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.     

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

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

Type II‘β-bend conformation of tert.-butyloxycarbonyl-L-amino-succinyl-L-alanyl-glycine methyl ester in the solid state

Boc-L-Asu-L-Ala-Gly-OMe crystallizes in the monoclinic space group P2₁ with cell dimensions a = 14.315(3) Å, b = 9.280(2) Å, c = 14.358(3) Å, β= 103.63 (1) d?, V= 1853.4(9) ų, with two molecules in the asymmetric unit. The conformation of the two molecules is characterized by a type II' β-bend, similar to that predicted earlier by potential energy calculations, stabilized by an intramolecular hydrogen bond. I.r. and ¹H-n.m.r. data show that the folded conformation is also stable in chloroform solution.     

Peptides from chiral Cα,α-disubstituted glycines

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

Structure of N-Boc-L-Pro-dehydro-Leu-NHCH3

The peptide N-Boc-L-Pro-dehydro-Leu-NHCH₃ was synthesized to examine the nature of β-bend as a result of dehydro-Leu in the sequence. The peptide crystallizes from methanol-water mixture at 4° in orthorhombic space group P22₁2₁ with a = 5.726(1)Å, b = 14.989(4) Å, c = 24.131(9) Å, V = 2071(1) ų, Z = 4, dm = 1.064(5)gcm^-3 and dc = 1.0886(5)gcm^-3. The structure was solved by direct methods using SHELXS 86 and it was refined by full-matrix least-squares procedure to an R value of 0.059 for 957 observed reflections. The peptide is found to adopt a β-bend type II conformation with φ₁=– 51(1)°, ψ₁= 133(1)°, φ₂= 74(2)° and ψ₂= 8(2)°. The β-bend is stabilized by an intra-chain hydrogen bond between the carbonyl oxygen of ith residue and the NH of (i + 3)th residue. The five-membered pyrrolidine ring of Pro-residue adopts an ideal C^γ-exo conformation with torsion angles of χ₁¹=– 25(1)°, χ₁²= 38(1)°, χ₂=– 34(1)°, χ₁⁴= 20(1)° and χ₁⁰= 2(1)°. The side chain conformation angles of dehydro-Leu residue are χ₂= 12(2)°, χ₂^2.1=– 112(2)° and χ₂^2.2= 136(2)°. The crystal structure is stabilized by a network of hydrogen bonds and van der Waals interactions.     

The preferred solid-state conformation of (αMe)Trp peptides

The two Z-l -Ala-d l -(xMe)Trp-NH₂ diastereomeric dipeptides were synthesized from (Z-l -Ala)₂O and H-dl -(xMe)Trp-NH₂. The latter racemate, prepared by phase-transfer catalyzed alkylation of the N^α-benzylidene derivative of alanine amide followed by acidic hydrolysis of the resulting Schiff base, was characterized by X-ray diffraction. The molecular and crystal structure of Z-l -Ala-l -(αMe)Trp-NH², separated from its diastereomer by silica-gel column chromatography, was determined by X-ray diffraction analysis. Both independent molecules in the asymmetric unit of the dipeptide adopt a type-II β-bend conformation. However, only the more regularly folded conformation of molecule B is stabilized by a 1←4 C=O…H—N intramolecular H bond. The present results indicate that: (i) the Cα-methylated (αMe)Trp residue is a strong β-bend and helix former, and (ii) the relationship between (αMe)Trp chirality and helix screw sense tends to be opposite to that of protein amino acids. The implications for the use of the (αMe)Trp residue in designing conformationally restricted analogs of bioactive peptides are briefly discussed. ©Munksgaard 1995.     

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.     

Conformationally restrained peptides: crystal structure of tert-butyloxycarbonyl-L-alanyl-D-alanyl-D-aminosuccinyl-glycyl-L-alanine methyl ester

The solid-state structure of a heterochiral peptide embodying a D-aminosuccinyl peptide (D-ASU) and a D-Ala was studied in order to analyse the effects of Asu and amino acids with inverse chirality on peptide conformation. The crystal structure has been determined by X-ray diffraction techniques and refined to a final R factor of 0.043. The molecule adopts an unusual overall 'S-shape’ conformation due to two consecutive type II β-turns. In this molecule it is possible to compare a type II β-bend conformation (L-Ala¹-D-Ala²) favoured by the presence of a D-residue at second corner to a type II β-turn (D-Asu³-Gly⁴) favoured by the presence of a D-ASU residue at first corner. In agreement with previous studies, this structure confirms that the Asu has a high propensity to adopt a type II or II′β-bend conformation and that it may be used as a strong determinant of these structural motifs. © Munksgaard 1996.     

