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.     

Influence of hydrophobic interactions on the conformational adaptability of the β‐Ala residue

Abstract: The chemical synthesis and X‐ray crystal structure analysis of a model peptide incorporating a conformationally flexible β‐Ala residue: Boc‐β‐Ala‐Pda, 1 (C₂₃H₄₆N₂O₃: molecular weight = 398.62) have been described. The peptide crystallized in the crystal system triclinic with space group P2₁: a = 5.116(3) Å, b = 5.6770(10) Å, c = 21.744(5) Å; α = 87.45°, β = 86.87°, γ = 90.0°; Z = 1. An attractive feature of the crystal molecular structure of 1 is the induction of a reasonably extended backbone conformation of the β‐Ala moiety, i.e. the torsion angles φ ≈ ?115°, µ ≈ 173° and ψ ≈ 122°, correspond to skew^?, trans and skew⁺ conformation, respectively, by an unbranched hydrophobic alkyl chain, Pda, which prefers an all‐anti orientation (θ₁ ≈ ?153°, θ₂ ≈ … θ₁₄ ≈ ±178°). The observation is remarkable because, systematic conformational investigations of short linear β‐Ala peptides of the type Boc‐β‐Ala‐Xaa‐OCH₃ (Xaa = Aib or Acc⁶) have shown that the chemical and stereochemical characters of the neighboring moieties may be critical in dictating the overall folded and/or unfolded conformational features of the β‐Ala residue. The overall conformation of 1 is typical of a ‘bar’. It appears convincing that, in addition to a number of hydrophobic contacts between the parallel arranged molecules, an array of conventional N‐H…O=C intermolecular H‐bonding interactions stabilize the crystal molecular structure. Moreover, the resulting 14‐membered pseudo‐ring motif, generated by the amide–amide interactions between the adjacent molecules, is completely devoid of nonconventional C?H…O interaction. The potentials of the conformational adaptation of the β‐Ala residue, to influence and stabilize different structural characteristics have been highlighted.     

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.     

Role of N‐t‐Boc group in helix initiation in a novel tetrapeptide

Abstract: Protecting groups in N‐ and C‐terminal positions play a decisive role in the conformational preference of smaller peptides. Conformational analysis of tetrapeptide derivatives containing Ala, Ile and Gly residues was performed. Peptide 1 , Boc‐Ala‐Ile‐Ile‐Gly‐OMe (Boc: tert‐butyloxycarbonyl) has a predominantly helical turn conformation in all the alcoholic solvents studied, whereas in the solid state it has a β‐sheet conformation. In contrast, peptide 2 , Ac‐Ala‐Ile‐Ile‐Gly‐OMe (Ac: acetyl) has a random coil conformation in solution. The FTIR spectrum of peptide 1 shows a lower frequency of urethane carbonyl, indicating involvement of the carbonyl group in hydrogen bonding in the helical turn.     

Conformational investigation of α, β-dehydropeptides

The crystal structure of Ac-Pro-ΔVal-NHCH₃ was examined to determine the influence of the α,β-dehydrovaline residue on the nature of peptide conformation. The peptide crystallizes from methanol-diethyl ether solution at 4° in needle-shaped form in orthorhombic space group P2₁2₁2₁ with a= 11.384(2) Å, b = 13.277(2) Å, c = 9.942(1) Å. V = 1502.7(4) ų Z = 4, D_m= 1.17 g cm^?3 and D_c=1.18 g cm^?3 The structure was solved by direct methods using SHELXS-86 and refined to an R value of 0.057 for 1922 observed reflections. The peptide is found to adopt a β-bend between the type I and the type III conformation with φ₁=?68.3(4)°, ψ₁=? 20.1(4)°, φ₂=?73.5(4)°= and Ψ₂=?14.1(4)°=. An intramolecular hydrogen bond between the carbonyl oxygen of ith residue and the NH of (i+ 3)th residue stabilizes the β-bend. An additional intermolecular N.,.O hydrogen bond joins molecules into infinite chains. In the literature described crystal structures of peptides having a single α,β-dehydroamino acid residue in the (i+ 2) position and forming a β-bend reveal a type II conformation.     

