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1.
In 1968 C. Venkatachalam (Biopolymers, Vol. 6, pp. 1425–1436) predicted the ideal forms of β-turns (type I, type II, etc.) based entirely on theoretical calculations. Subsequently, over a thousand x-ray structures of different globular proteins have been analyzed, with results suggesting that the most important form among the hairpin conformers is the type I β-turn. For the latter type of hairpin conformation, the original computations had predicted ϕi+1 = −60°, ψi+1 = −30°, ϕi+2 = −90°, and ψi+2 = 0° as backbone torsion angle values, and these have been used from that time as reference values for the identification of the type I β-turn. However, it has never been clarified whether these “ideal” backbone torsion angle values exist in real structures, or whether these torsion angles are only “theoretical values.” Using the most recent release of the Protein Data Bank (1994), a survey has been made to assign amino acid pairs that approach the ideal form of the type I β-turn. The analysis resulted in four sequences where the deviation from ideal values for any main-chain torsion angles was less than 2°. In order to determine whether such a backbone fold is possible only in proteins owing to fortuitous cooperation of different folding effects, or whether it occurs even in short peptides, various attempts have been made to design the optimal amino acid sequence. Such a peptide model compound adopting precisely the predicted torsion angle values [ϕi+1 = −60°, ψi+1 = −30°, ϕi+2 = −90°, and ψi+2 = 0°] could provide valuable information. The solid state conformation of cyclo[(δ) Ava-Gly-Pro-Thr (O1Bu)-Gly] reported herein, incorporating the -Pro-Thr- subunit, yields values suggesting that the “ideal” type I β-turn is even possible for a peptide where there are no major environmental effects present. © 1996 John Wiley & Sons, Inc.  相似文献   

2.
Pivaloyl-L -Pro-Aib-N-methylamide has been shown to possess one intramolecular hydrogen bond in (CD3)2SO solution, by 1H-nmr methods, suggesting the existence of β-turns, with Pro-Aib as the corner residues. Theoretical conformational analysis suggests that Type II β-turn conformations are about 2 kcal mol?1 more stable than Type III structures. A crystallographic study has established the Type II β-turn in the solid state. The molecule crystallizes in the space group P21 with a = 5.865 Å, b = 11.421 Å, c = 12.966 Å, β = 97.55°, and Z = 2. The structure has been refined to a final R value of 0.061. The Type II β-turn conformation is stabilized by an intramolecular 4 → 1 hydrogen bond between the methylamide NH and the pivaloyl CO group. The conformational angles are ?Pro = ?57.8°, ψPro = 139.3°, ?Aib = 61.4°, and ψAib = 25.1°. The Type II β-turn conformation for Pro-Aib in this peptide is compared with the Type III structures observed for the same segment in larger peptides.  相似文献   

3.
The crystal and molecular structure of N-benzyloxycarbonyl-α-aminoisobutyryl-L -prolyl methylamide, the amino terminal dipeptide fragment of alamethicin, has been determined using direct methods. The compound crystallizes in the orthorhombic system with the space group P212121. Cell dimensions are a = 7.705 Å, b = 11.365 Å, and c = 21.904 Å. The structure has been refined using conventional procedures to a final R factor of 0.054. The molecular structure possesses a 4 → 1 intramolecular N-H—O hydrogen bond formed between the CO group of the urethane moiety and the NH group of the methylamide function. The peptide backbone adopts the type III β-turn conformation, with ?2 = ?51.0°, ψ2 = ?39.7°, ?3 = ?65.0°, ψ3 = ?25.4°. An unusual feature is the occurrence of the proline residue at position 3 of the β-turn. The observed structure supports the view that Aib residues initiate the formation of type III β-turn conformations. The pyrrolidine ring is puckered in Cγ-exo fashion.  相似文献   