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.     

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.     

SPECTROSCOPIC STUDY OF THE CONFORMATIONS OF PROLINE-CONTAINING OLIGOPEPTIDES IN THE CRYSTALLINE STATE AND IN SOLUTION

A conformational study has been carried out on a series of linear proline-containing oligopeptides (ZGP, ZGPL, ZGPLG and ZGPLGP) in both the crystalline state and in DMSO-d₆, solution, using Raman and n.m.r. spectroscopy. The amide I and HI bands in the Raman spectra of the crystalline forms indicate the presence of a type I β-bend conformation in ZGPLG and ZGPLGP, but not in ZGP and ZGPL. This result is in agreement with X-ray data on these mole cules. The Raman spectra of these peptides in solution indicate that more than one conformation is present, i.e. that the β-bend structure of the solid form of ZGPLG and ZGPLGP is destabilized by DMSO-d₆. ¹³C and ¹H n.m.r. data also demonstrate the presence of more than one conformation in ZGP, ZGPL, ZGPLG and ZGPLGP in DMSO-d₆ solution. These isomers differ in their con formation (cis and trans) about their Gly-Pro peptide bonds and possibly about the Cα-C' bond of the C-terminal proline in ZGPLGP. For ZGP, ZGPL, ZGPLG and ZGPLGP, the ensemble of conformations in DMSO-d₆ includes C₅ and C₇ hydrogen-bonded rings; in addition, ZGPLGP may contain a small percentage of a β-bend conformation (at Pro₂-Leuj₃) with trans peptides in both Gly-Pro moieties.     

Crystal structure of a dipeptide Boc-Aib-Phe-OMe

In order to understand the effect of the restrictions posed by the Aib residue on peptide conformation we studied the crystal structure of a dipeptide tBoc-Aib-Phe-OMe. Crystals of this compound are triclinic, space group PI with a= 9.600(1) Å, b=10.262(1) Å, c= 10.799(1) Å, α= 98.43°(1), β=99.18°(1), °=98.87°(1), V= 1021.69(18) ų and Z=2. The structure was solved by direct methods and refined to an R-factor of 4.98%. The backbone conformational angles for the Aib residue in molecule A are in the left-handed helical region, while in molecule B they are in the right-handed helical region. The Phe residue in molecule A is in the right-handed helical conformation, while in molecule B it is in the β-region. The peptide units are trans and show significant deviation from planarity [(ω₁= 166.67(5)° and ω₂=–177.9(5)]. © Munksgaard 1997.     

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.     

Design of peptides withα,β-dehydro-residues: synthesis,and crystal and molecular structure of a310-helical tetrapeptide Boc-l-Val-ΔPhe-ΔPhe-l-Ile-OCH3

Abstract: The peptide Boc-l -Val-ΔPhe-ΔPhe-l -Ile-OCH₃ was synthesized using the azlactone method in the solution phase, and its crystal and molecular structures were determined by X-ray diffraction. Single crystals were grown by slow evaporation from solution in methanol at 25°C. The crystals belong to an orthorhombic space group P2₁2₁2₁ with a = 12.882(7) Å, b = 15.430(5) Å, c = 18.330(5) Å and Z = 4. The structure was determined by direct methods and refined by a least-squares procedure to an R-value of 0.073. The peptide adopts a right-handed 3₁₀-helical conformation with backbone torsion angles: φ₁ = 56.0(6)°, ψ₁ = –38.0(6)°, φ₂ = –53.8(6)°, ψ₂ = 23.6(6)°, φ₃ = –82.9(6)°, ψ₃ = –10.6(7)°, φ₄ = 124.9(5)°. All the peptide bonds are trans. The conformation is stabilized by intramolecular 4→1 hydrogen bonds involving Boc carbonyl oxygen and NH of ΔPhe³ and CO of Val¹ and NH of Ile⁴. It is noteworthy that the two other chemically very similar peptides: Boc-Val-ΔPhe-ΔPhe-Ala-OCH₃ (i) and Boc-Val-ΔPhe-ΔPhe-Val-OCH₃ (ii) with differences only at the fourth position have been found to adopt folded conformations with two overlapping β-turns of types II and III′, respectively, whereas the present peptide adopts two overlapping β-turns of type III. Thus the introduction of Ile at fourth position in a sequence Val-ΔPhe-ΔPhe-X results in the formation of a 3₁₀-helix. The crystal structure is stabilized by intermolecular hydrogen bonds involving NH of Val¹ and carbonyl oxygen of a symmetry related (–x, y – 1/2, 1/2 + z) ΔPhe² and NH of ΔPhe² with carbonyl oxygen of a symmetry related (x, y1/2, 1/2 + z) Ile⁴. This gives rise to long columns of helical molecules linked head to tail running along [010] direction.     