Synthetic protein design: construction of a four‐stranded β‐sheet structure and evaluation of its integrity in methanol–water systems

Abstract: The characterization of a four‐stranded β‐sheet structure in a designed 26‐residue peptide Beta‐4 is described. The sequence of Beta‐4 (Arg‐Gly‐Thr‐Ile‐Lys‐^Dpro‐Gly‐Ile‐Thr‐Phe‐Ala‐^DPro‐Ala‐Thr‐Val‐Leu‐Phe‐Ala‐Val‐^DPro‐Gly‐Lys‐Thr‐Leu‐Tyr‐Arg) was chosen such that three strategically positioned ^DPro‐Xxx segments nucleate type II′β‐turns, which facilitate hairpin extension. A four‐stranded β‐sheet structure is determined in methanol from 500 MHz ¹H NMR data using a total of 100 observed NOEs, 11 dihedral restraints obtained from vicinal J_CαH‐NH values and 10 hydrogen bonding constraints obtained from H/D exchange data. The observed NOEs provide strong evidence for a stable four‐stranded sheet and a nonpolar cluster involving Ile⁸, Phe¹⁰, Val¹⁵ and Phe¹⁷. Circular dichroism studies in water–methanol mixtures provide evidence for melting of the β‐sheet structure at high water concentrations. NMR analysis establishes that the four‐stranded sheet in Beta‐4 is appreciably populated in 50% (v/v) aqueous methanol. In water, the peptide structure is disorganized, although the three β‐turn nuclei appear to be maintained.     

Structural analysis of fertilinβ cyclic peptide mimics that are ligands for α6β1 integrin

Abstract: The NMR structural analysis of two fertilinβ mimics cyclo(EC²DC¹)YNH_2, 1 , and cyclo(D²EC²D¹C¹)YNH_2, 2 is described. Both of these mimics are moderate inhibitors of sperm?egg binding with IC₅₀ values of 500 µm in a mouse in vitro fertilization assay. For peptide 1 , the optimized conformations that best match the NMR data have a pseudo‐type II′β‐turn with the linker and Glu at the i+1 and i+2 positions, respectively. The EC²D¹C¹ sequence is in a nonclassical (type IV) β‐turn. For peptide 2 , the conformation that best matches the NMR data has two turns: a pseudo‐type II′β‐turn in the D²EC²D¹ sequence followed by a nonclassical β‐turn in the EC²D¹C¹ sequence. The Cβ?Cβ distance between E and D¹ in peptide 1 is 9.1 Å, in peptide 2 , it is 7.7 Å. Thus, one possibility for the high IC₅₀ values of these cyclic peptides is that the acidic residues are not constrained to a sufficiently tight turn, and thus much entropy must still be lost upon binding to the α₆β₁ integrin. This explains why the cyclic peptides are the same as linear peptides at inhibiting sperm?egg binding.     

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).     

Synthesis of dermorphin‐(1–4) derivatives catalyzed by proteases in organic solvents*

Abstract: In order to extend the use of proteases to organic synthesis and seek the rules of enzymatic reactions in organic media, we focused on unnatural substrates for proteases to form amide bonds. In this paper, the study of unnatural substrates containing d ‐amino acid residue, which act as acyl acceptors as well as acyl donors for proteases in organic media, is reported. Dermorphin is a heptapeptide (H‐Tyr‐d ‐Ala‐Phe‐Gly‐Tyr‐Pro‐Ser‐NH₂) with potent analgesic activity. The N‐terminal tetrapeptide is the minimum sequence that retains dermorphin activity, and is selected as the model compound in our study. Two dermorphin‐(1–4) derivatives, Boc‐Tyr‐d ‐Ala‐Phe‐Gly‐N₂H₂Ph and Boc‐Tyr‐d ‐Ala‐Phe‐Gly‐NH₂, which contained a d ‐amino acid residue, were synthesized by proteases in organic media for the first time. The synthesis of these two dermorphin‐(1–4) derivatives could be catalyzed by subtilisin with Boc‐Tyr‐d ‐Ala‐OCH₂CF₃ as an acyl donor substrate in AcOEt. The synthesis of dermorphin‐(1–2) derivative Boc‐Tyr‐d ‐Ala‐N₂H₂Ph was catalyzed by α‐chymotrypsin in different organic solvents and d ‐Ala‐N₂H₂Ph was used as an acyl acceptor substrate. Factors influencing the above enzymatic reactions were systematically studied.     