4.
The influence of amino acids with contrasting conformational tendencies on the stereochemistry of oligopeptides has been investigated using an octapeptide Boc-Leu-Aib-Val-Gly-Gly-Leu-Aib-Val-OMe, which contains two helix-promoting Aib residues and a central helix-destabilizing Gly-Gly segment. Single crystal x-ray diffraction studies reveal that a 3 10-helix is formed up to the penultimate Aib residue, at which point there is a helix reversal in the backbone, reminiscent of a C-terminal 6 → I hydrogen bond. The curious feature in the crystal is the solvation of the possible 6 → 1 bond by a CH3OH molecule, where the OH is inserted between O(3) and N(8) and participates in hydrogen bonds with both. The cell parameters are as follows: space group P212121, a = 10.649(4) Å, b = 15.694(5) Å, c = 30.181(8) Å, R = 6.7% for 3427 data (| F0| > 3σF) observed to 0.9 Å. Nuclear magnetic resonance studies in CDCl3 using NH group solvent accessibility and nuclear Overhauser effects as probes are consistent with a 3 10-helical conformation. In contrast, in (CD3)2SO, unfolding of the central segment results in a multiple β-turn structure, with β-turn conformations populated at residues 1–2, 3–4, and 6–7. CD studies in methanol-2,2,2-trifluoroethanol (TFE) mixtures also provide evidence for a solvent-dependent structural transition. Helical conformations are populated in TFE, while type II β-turn structures are favored in methanol. © 1996 John Wiley & Sons, Inc.  相似文献   

5.
The peptide Boc-L-Val-ΔPhe-ΔPhe-L-Val-OCH3 was synthesized by the azlactone method in solution phase, and its crystal and molecular structures were determined by x-ray diffraction method. Single crystals were grown by slow evaporation from a methanol/water solution at 6°C. The crystals belong to an orthorhombic space group P212121 with a = 10.478 (6) Å, b = 13.953 (1), c = 24.347 (2) and Z = 4. The structure was determined by direct methods and refined by least squares procedure to an R value of 0.052. The structure consists of a peptide and a water molecule. The peptide adopts two overlapping β-turn conformations of Types II and I′ with torsion angles: ϕ1 = -54.8 (6), ψ1 = 130.5 (4), ϕ2 = 65.8 (5), ψ2 = 12.8 (6), ϕ3 = 79.4 (5), ψ3 = 3.9 (7)°. The conformation is stabilized by intramolecular hydrogen bonds involving Boc CO and NH of ΔPhe3 and CO of Val1 and NH of Val4. The molecules are tightly packed in the unit cell. The crystal structure is stabilized by hydrogen bonds involving NH of ΔPhe2 and CO of a symmetry related (x-½, ½ -y, -z) ΔPhe2. The solvent-water molecule forms two hydrogen bonds with peptide molecule involving NH of Val1 as an acceptor and another with CO of a symmetry related (1 -x, y-½, ½ -z) ΔPhe3 as a donor. These studies indicate that a tetrapeptide with two consecutive ΔPhe residues sequenced with valines on both ends adopts two overlapping β-turns of Types II and I′. © 1996 John Wiley & Sons, Inc.  相似文献   

6.
An Nα-protected model pentapeptide containing two consecutive ΔPhe residues, Boc-Leu-ΔPhe-ΔPhe-Ala-Phe-NHMe, has been synthesized by solution methods and fully characterized. 1H-nmr studies provided evidence for the occurrence of a significant population of a conformer having three consecutive, intramolecularly H-bonded β-bends in solution. The solid state structure has been determined by x-ray diffraction methods. The crystals grown from aqueous methanol are orthorhombic, space group P212121, a = 11.503(2), b = 16.554(2), c = 22.107(3) Å, V = 4209(1) Å,3 and Z = 4. The x-ray data were collected on a CAD4 diffractometer using CuKa radiation (λ = 1.5418 Å). The structure was determined using direct methods and refined by full-matrix least-squares procedure. The R factor is 5.3%. The molecule is characterized by a right handed 310-helical conformation (〈ϕ〉 = −68.2°, 〈ψ〉 = −26.3°), which is made up of two consecutive type III β-bends and one type I β-bend. In the solid state the helical molecules are aligned head-to-tail, thus forming long rod like structures. A comparison with other peptide structures containing consecutive ΔPhe residues is also provided. The present study confirms that the -ΔPhe-ΔPhe-sequence can be accommodated in helical structures. © 1997 John Wiley & Sons, Inc. Biopoly 42: 373–382, 1997  相似文献   