Backbone side chain interactions in peptides.

The preferential occurrence of amino-acid residues having short polar side-chain within β-folded regions of crystallized proteins suggests the existence of some stabilizing interaction involving the side polar function. Three model dipeptides tBuCO-l -Pro-l -Ser-NHMe 1 , tBuCO-l -Pro-d -Ser-NHMe 2 in the pure enantiomeric a and racemic b forms, and iPrCO-l -Pro-d -Ser-OMe 3 have been investigated in the solid state by X-ray crystallography. Homo and heterochiral sequences 1 and 2 are folded in the βI and βII types, respectively, whereas 3 obviously accommodates an open conformation. Besides the i + 3 →i hydrogen bond typical of β-bends in 1 , 2a , and 2b , the Ser NH group in all four crystal structures is a proton donor to the lone orbitals of the Ser O^γ oxygen atom. The result is that the disposition of the Ser C^α–C^β bond corresponds to the rotamer III (χ¹? 60°). As shown by the crystal structure of 3 , the intra-Ser NH.O^γ hydrogen bonding is not restricted to β-folded Pro-Ser sequences. Therefore, this interaction is not only a stabilizing factor for β-turns but it is also probably responsible for the already mentioned stability of rotamer III for the Ser C^α–C^β bond in peptides and protein.     

Synthesis,crystal structure and molecular conformation of the tBuCO-D,L-Ala-Δz-Phe-NhiPr α,β-unsaturated dipeptide

The crystal structure of the tBuCO-d,l -Ala-Δ^z-Phe-NHiPr dipeptide has been solved by X-ray diffraction. The peptide crystallizes in monoclinic space group P2JC with a = 13.445 (3) Å, b = 35.088 (4) Å, c = 14.755(3) Å, β= 116.73(1)°, Z = 12 and d_c= 1.151 g.cm^?3. The three independent molecules per asymmetric unit accommodate a βII-folded conformation, but only one of them contains the typical i + 3 → i interaction characterizing a β-turn. In the other two molecules, the N…O distance exceeds 3.2 Å, a value generally considered the upper limit for hydrogen bonds in peptides. In solution, the βII-turn conformation is largely predominant.     

Conformation—activity correlations for chemotactic tripeptide analogs incorporating dialkyl residues with linear and cyclic alkyl sidechains at position 2

Five stereochemically constrained analogs of the chemotactic tripeptide incorporating l-aminocycloalkane-l-carboxylic acid (Ac_nc) and α, α-dialkylglycines (Deg, diethylglycine; Dpg, N, N-dipropylglycine and Dbg, N, N-dibutylglycine) at position 2 have been synthesized. NMR studies of peptides For-Met-Xxx-Phe-OMe (Xxx = Ac₇c. I: Ac₈c. II: Deg, III; Dpg, IV and Dbg, V; For, formyl) establish that peptides with cycloalkyl residues, I and II, adopt folded β-turn conformations in CDCl₃, and (CD³)₂SO. In contrast, analogs with linear alkyl sidechains, III-V, favour fully extended (C₅) conformations in solution. Peptides I-V exhibit high activity in inducing β-glucosaminidase release from rabbit neutrophils, with ED₅₀ values ranging from 1.4–8.0 × 10–¹¹. M. In human neutrophils the Dxg peptides III-V have ED₅₀ values ranging from 2.3 × 10^?8 to 5.9 × 10^?10 M, with the activity order being V>IV>III. While peptides I-IV are less active than the parent. For-Met-Leu-Phe-OH, in stimulating histamine release from human basophils, the Dbg peptide V is appreciably more potent, suggesting its potential utility as a probe for formyl peptide receptors. © Munksgaard 1996.     