β‐turn formation by a six‐residue linear peptide in solution

Abstract: A model peptide AAGDYY‐NH₂ (B1), which is found to adopt a β‐turn conformation in the TEM‐1 β‐lactamase inhibitor protein (BLIP) in the TEM‐1/BLIP co‐crystal, was synthesized to elucidate the mechanism of its β‐turn formation and stability. Its structural preferences in solution were comprehensively characterized using CD, FT‐IR and ¹H NMR spectroscopy, respectively. The set of observed diagnostic NOEs, the restrained molecular dynamics simulation, CD and FT‐IR spectroscopy confirmed the formation of a β‐turn in solution by the model peptide. The dihedral angles [(φ₃, ?₃) (φ₄, ?₄)] of [(?52°, ?32°) (?38°, ?44°)] of Gly‐Asp fragment in the model peptide are consistent with those of a type III β‐turn. In a conclusion, the conformational preference of the linear hexapeptide B1 in solution was determined, and it would provide a simple template to study the mechanism of β‐turn formation and stability.     

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.     

Synthesis,crystal structure and molecular conformation of Nα‐Boc‐l‐leucyl‐(Z)‐β‐(3‐pyridyl)‐α,β‐ dehydroalanyl‐l‐leucine methyl ester*

Abstract: A protected tridehydropeptide containing (Z)‐β‐(3‐pyridyl)‐α,β‐dehydroalanine (Δ^Z3Pal) residue, Boc‐Leu‐Δ^Z3Pal‐Leu‐OMe ( 1 ), was synthesized via Erlenmeyer azlactone method. X‐ray crystallographic analysis revealed that the peptide 1 adopts an extended conformation, which is similar to that of a Δ^ZPhe analog, Boc‐Leu‐Δ^ZPhe‐Leu‐OMe ( 2 ).     

Ac10c: a medium‐ring,cycloaliphatic Cα,α‐disubstituted glycine. Incorporation into model peptides and preferred conformation

Abstract: Two complete series of N‐protected oligopeptide esters to the pentamer level from 1‐amino‐cyclodecane‐1‐carboxylic acid (Ac₁₀c), an α‐amino acid conformationally constrained through a medium‐ring C^α_i ? C^α_i cyclization, and either the l ‐Ala or Aib residue, along with the N‐protected Ac₁₀c monomer and homo‐dimer alkylamides, were synthesized using solution methods and fully characterized. The preferred conformation of these model peptides was assessed in deuterochloroform solution using FT‐IR absorption and ¹H NMR techniques. Furthermore, the molecular structures of two derivatives (Z‐Ac₁₀c‐OH and Fmoc‐Ac₁₀c‐OH) and two peptides (the dipeptide ester Z‐Ac₁₀c‐l ‐Phe‐OMe and the tripeptide ester Z‐Aib‐Ac₁₀c‐Aib‐OtBu) were determined in the crystal state using X‐ray diffraction. The experimental results support the view that β‐bends and 3₁₀‐helices are preferentially adopted by peptides rich in Ac₁₀c, the third largest cycloaliphatic C^α,α‐disubstituted glycine known. This investigation allowed us to complete a detailed conformational analysis of the whole 1‐amino‐cycloalkane‐1‐carboxylic acid (Ac_nc, with n = 3–12) series, which represents the prerequisite for our recent proposal of the ‘Ac_nc scan’ concept.     

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.     

Isopeptide bonds in chemotactic tripeptides. Synthesis and activity of lysine‐containing fMLF analogs

Abstract: In order to explore the properties of chemotactic N‐formylpeptides containing isopeptide bonds within their backbones, a group of lysine‐containing analogs of the prototypical chemotactic tripeptide N‐formylmethionyl‐leucyl‐phenylalanine (fMLF) was synthesized. The new analogs were designed by adding to the HCO‐Met or Boc‐Met residue a dipeptide fragment made up of Lys and Phe residues joined through Lys N^α or N^ε bonds, in all possible combinations. Thus, the following six pairs of tripeptides were synthesized and examined for their bioactivity: RCO‐Met‐Lys(Z)‐Phe‐OMe ( 2a , b ), RCO‐Met‐Lys(Z‐Phe)‐OMe ( 3a , b ), Z‐Lys(RCO‐Met)‐Phe‐OMe ( 4a , b ), Z‐Phe‐Lys(RCO‐Met)‐OMe ( 5a , b ), RCO‐Met‐Phe‐Lys(Z)‐OMe ( 6a , b ) and Z‐Lys(RCO‐Met‐Phe)‐OMe ( 7a , b ), with R=OC(CH₃)₃ and R=H for compounds a and b , respectively. All the new models were characterized fully and their activity (chemotaxis, superoxide anion production and lysozyme release) on human neutrophils determined as agonists (compounds b ) and antagonists (compounds a ). All N‐formyl derivatives 2b ? 7b are less potent than fMLF‐OMe as chemoattractants, but compound 7b exhibits selective activity as superoxide anion producer. Derivatives 2a ? 7a do not show antagonistic activity towards fMLF induced chemotaxis and O₂^? production, however, all these compounds except 4a antagonize lysozyme release by 60%.     

Reactivity toward deamidation of asparagine residues in β‐turn structures

Mimetics of β‐turn structures in proteins have been used to calibrate the relative reactivities toward deamidation of asparagine residues in the two central positions of a β‐turn and in a random coil. N‐Acetyl‐Asn‐Gly‐6‐aminocaproic acid, an acyclic analog of a β‐turn mimic undergoes deamidation of the asparaginyl residue through a succinimide intermediate to generate N‐acetyl‐Asp‐N‐Gly‐6‐aminocaproic acid (6‐aminocaproic acid, hereafter Aca) and N‐acetyl‐l ‐iso‐aspartyl (isoAsp)‐Gly‐Aca (pH 8.8, 37 °C) ≈ 3‐fold faster than does the cyclic β‐turn mimic cyclo‐[L‐Asn‐Gly‐Aca] with asparagine at position 2 of the β‐turn. The latter compound, in turn, undergoes deamidation ≈ 30‐fold faster than its positional isomer cyclo‐[Gly‐Asn‐Aca] with asparagine at position 3 of the β‐turn. Both cyclic peptides assume predominantly β‐turn structures in solution, as demonstrated by NMR and circular dichroism characterization. The open‐chain compound and its isomer N‐acetyl‐Gly‐Asn‐Aca assume predominantly random coil structures. The latter isomer undergoes deamidation 2‐fold slower than the former. Thus the order of reactivity toward deamidation is: asparagine in a random coil ≈ 3× asparagine in position 2 of a β‐turn ≈ 30× asparagine in position 3 of a β‐turn.     

Aromatic interactions in tryptophan‐containing peptides: crystal structures of model tryptophan peptides and phenylalanine analogs*