7.
The crystal structure of a dipeptide L -leucyl–L -leucine (C12H24N2O3) has been determined. The crystals are monoclinic, space group P21, with a = 5.434(4) Å, b = 15.712(7) Å, c = 11.275(2) Å, β = 100.41(1)°, and Z = 2. The crystals contain one molecule of dimethyl sulfoxide (DMSO) as solvent of crystallization for each dipeptide molecule. The structure has been solved by direct methods and refined to a final R index of 0.059 for 920 reflections (sinθ/λ ? 0.60 Å?1) with I ? 2σ (I). The trans peptide unit shows substantial degree of non-planarity (Δω = 14°). The peptide backbone adopts an extended conformation with torsion angles of ψ1 = 138(1)°, ω1 = 166(1)°, ?2 = ? 149.3(7)°, ψ21 = 164.2(7)°, and ψ22 = ? 15(1)°. For the first leucyl residue, the side-chain conformation is specified by the torsion angles 1χ1 = 176.7(7)°, 1χ21 = 62(1)°, 1χ22 = ? 177.4(8)°; the second leucyl residue adopts a Sterically unfavorable conformation with 2χ1 = 61(1)°, 2χ21 = 97(1)°, and 2χ22 = ?151(1)°. The packing involves head-to-tail interaction of peptide molecules and segregation of polar and nonpolar regions. The DMSO molecule is strongly hydrogen bonded to the terminal NH group. © 1994 John Wiley & Sons, Inc.  相似文献   

8.
Many of the theoretical methods used for predicting the occurrence of α-helices in peptides are based on the helical preferences of amino acid monomer residues. In order to check whether the helix-forming tendencies are based on helical preferences of monomers only or also on their sequence contexts, we synthesized permuted sequences of the tripeptides GAP, GAV, and GAL that formed crystalline helices with near α-helical conformation. The tripeptides AFG and FAG formed good crystals. The x-ray crystallographic studies of AFG and FAG showed that though they contain the same amino acids as GAF but in different sequences, they do not assume a helical conformation in the solid state. On the other hand, AFG and FAG, which contain the same amino acids but in a different sequence, exhibit nearly the same backbone torsion angles corresponding to an incipient formation of a β-bulge, and exhibit nearly identical unit cells and crystal structures. Based on these results, it appears that the helix-forming tendencies of amino acids depend on the sequence context in which it occurs in a polypeptide. The synthetic peptides AFG (L -Ala-L -Phe-Gly) and FAG (L -Phe-L -Ala-Gly), C14H19N3O4, crystallize in the orthorhombic space group P212121, with a = 5. 232(1), b = 14. 622(2), c = 19. 157(3) Å, Dx = 1.329 g cm?3, Z = 4, R = 0.041 for 549 reflections for AFG, and with a = 5. 488(2), b = 14.189 (1), c = 18.562(1) Å, Dx = 1.348 g cm?3, Z = 4, R = 0.038 for 919 reflections for FAG. Unlike the other tripeptides GAF, GGV, GAL, and GAI, the crystals of AFG and FAG do not contain water molecule, and the molecules of AFG or FAG do not show the helical conformation. The torsion angles at the backbone of the peptide are ψ1 = 144. 5(5)°; ?2, ψ2 = ?98.1(6)°, ?65.2(6)° ?3, ψ13, ψ31 = 154.1(6)°, ?173.6(6)°, 6.9(8)° for AFG; and ψ1 = 162.6(3)°; ?2, ψ2 = ?96.7(4)°, ?46.3(4)°; ?3, ψ13, ψ31 = 150.1(3)°, ?168.7(3)°, 12.2(5)° for FAG. The conformation angles (? ψ) for residues 2 and 3 for both AFG and FAG show incipient formation of an β-bulge. © 1993 John Wiley & Sons, Inc.  相似文献   

9.
Incorporation of easily available achiral ω-amino acid residues into an oligopeptide results in substitution of amide bonds by polymethylene units of an aliphatic chain, thereby providing a convenient strategy for constructing a peptidomimetic. The central Gly-Gly segment of the helical octapeptide Boc-Leu-Aib-Val-Gly-Gly-Leu-Aib-Val-Ome(1) has been replaced by δ-amino-valeric acid (δ-Ava) residue in the newly designed peptide Boc-Leu-Aib-Val-δ-Ava-Leu-Aib-Val-OMe(2). 1H-nmr results clearly suggest that in the apolar solvent CDCl3, the δ-Ava residue is accommodated into a folded helical conformation, stabilized by successive hydrogen bonds involving the NH groups of Val(3), δ-Ava(4), and Leu(5). The δ-Ava residue must adopt a gauche-gauche-trans-gauche-gauche conformation along the central polymethylene unit of the aliphatic segment, a feature seen in an energy-minimized model conformation based on nmr parameters. The absence of hydrogen bonding functionalities, however, limits the elongation of the helix. In fact, in CDCl3, the folded conformation consists of an N-terminal helix spanning residues 1–4, followed by a Type II β-turn at residues 5 and 6, whereas in strongly solvating media like (CD3)2SO, the unfolding of the N-terminal helix results in β-turn conformations at Leu(1)-Aib(2). The Type II β-turn at the Leu(5)-Aib(6) segment remains intact even in (CD3)2SO. CD comparisons of peptides 1 and 2 reveal a “nonhelical” spectrum for 2 in 2,2,2-trifluoroethanol. © 1996 John Wiley & Sons, Inc.  相似文献   

