is stable in strongly solvating media. © Munksgaard 1996.     

Synthesis and solution conformation of c(D-Trp-D-Cys(SO3-Na+)-Pro-D-Val-Leu), a potent endothelin-A receptor antagonist

The endothelin family of polypeptides are known to exert potent physiological effects which include cardiovascular regulation. The solution conformation and dynamics of c(D-Trp-D-Cys(SO₃-Na⁺)-Pro-D-Val-Leu), a potent endothelin-A receptor-selective antagonist, were characterized in aqueous solution by NMR spectroscopy and molecular modeling. NMR-derived conformational constraints were combined with computer-assisted molecular modeling using distance geometry calculations and energy minimization. The pentapeptide backbone is shown to adopt a single conformation in solution comprising a type II β-turn and an inverse γ-turn, with each residue in the trans conformation. Molecular dynamics were explored using relaxation measurements and low-temperature studies, and indicate that the peptide backbone is highly constrained with little conformational mobility present.     

Synthesis, conformation analysis, and anxiolytic activity of retropeptide analogs of 4-cholecystokinin

Potential dipeptide anxiolytics with the general formula Ph(CH₂)₅ CO-X-L-Trp-NH₂(X = Gly, β-Ala, GABA, L-Pro) have been synthesized on the basis of the 4-cholecystokinin structure. These compounds exhibit anxiolytic activity in the elevated plus-maze test in rats at doses of 0.01–0.5 mg/kg (i.p.). The activity among these derivatives increases in the following order: GABA → β-Ala → Gly → Pro. Conformation analysis of dipeptides in solution by ¹H NMR method with the use of the nuclear Overhauser effect and peptide vicinal ³J(H-NC^α-H) coupling constants has been carried out. It is established that the percentage of βI-turn conformation stabilized by hydrogen bonds with the participation of C-end amide group proton also increases in the order β-Ala → Gly → Pro derivatives. It is concluded that the βI-turn conformation is probably the biologically active one in the synthesized substituted dipeptides. __________ Translated from Khimiko-Farmatsevticheskii Zhurnal, Vol. 40, No. 7, pp. 21–26, July, 2006.     

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.     

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.     

Conformations of peptides containing 1-aminocyclohexanecarboxylic acid (Acc6). Crystal structures of two model peptides

The crystal structures of two peptides containing 1-aminocyclohexanecarboxylic acid (Acc⁶) are described. Boc-Aib-Acc⁶-NHMe · H₂O adopts a β-turn conformation in the solid state, stabilized by an intramolecular 4 → 1 hydrogen bond between the Boc CO and methylamide NH groups. The backbone conformational angles (φ_Aib = – 50.3°, ψ_Aib = – 45.8°; φ_Acc6 = – 68.4°, ψ_Acc6 = – 15°) lie in between the values expected for ideal Type I or III β-turns. In Boc-Aib-Acc⁶-OMe, the Aib residue adopts a partially extended conformation (φ_Aib = – 62.2°, ψ_Aib = 143°) while the Acc⁶residue maintains a helical conformation (φ_Acc6 = 48°, ψ_Acc⁶= 42.6°). ¹H n.m.r. studies in CDCl₃ and (CD₃)₂SO suggest that Boc-Aib-Acc⁶-NHMe maintains the β-turn conformation in solution.     

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.     

NMR analysis of a potent decapeptide agonist of human C5a anaphylatoxin

A NMR investigation in H₂0, TFE and DMSO of a conformationally constrained, potent decapeptide agonist of human C5a, YSFKDMPLaR (C5a₆₅-₇₄, Y65, F67, P71, d -Ala73) showed that its N-terminal region (YSFKD) exhibited an extended backbone conformation in H₂O and a more twisted conformation in both TFE/H₂O (30:70, v/v; referred to as TFE) and DMSO. The C-terminal region (MPLaR) of the peptide adopted compact, turn-like structures. In H₂O, the C-terminal region adopted a type II β-turn or a distorted type V/II β-turn involving residues PLaR. In the distorted type V/II β-turn, Leu72 exhibited a conformation typical of a type V β-turn, whereas D -Ala73 exhibited a conformation typical of a type II β-turn. The distorted type V/II β-turn overlapped with an inverse γ-turn involving residues MPL. In DMSO, the C-terminal region had the analogous inverse y-turn and the V/II γ-turn found in H₂O. In many of the DMSO structures, two inverse γ-turns in the MPL and PLa positions formed a double-inverse γ-turn. None of the turns observed in H₂O were present in TFE. However, in TFE, the PLa residues formed an inverse γ-turn. Overall, the turn-like structural motifs in the C-terminal region of the peptide in both H₂O and DMSO (but not in TFE) agreed with the biologically important conformations obtained earlier by the structure-function analysis of a panel of C5a agonist peptides. These motifs may represent key structural elements important for C5a agonist activity and may be used to design the next generation of C5a agonist and antagonist analogues. © Munksgaard 1998.     

Solution structure of a biologically active cyclic LDV peptide analogue containing a type II′β-turn mimetic

The solution structure of cyclo-[Gly-Leu-Asp-Val-BTD] (BTD=β-turn dipeptide) has been determined by two-dimensional ¹H-NMR (nuclear magnetic resonance) spectroscopy and systematic conformational searching combined with molecular dynamics studies. The structure contains two hydrogen bonds between the Gly and Val residues, and a type I β-turn with Leu and Asp at the (i+ 1) and (i+ 2) positions of the turn. The cyclic compound shows activity in a scintillation proximity assay (SPA) for the inhibition of the interaction between the integrin α₄β₁ and vascular cell adhesion molecule-1 (VCAM-1). The structure-activity relationship of the LDV sequence is discussed. © Munksgaard 1996.