Abstract: The crystal structures of the peptides, Boc‐Leu‐Trp‐Val‐OMe ( 1) , Ac‐Leu‐Trp‐Val‐OMe ( 2a and 2b), Boc‐Leu‐Phe‐Val‐OMe ( 3 ), Ac‐Leu‐Phe‐Val‐OMe ( 4 ), and Boc‐Ala‐Aib‐Leu‐Trp‐Val‐OMe ( 5 ) have been determined by X‐ray diffraction in order to explore the nature of interactions between aromatic rings, specifically the indole side chain of Trp residues. Peptide 1 adopts a type I β‐turn conformation stabilized by an intramolecular 4→1 hydrogen bond. Molecules of 1 pack into helical columns stabilized by two intermolecular hydrogen bonds, Leu(1)NH…O(2)Trp(2) and IndoleNH…O(1)Leu(1). The superhelical columns further pack into the tetragonal space group P4₃ by means of a continuous network of indole–indole interactions. Peptide 2 crystallizes in two polymorphic forms, P2₁ ( 2a ) and P2₁2₁2₁ ( 2b ). In both forms, the peptide backbone is extended, with antiparallel β‐sheet association being observed in crystals. Extended strand conformations and antiparallel β‐sheet formation are also observed in the Phe‐containing analogs, Boc‐Leu‐Phe‐Val‐OMe ( 3 ) and Ac‐Leu‐Phe‐Val‐OMe ( 4 ). Peptide 5 forms a short stretch of 3₁₀‐helix. Analysis of aromatic–aromatic and aromatic–amide interactions in the structures of peptides, 1 , 2a , 2b are reported along with the examples of 14 Trp‐containing peptides from the Cambridge Crystallographic Database. The results suggest that there is no dramatic preference for a preferred orientation of two proximal indole rings. In Trp‐containing peptides specific orientations of the indole ring, with respect to the preceding and succeeding peptide units, appear to be preferred in β‐turns and extended structures.     

The linked φψ chain plot for visual comparison of the backbone conformation of peptides and proteins

A new graphic method is described for presenting in two dimensions the φ and ψ dihedral angles that describe the backbone conformation of a peptide or protein chain. For each residue in sequence, φ and ψ are plotted as dots on the y-axis above the next two points on the x-axis representing the residue number. Each dot is linked to the next dot by a slanting line segment (link) and each cis-peptide bond (ω～0°) between residues X and Y is indicated by marking dots ψx and φy with a diamond. This linked φ and ψ chain plot is more useful than an unlinked φ and ψ chain plot for visually recognizing helices, sheets and turns and for graphically comparing several protein structures. Overlaying the linked φ and ψ chain plots for 15 β-hairpins classified as type-I' β-turns revealed that three were significantly different from the rest. The dihedral angles (mean f standard deviation) of the loop residues (L1, L2) for a cluster of 12 β-hairpins with an inverse-common, type-I′β-turn (φ_L1= 52±7°, ψ_L1=40±8°, φ_L2=80±9°, ψ_L2= -1±13°) are similar to the standard dihedral angles for the type-1′ turn (60, 30, 90 and 0°, respectively).     

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.     

Design of peptides: synthesis,crystal structure and molecular conformation of N-Boc-L-Val-δPhe-L-Ile-OCH3

The dehydro-peptide Boc-L-Val-δPhe-L-Ile-OCH₃ was synthesized by the azlactone method in the solution phase. The peptide crystallized from a methanol/dimethyl sulfoxide (95:5) mixture in space group P6₁, with a=b= 15.312(1), c= 22.164(5) Å. The structure was determined by direct methods and refined to an R value of 0.098 for 1589 observed reflections [I≥ 1.5 σ(I)]. The peptide adopts an S-shaped conformation with torsion angles: ø₁=-127(1), ψ₁= -44(1), ø₂, = 67(1), ψ₂, = 37(1), ø₃,=-82(1)°. The side-chain torsion angles in δPhe of X¹₂= 1(2), X^2.1₂= 7(2) and X_2.22 = 177(1)° indicate that the δPhe residue is essentially planar. In valyl residue the two side-chain torsion angles are X¹₁= -65(1) and X²₁= 177(1), whereas the torsion angles in Ile are X^1,1₃= 72(2), X^1,2₃= -159(2), X²₃= 150(2)°. This is the first peptide which does not adopt a folded conformation for a sequence with a δPhe at the (i+ 2) position. The molecular packing in the crystals is stabilized by several hydrogen bonds: N₁-H₁?O₁’= 2.77(1) Å, N₂-H₂?O₁’= 2.95(1) Å, N₃-H₃?O₂=2.85(1) Å and a possible weak interaction N₂-H₂?O₁’3.29(1) Å- within the columns of molecules along the c-axis and van der Waals forces between the columns. © Munksgaard 1996.