10.
The protected dipeptide Boc-Aib-Pro-OBzl, C21H30N2O5, crystallizes in the orthorhombic space group P212121, with a = 12.820, b = 10.529, c = 16.548Å, and Z = 4. The crystal structure has been solved by direct methods and refined to an R value of 0.074 for 1352 reflections. The Boc-Aib-Pro-OBzl molecule has been shown to adopt an unfolded conformation in the solid state with ?Aib = 50.5°, ΨAib = 45.3°, ?Pro = ?64.6°, and ΨPro = 148.1°. The result is in marked contrast with the reported crystal structure of Cbz-Aib-Pro-NHMe, which adopts an intramolecularly hydrogen-bonded β-turn conformation. Comparison with 13 reported conformations of Aib-Pro sequences in the crystalline state revealed that the Aib-Pro sequence adopts an unfolded conformation if the residue that immediately follows the dipeptide sequence possesses no hydrogen available for hydrogen bonding, while a β-turn conformation is preferred if the Pro residue is followed by an NH group. Correlation between pyrrolidine ring puckering of the Pro residue and main-chain conformation in Aib-Pro sequences is discussed.  相似文献   

11.
Abstract

The crystal structure of the dehydro octapeptide Boc-Val-ΔPhe-Phe-Ala-Leu-Ala-ΔPhe-Leu-OH has been determined to atomic resolution by X-ray crystallographic methods. The crystals grown by slow evaporation of peptide solution in methanol/water are orthorhombic, space group P212121. The unit cell parameters are a= 8.404 (3), b= 25.598(2) and c = 27.946(3) Å, Z=4. The agreement factor is R= 7.58% for 3636 reflections having (IF0I) ≥ 3σ (IF0I). The peptide molecule is characterised by a 310-helix at the N-terminus and a π-turn at the C-terminus. This conformation is exactly similar to the helix termination features observed in proteins. The π-turn conformation observed in the octapeptide is in good agreement with the conformational features of π-turns seen in some proteins. The αL-position in the π-turn of the octapeptide is occupied by ΔPhe7, which shows that even bulky residues can be accommodated in this position of the π-turns. In proteins, it is generally seen that aL- position is occupied by glycine residue. No intermolecular head-to-tail hydrogen bonds are observed in solid state structure of the octapeptide. A water molecule located in the unit cell of the peptide molecule is mainly used to hold the peptide molecule together in the crystal. The conformation observed for the octapeptide might be useful to understand the helix termination and chain reversal in proteins and to construct helix terminators for denovo protein design.  相似文献   

12.
An apolar synthetic octapeptide, Boc-(Ala-Aib)4-OMe, was crystallized in the triclinic space group P1 with cell dimensions a = 11.558 Å, b = 11.643 Å, c = 9.650 Å, α = 120.220°, β = 107.000°, γ = 90.430°, V = 1055.889 Å3, Z = 1, C34H60O11N8·H2O. The calculated crystal density was 1.217 g/cm3 and the absorption coefficient ? was 6.1. All the intrahelical hydrogen bonds are of the 310 type, but the torsion angles, ? and ψ, of Ala(5) and Ala(7) deviate from the standard values. The distortion of the 310-helix at the C-terminal half is due to accommodation of the bulky Boc group of an adjacent peptide in the nacking. A water molecule is held between the N-terminal of one peptide and the C-terminal of the other. The oxygen atom of water forms hydrogen bonds with N (1) -H and N (2) -H, which are not involved in the intrahelical hydrogen bonds. The hydrogen atoms of water also formed hydrogen bonds with carbonyl oxygens of the adjacent peptide molecule. On the other hand, 1H-nmr analysis revealed that the octapeptide took an α-helical structure in a CD3CN solution. The longer peptides, Boc-(Ala-Aib)6-OMe and Boc-(Ala-Aib)8-OMe, were also shown to take an α-helical structure in a CD3CN solution. An α-helical conformation of the hexadecapeptide in the solid state was suggested by x-ray analysis of the crystalline structure. