Conformation d'hétéropeptides à l'aide de la spectrométrie Raman

The conformational analysis of some heteropeptides Ala_x Pro Ala_y (x = 2 or 3; y = 1, 2 or 3) has been investigated using Raman spectrometry. Vibrational assignments of characteristic amide I and III bands are made for C⁵, C⁷, C⁷C⁵, β turn conformations. Some characteristic end-group vibrations are listed also. The proline group has been shown to induce a β turn in a peptide chain. Raman speclroscopy can give the molecular conformation of solid oligopeptides with only 0.3–0.4 mg of sample.     

A linear free-energy correlation in the low-energy tandem mass spectra of protonated tripeptides Gly–Gly–Xxx

The backbone cleavages of protonated tripeptide ions of the series Gly—Gly—Xxx, where Xxx ? Gly, Ala, Val, d-Leu, l-Leu, Ile, Phe, Tyr, Trp, Pro, Met and Glu, were studied in a hybrid tandem mass spectrometer. C-Terminal y-type ions and N-terminal a- and b-type ions were noted. A linear relationship between log (y₁/b₂) and the proton affinity of the C-terminal amino acid substituents was found: as the proton affinity of the C-terminal residue increases, the fraction of y₁ ion formation increases. When the C-terminal substituent was more basic than Trp, the b₂ ion was not observed. It is likely that the site of protonation changes from peptide bond to side-chain for just these residues, Lys, His and Arg.     

Influence of Dab2 and Pro3 configuration of [Leu]-enkephalins on the interactions with β-cyclodextrin studied by fluorescence spectroscopy, microcalorimetry and 1H NMR spectroscopy

Natural enkephalins and their analogues are very important as potential therapeutic agents (analgetics). Herein we describe the influence of Dab and Pro chirality of cyclic [Leu]enkephalins (X¹-c[Dab²-Pro³-βNal(2)⁴-Leu⁵], where X = Tyr or Phe) on the binding constant with β-cyclodextrin and spatial and mutual orientation of guest and host molecules. The formation of complexes is enthalpy driven for all cyclic [Leu]enkephalins studied as well as for Nal and AcNalNH₂. Moreover, change of Dab residue configuration has a greater influence on changes of the binding constant of cyclic enkephalin with β-CD than change of Pro chirality has. Also, the replacement of Tyr¹ residue by Phe¹ substantially changes the peptide chain conformation. An analysis of 2D NMR spectra reveals that, apart from inclusion complex formed by penetration of cyclodextrin cavity from wider and narrow rims by Nal, Tyr or Phe or Leu residue, a side and/or bottom association complexes are formed.     

Boc–Pro–Hyp–Gly–OBzl and Boc–Ala–Hyp–Gly–OBzl,two repeating triplets found in collagen

The protected tripeptides benzyl N‐{2‐[N‐(tert‐butoxy­carbon­yl)­prol­yl]‐4‐hydroxy­prol­yl}glycinate or Boc–Pro–Hyp–Gly–OBzl, C₂₄H₃₃N₃O₇, and benzyl N‐{2‐[N‐(tert‐butoxy­carbon­yl)­alan­yl]‐4‐hydroxy­prol­yl}glycinate or Boc–Ala–Hyp–Gly–OBzl, C₂₂H₃₁N₃O₇, are the minimum repeating triplets found in collagen. Within the crystal structure of each are two independent peptide mol­ecules with similar structures. The peptides are arranged anti­parallel to one another and inter­act through hydrogen bonds involving the main chains and the 4‐hydroxy­prolyl groups. The structures exhibit characteristics of a triple helix, but the peptides tend to assume a sheet‐like structure.     