Conformational study on trans- and cis-N-acetyl-N′-methylamides of Pro-Xaa dipeptides

Conformational free energy calculations using an empirical potential ( ECEPP /2) and the hydration shell model were carried out on the N-acetyl-N′-methylamides of Pro-Xaa dipeptides (Xaa = Ala, Leu, Val, Gly, Cys, Met, Phe, Tyr, Asn, Asp, and Ser) with trans and cis peptide bonds preceding proline residue in the unhydrated and hydrated states. As compared with the results obtained by using the earlier version of ECEPP, the values of β-bend probabilities are about doubled. The average calculated population of cis-dipeptide is about 4%, which is close to the abundance obtained from the analysis of X-ray crystal structures of proteins. The β-bends are the most dominant structures of cis-dipeptides. Type I, usually having intramolecular hydrogen bonds, contributes greatly to the β-bend conformations of trans- and cis-dipeptides. However, type I β-bends of cis-dipeptides do not have any hydrogen bonds. By including the hydration, the β-bend probabilities for trans- and cis-dipeptides decreased, indicating that the interactions of water molecules with a backbone or side-chain may force the dipeptides to be more distorted or extended. In particular, type II is found to be a dominant β-bend conformation of trans- and cis-Pro-Gly dipeptides in both the unhydrated and hydrated states. In general, the calculated propensities for Pro-Xaa dipeptides to adopt β-bend conformations are reasonably consistent with available experimental data. From comparing conformations of Pro and Xaa residues in the dipeptides and single residues, we found that inter-residue interactions and hydration are of importance in determining the conformational properties of the Pro-Xaa dipeptide.     

Conformations of dehydrophenylalanine containing peptides Nuclear Overhauser effect study of two acyclic tetrapeptides

Two isomeric, acyclic tetrapeptides containing a Z-dehydrophenylalanine residue (Δ^z-Phe) at position 2 or 3, Boc-Leu-Ala-Δ^z-Phe-Leu-OMe (1) and Boc-Leu-Δ^z-Phe-Ala-Leu-OMe (2), have been synthesized and their solution conformations investigated by 270MHz ¹H n.m.r. spectroscopy. In peptide 1 the Leu(4) NH group appears to be partially shielded from solvent, while in peptide 2 both Ala(3) and Leu(4) NH groups show limited solvent accessibility. Extensive difference nuclear Overhauser effect (n.O.e.) studies establish the occurrence of several diagnostic inter-residue n.O.e.s (Cα_jH ? N_i+1H and N_iH ? N_i+1H) between backbone protons. The simultaneous observation of “mutually exclusive” n.O.e.s suggests the presence of multiple solution conformations for both peptides. In peptide 1 the n.O.e. data are consistent with a dynamic equilibrium between an -Ala-Δ^z-Phe- Type II β-turn structure and a second species with Δ^z-Phe adopting a partially extended conformation with Ψ values of ± 100° to ± 150°. In peptide 2 the results are compatible with an equilibrium between a highly folded consecutive β-turn structure for the -Leu-Δ^z-Phe-Ala- segment and an almost completely extended conformation.     

Crystal structure and conformation of polypeptides: L-leucylglycylglycylglycine

Crystals of L-leucylglycylglycylglycine, LGGG (C₁₂H₂₂N₄O₅), grown from an ethanol-water solution, are orthorhombic, space groups P2₁2₁2₁, with unit cell dimensions (at 22 ± 3°) a = 9.337(1), b = 10.995(1), c = 15.235(1)Å, v = 1563.4 ų, Z = 4 with a density of D_obs= 1.29 g-cm^-3 and D_calc= 1.279 g°cm^-3. The crystal structure was solved by the application of direct methods and refined to an R value of 0.029 for 1018 reflections with I ± 2s?. The molecule exists as a zwitterion in the crystal. The trans peptide backbone takes up a folded conformation at the middle glycylglycyl link accompanied by a significant nonplanarity up to Δω of 8° at the middle peptide and is relatively more extended at the two ends. The molecules are linked together intermolecularly in an infinite sequence of head to tail 1–4′ hydrogen bonds, as is typical of charged peptides. It is interesting to note that while glycylglycylglycine takes up an extended β-sheet conformation, addition of Leu to the N-terminal results in a bent conformation.