N‐(tert‐Butoxycarbonyl)‐O‐allyl‐l‐seryl‐α‐aminoisobutyryl‐l‐valine methyl ester: a protected tripeptide with an allylated serine residue

The title compound [systematic name (6S,12S)‐methyl 6‐(allyloxymethyl)‐12‐isopropyl‐2,2,9,9‐tetramethyl‐4,7,10‐trioxo‐3‐oxa‐5,8,11‐triazatridecan‐13‐oate], C₂₁H₃₇N₃O₇, containing the little studied O‐allyl‐l ‐serine residue [Ser(All)], crystallizes in the monoclinic space group C2 with one molecule in the asymmetric unit. The compound is an analogue of the Ser140‐Val142 segment of the water channel aquaporin‐4 (AQP4). It forms a distorted type‐II β‐turn with a P_II–3_10L–P_II backbone conformation (P_II is polyproline II). The overall backbone conformation is markedly different from that of the CO(Pro139)–Val142 stretch of rat AQP4, but is quite similar to the corresponding segment of human AQP4, despite significant differences at the level of the individual residues. The side chain of the Ser(All) residue adopts a gauche conformation relative to the backbone CO—C^α and C^α—N bonds. The H atoms of the two CH₂ groups in the Ser(All) side chain are almost eclipsed. The crystal packing of the title compound is divided into one‐molecule‐thick layers, each layer having a hydrophilic core and distinct hydrophobic interfaces on either side.     

Synthesis of a Type-VIβ-Turn Peptide Mimetic and Its Incorporation into a High-Affinity Somatostatin Receptor Ligand

The synthesis of a cis-Phe-Pro dipeptide mimetic is described, which adopts a type-VIβ-turn conformation. In this mimetic, the α-positions of Phe and Pro are joined by a CH₂CH₂ bridge, thereby forming a fused bicyclic system, and fixing a geometry similar to that seen in cis-Phe-Pro units in protein crystal structures. The dipeptide mimetic 20 was synthesized in optically pure form starting from (R)-α-allylproline ( 6 ; Schemes 1, 3, and 4), with a free carboxylic acid and an Fmoc-protected N-terminus, thereby allowing its incorporation into linear and cyclic peptides using standard solid-phase methods. The mimetic 20 was incorporated into the cyclic somatostatin analogue cyclo(-Phe = Pro-Phe-D -Trp-Lys-Thr-), where Phe = Pro represents the mimetic. This analogue shows a high affinity (pIC₅₀ 8.6) for somatostatin receptors on rat-brain cortex membranes. Based on NMR studies in aqueous solution, likely low-energy conformations for this analogue were deduced using restrained dynamic simulated annealing. The conformations found, which include a distorted type-II′ turn at D -Trp-Lys, are similar to low-energy conformations deduced elsewhere for cyclo(-Phe-Pro-Phe-D -Trp-Lys-Thr-), as well as to those seen in crystal structures of the somatostatin analogue octreotide.     

New Open‐Chain and Cyclic Tetrapeptides,Consisting of α‐, β2‐, and β3‐Amino‐Acid Residues,as Somatostatin Mimics – A Survey

Cyclo‐β‐tetrapeptides are known to adopt a conformation with an intramolecular transannular hydrogen bond in solution. Analysis of this structure reveals that incorporation of a β²‐amino‐acid residue should lead to mimics of ‘α‐peptidic β‐turns’ (cf. A, B, C ). It is also known that short‐chain mixed β/α‐peptides with appropriate side chains can be used to mimic interactions between α‐peptidic hairpin turns and G protein‐coupled receptors. Based on these facts, we have now prepared a number of cyclic and open‐chain tetrapeptides, 7 – 20 , consisting of α‐, β²‐, and β³‐amino‐acid residues, which bear the side chains of Trp and Lys, and possess backbone configurations such that they should be capable of mimicking somatostatin in its affinity for the human SRIF receptors (hsst_1–5). All peptides were prepared by solid‐phase coupling by the Fmoc strategy. For the cyclic peptides, the three‐dimensional orthogonal methodology (Scheme 3) was employed with best success. The new compounds were characterized by high‐resolution mass spectrometry, NMR and CD spectroscopy, and, in five cases, by a full NMR‐solution‐structure determination (in MeOH or H₂O; Fig. 4). The affinities of the new compounds for the receptors hsst_1–5 were determined by competition with [¹²⁵I]LTT‐SRIF₂₈ or [¹²⁵I] [Tyr¹⁰]‐CST₁₄. In Table 1, the data are listed, together with corresponding values of all β‐ and γ‐peptidic somatostatin/Sandostatin^® mimics measured previously by our groups. Submicromolar affinities have been achieved for most of the human SRIF receptors hsst_1–5. Especially high, specific binding affinities for receptor hsst₄ (which is highly expressed in lung and brain tissue, although still of unknown function!) was observed with some of the β‐peptidic mimics. In view of the fact that numerous peptide‐activated G protein‐coupled receptors (GPCRs) recognize ligands with turn structure (Table 2), the results reported herein are relevant far beyond the realm of somatostatin: many other peptide GPCRs should be ‘reached’ with β‐ and γ‐peptidic mimics as well, and these compounds are proteolytically and metabolically stable, and do not need to be cell‐penetrating for this purpose (Fig. 5).     

[Leu2]Gramicidin S preserves the structural properties of its parent peptide and forms helically aligned β‐sheets

For crystallographic analysis, Leu was substituted for Orn in Gramicidin S (LGS) to suppress interactions with hydrophilic solvent molecules, which increased the flexibility of the Orn side chains, leading to disorder within the crystals. The asymmetric unit (C₆₂H₉₄N₁₀O₁₀·1.296C₃H₈O·1.403H₂O) contains three LGS molecules (A, B and C) forming β‐turn and intramolecular β‐sheet structures. With the exception of one motif in molecule C, d ‐Phe‐Pro turn motifs (Phe is phenylalanine and Pro is proline) were classed as type II′ β‐turns. The peptide backbones twist slightly to the right along the long axis of the molecule. The puckering of Pro is in a Cγ‐endo or twisted Cγ‐endo–Cβ‐exo form. Flanking molecules are arranged such that the angles (A…B = 104°, B…C = 139° and C…A = 117°) form helical β‐sheets. Solvent molecules interact with the peptide backbones supporting the β‐sheets. The forms of the replacement Leu side chains are consistent with the e‐form of the Orn side chain in GS analogues. No hydrophilic region composed of solvent molecules, such as that observed in Gramicidin S hydrochloride (GS·HCl) crystals, was found. The perturbation of αH chemical shifts and coupling constants of CONH showed that the structural properties of GS·HCl and LGS are similar to each other in solution. CD spectra also supported the structural similarity. With the sequence cyclo(–Val–Leu–Leu–d ‐Phe–Pro–)₂ (Val is valine and Leu is leucine), LGS lacks the amphiphilicity and antimicrobial activity of parental Gramicidin S (GS). However, the structure of LGS reflects the structural characteristics of GS and no disordering inconvenient for structural analysis was found. Thus, LGS could be a novel scaffold useful for studying β‐turn and sheet structures.     

β‐Turn Analogues in Model αβ‐Hybrid Peptides: Structural Characterization of Peptides Containing β2,2Ac6c and β3,3Ac6c Residues

The effect of gem‐dialkyl substituents on the backbone conformations of β‐amino acid residues in peptides has been investigated by using four model peptides: Boc‐Xxx‐β^2,2Ac₆c(1‐aminomethylcyclohexanecarboxylic acid)‐NHMe (Xxx=Leu ( 1 ), Phe ( 2 ); Boc=tert‐butyloxycarbonyl) and Boc‐Xxx‐β^3,3Ac₆c(1‐aminocyclohexaneacetic acid)‐NHMe (Xxx=Leu ( 3 ), Phe ( 4 )). Tetrasubstituted carbon atoms restrict the ranges of stereochemically allowed conformations about flanking single bonds. The crystal structure of Boc‐Leu‐β^2,2Ac₆c‐NHMe ( 1 ) established a C₁₁ hydrogen‐bonded turn in the αβ‐hybrid sequence. The observed torsion angles (α(?≈?60°, ψ≈?30°), β(?≈?90°, θ≈60°, ψ≈?90°)) corresponded to a C₁₁ helical turn, which was a backbone‐expanded analogue of the type III β turn in αα sequences. The crystal structure of the peptide Boc‐Phe‐β^3,3Ac₆c‐NHMe ( 4 ) established a C₁₁ hydrogen‐bonded turn with distinctly different backbone torsion angles (α(?≈?60°, ψ≈120°), β(?≈60°, θ≈60°, ψ≈?60°)), which corresponded to a backbone‐expanded analogue of the type II β turn observed in αα sequences. In peptide 4 , the two molecules in the asymmetric unit adopted backbone torsion angles of opposite signs. In one of the molecules, the Phe residue adopted an unfavorable backbone conformation, with the energetic penalty being offset by a favorable aromatic interaction between proximal molecules in the crystal. NMR spectroscopy studies provided evidence for the maintenance of folded structures in solution in these αβ‐hybrid sequences.     

Relaxation–structure relationship in bulk and plasticized polyamide 11

A series of blends of high molecular weight polyamide 11 (PA11) and three different polar molecules [i.e., N-butyl benzene sulfonamide (BBSA), δ-valerolactam, and ω-laurolactam] have been studied by differential scanning calorimetry (DSC), Fourier transform infrared microscopy (FTIR), differential mechanical thermal analysis (DMTA), and wide- and small-angle X-ray scattering (WAXS and SAXS) experiments. FTIR analysis shows that the concentration of free NH groups in bulk or plasticized PA11 is lower than 15% up to 260°C and less than 1% at room temperature. DMTA data show a β relaxation for dry PA11 but not for dry PA6 and PA12. On the other hand, when ω-laurolactam, which allows trans conformation of amide groups, is added to PA11 the intensity of the α relaxation increases and a strong antiplasticizing effect is observed. This effect is associated with a decrease of the PA11 free volume by increasing the chains packing. On the contrary, when δ-valerolactam, which allows cis amide groups conformations, and BBSA are blended with PA11, the α relaxation temperature and β peak intensity decrease as a function of added molecules concentration. This is associated with a plasticizing effect. It is suggested to attribute the bulk β relaxation to segmental motions involving H-bonded CONH in a cis conformation in amorphous domains of the PA11. In turn, the α relaxation is related to segmental motions involving CONH groups in a trans conformation. Therefore, antiplasticizing and plasticizing effects depend upon the ability of the additive molecule to change the initial conformational structure of polyamide (i.e., the ratio cis over trans H-bonding of the amide group conformations). Overall, a point of interest to note seems to be the difference between the trans and cis conformations of the amide groups in term of bonding with each other in a PA11 chain and additives such as BBSA. In addition, the limited influence of BBSA on the crystalline microstructure of PA11 is explained by the fact that 85% of the PA11 amorphous phase is intraspherulitic and that a great part of the plasticizer is located in these domains. © 1996 John Wiley & Sons, Inc.     

Three‐Residue Turn in β‐Peptides Nucleated by a 12/10 Helix

A new three‐residue turn in β peptides nucleated by a 12/10‐mixed helix is presented. In this design, β peptides were derived from the 1:1 alternation of C‐linked carbo‐β‐amino acid ester [BocNH‐(R)‐β‐Caa_(r)‐OMe] (Boc=tert‐butyloxycarbonyl), which consisted of a D ‐ribo furanoside side chain, and β‐hGly residues. The hexapeptide with (R)‐β‐Caa_(r) at the N terminus showed the ‘turn’ stabilized by a 14‐membered NH(4) ??? CO(6) hydrogen bond at the C terminus nucleated by a robust 12/10‐mixed helix, thus providing a ‘helix‐turn’ (HT) motif. The turn and the helix were additionally stabilized by intraresidue electrostatic interaction between the furan oxygen in the carbohydrate side chain and NH in the backbone. However, the hexapeptide with a β‐hGly residue at the N terminus demonstrated the presence of a 10/12 helix through its entire length, which again showed the intraresidue interaction between NH and furan oxygen. The intraresidue NH ??? O? Me electrostatic interactions observed in the monomer, however, were absent in the peptides.     

Conformational structure of peptides containing dehydroalanine: Formation of β‐bend ribbon structure

α,β‐Unsaturated amino acids (dehydroamino acids) have been found in naturally occurring antibiotics of microbial origin and in some proteins. Due to the presence of the C_αC_β double bond, the dehydroamino acids influence the main‐chain and the side‐chain conformations. The lowest‐energy conformational state of the model tripeptides, Ac–X–ΔAla–NHMe, (X=Ala, Val, Leu, Abu, or Phe) corresponds to ϕ₁=−30°, ψ₁=120° and ϕ₂=ψ₂=30°. This structure is stabilized by the hydrogen bond between CO of the acetyl group and the NH of the amide group, resulting in the formation of a 10‐membered ring. In the model heptapeptide containing ΔAla at alternate position with Ala, Abu, and Leu, the lowest‐energy conformation corresponds to ϕ=−30° and ψ=120° for all the Ala, Abu, and Leu residues and ϕ=ψ=30° for all ΔAla residues. A graphical view of the molecule in this conformation reveals the formation of three hydrogen bonds involving the CO moiety of the ith residue and the NH moiety of the i+3th residue, resulting in a 10‐membered ring formation. In this structure, only alternate peptide bonds are involved in the intramolecular hydrogen‐bond formation unlike the helices and it has been named the β‐bend ribbon structure. The helical structures were predicted to be the most stable structures in the heptapeptide Ac–(Aib–ΔAla)₃–NHMe with ϕ=±30°, ψ=±60° for Aib residues and ϕ=ψ=±30° for ΔAla residues. The computational results reveal that the ΔAla residue does not induce an inverse γ‐turn in the preceding residue. It is the competitive interaction of small solvent molecules with the hydrogen‐bonding sites of the peptide which gives rise to the formation of an inverse γ‐turn (ϕ₁=−54°, ψ₁=82°; ϕ₂=44°, ψ₂=3°) in the preceding residue to ΔAla. The computational studies for the positional preference of ΔAla in the peptide containing one ΔAla and nine Ala residues reveals the formation of a 3₁₀ helical structure in all the cases with the terminal preferences for ΔAla, consistent with the position of ΔAla in the natural antibiotics. The extended structures is found to be the most stable for poly‐ΔAla. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 72: 15–23, 1999     

Bonding of copper(II) ions by proctolin analogues modified in fifth position of the peptide chain

The proctolin (Arg–Tyr–Leu–Pro–Thr, RYLPT) analogues modified in fifth position of the peptide chain (RYLPP, RYLPI, RYLP–Dab, where Dab – 2,4-diamminobutyric acid) have been synthesised and their complexes with H⁺ and Cu²⁺ studied by potentiometry and spectroscopy (UV–Vis, CD and EPR) at 25 °C and I = 0.10 mol dm⁻³ (KNO₃). The results obtained support the earlier suggestion on the specific role of a proline residue as a “break-point” in copper complex formation with peptides. The presence of a proline residue into the fourth position of the proctolin analogues (RYLPP, RYLPI) leads in wide pH range of the existence the CuL and CuH₋₁L complexes with expected stabilities. Spectroscopic studies confirm that these are 2N {NH₂, N⁻, CO} and 3N {NH₂, 2N⁻, CO} species, respectively. The amine group of the Dab residue of the RYLP–Dab proctolin analogue, in whole pH range (2.5–10.5) is coordinated to the copper(II) ions, and the deprotonation and coordination of the second amide nitrogen atom to the metal ion is prevented. In solution in wide pH range (5–10.5) the 3N {NH₂, N⁻, CO, NH_2Dab} complex is present. Proctolin and its analogues modified in fifth position contain in the second position of the peptide sequence the Tyr residue and the CD results show that TyrO⁻–Cu²⁺ bonding is present at pH above 8.     

Cyclic Heptapeptides Axinastatin 2, 3, and 4: Conformational analysis and evaluation of the biological potential

The conformational analysis of naturally occurring cytostatic cyclic heptapeptides axinastatin 2, 3, and 4 was carried out by two-dimensional NMR spectroscopy in combination with distance-geometry (DG) and molecular-dynamics (MD) calculations in explicit solvents. The synthesized secondary metabolites were examined in (D₆)DMSO. Axinastatin 2 was also investigated in CD₃OH. In all structures, Pro² is in the i + 1 position of a βI turn and Pro⁶ occupies the i + 2 position of a βVIa turn about the cis amide bond between residue 5 and Pro^6. In all peptides, a bifurcated H-bond occurs between residue 4 CO and the amide protons of residue 1 and 7. For axinastatin 2 and 3, an Asn I_g turn was found about Asn¹ and Pro^2. We compared these structures with conformations of cyclic heptapeptides obtained by X-ray and NMR studies. A β-bulge motif with two β turns and one bifurcated H-bond is found as the dominating backbone conformation of cyclic all-L-heptapeptides. Axinastatin 2, 3, and 4 can be characterized by six trans and one cis amide bond resulting in a β/βVI(a)-turn motif, a conformation found for many cyclic heptapeptides. Detailed biological tests of the synthetic compounds in different human cancer cell lines indicates these axinastatins to be inactive or of low activity.     

The peptide (Z)‐Pro–Leuol

The structure of the synthetic protected dipeptide (Z)‐Pro–Leuol [systematic name: benzyl 2‐(1‐hydroxy­methyl‐3‐methyl­butyl­amino­carbonyl)­pyrrolidine‐1‐carboxyl­ate], C₁₉H₂₈N₂O₄, was determined by X‐ray crystallography. The peptide adopts a novel backbone conformation compared with other longer oligopeptides containing Pro–Leuol.     

Second‐Generation Inhibitors for the Metalloprotease Neprilysin Based on Bicyclic Heteroaromatic Scaffolds: Synthesis,Biological Activity,and X‐Ray Crystal‐Structure Analysis

A new class of nonpeptidic inhibitors of the Zn^II‐dependent metalloprotease neprilysin with IC₅₀ values in the nanomolar activity range (0.034–0.30 μM ) were developed based on structure‐based de novo design (Figs. 1 and 2). The inhibitors feature benzimidazole and imidazo[4,5‐c]pyridine moieties as central scaffolds to undergo H‐bonding to Asn542 and Arg717 and to engage in favorable π‐π stacking interactions with the imidazole ring of His711. The platform is decorated with a thiol vector to coordinate to the Zn^II ion and an aryl residue to occupy the hydrophobic S1′ pocket, but lack a substituent for binding in the S2′ pocket, which remains closed by the side chains of Phe106 and Arg110 when not occupied. The enantioselective syntheses of the active compounds (+)‐ 1 , (+)‐ 2 , (+)‐ 25 , and (+)‐ 26 were accomplished using Evans auxiliaries (Schemes 2, 4, and 5). The inhibitors (+)‐ 2 and (+)‐ 26 with an imidazo[4,5‐c]pyridine core are ca. 8 times more active than those with a benzimidazole core ((+)‐ 1 and (+)‐ 25 ) (Table 1). The predicted binding mode was established by X‐ray analysis of the complex of neprilysin with (+)‐ 2 at 2.25‐Å resolution (Fig. 4 and Table 2). The ligand coordinates with its sulfanyl residue to the Zn^II ion, and the benzyl residue occupies the S1′ pocket. The 1H‐imidazole moiety of the central scaffold forms the required H‐bonds to the side chains of Asn542 and Arg717. The heterobicyclic platform additionally undergoes π‐π stacking with the side chain of His711 as well as edge‐to‐face‐type interactions with the side chain of Trp693. According to the X‐ray analysis, the substantial advantage in biological activity of the imidazo‐pyridine inhibitors over the benzimidazole ligands arises from favorable interactions of the pyridine N‐atom in the former with the side chain of Arg102. Unexpectedly, replacement of the phenyl group pointing into the deep S1′ pocket by a biphenyl group does not enhance the binding affinity for this class of inhibitors.     

1H and 13C NMR study of phosphopeptides: 2—Ac–PSer–Gly,Ala–PSer–Gly and Gly–PSer–Phe

¹H and ¹³C NMR spectra of AC–PSer–Gly, Ala–PSer–Gly and Gly–PSer–Phe have been measured and analysed as a function of pD. The NMR parameters of the PSeryl side chain are a function of the sequence. The second titration step of the phosphate group (pK₂ = 5.7) is much more difficult to detect in Ac–PSer–Gly and Ala–PSer–Gly than in Gly–PSer–Phe. The conformation in which H-α? C-α? C-β? O? P forms a planar W-type arrangement predominates only for Ala–PSer–Gly. In the other two phosphopeptides the gauche conformations contribute increasingly, in particular for Gly–PSer–Phe.     

A Designed Three‐Stranded β‐Sheet in an α/β Hybrid Peptide

The incorporation of β‐amino acid residues into the antiparallel β‐strand segments of a multi‐stranded β‐sheet peptide is demonstrated for a 19‐residue peptide, Boc‐LV^βFV^DPGL^βFVVL^DPGLVL^βFVV‐OMe (BBH19). Two centrally positioned ^DPro–Gly segments facilitate formation of a stable three‐stranded β‐sheet, in which β‐phenylalanine (^βPhe) residues occur at facing positions 3, 8 and 17. Structure determination in methanol solution is accomplished by using NMR‐derived restraints obtained from NOEs, temperature dependence of amide NH chemical shifts, rates of H/D exchange of amide protons and vicinal coupling constants. The data are consistent with a conformationally well‐defined three‐stranded β‐sheet structure in solution. Cross‐strand interactions between ^βPhe3/^βPhe17 and ^βPhe3/Val15 residues define orientations of these side‐chains. The observation of close contact distances between the side‐chains on the N‐ and C‐terminal strands of the three‐stranded β‐sheet provides strong support for the designed structure. Evidence is presented for multiple side‐chain conformations from an analysis of NOE data. An unusual observation of the disappearance of the Gly NH resonances upon prolonged storage in methanol is rationalised on the basis of a slow aggregation step, resulting in stacking of three‐stranded β‐sheet structures, which in turn influences the conformational interconversion between type I′ and type II′ β‐turns at the two ^DPro–Gly segments. Experimental evidence for these processes is presented. The decapeptide fragment Boc‐LV^βFV^DPGL^βFVV‐OMe (BBH10), which has been previously characterized as a type I′ β‐turn nucleated hairpin, is shown to favour a type II′ β‐turn conformation in solution, supporting the occurrence of conformational interconversion at the turn segments in these hairpin and sheet structures.     

Characterization of structural transitions from aqueous solution to a lipid phase for α-MSH

Fluorescence spectroscopy results show that the α-melanocyte-stimulating hormone peptide (α-MSH) interacts with acidic lipid vesicles. Detectable structural changes are concomitant with the passage of a tryptophan residue from aqueous to lipidic media. The observed multiexponential decay of fluorescence, rationalized as originating from three rotameric populations of the tryptophan residue, has been used together with a matrix algorithm to find the most probable conformational families of α-MSH in water and lipid environments. A model is discussed in which the same conformational families occur in various phases, although with different probabilities. A conformational family in which χ₁ of the Trp9 side chain is in the trans-rotameric conformation is shown to have structural features highly appropriate to interact with negatively charged biological membranes, which are also in accordance with previous molecular dynamics simulations and with structures engineered in α-MSH analogs that show an increased potency in biological essays. The gauche minus and gauche plus side-chain conformations of Trp9, on the other hand, yield conformations more likely to predominate in aqueous solution. NMR spectroscopy measurements of α-MSH analogs indicate the existence in aqueous solution of a β strand in the vicinity of Trp9. A similar structural feature was found in the present conformational analysis for the gauche minus and gauche plus side-chain rotamers of Trp9. © 1995 John Wiley & Sons, Inc.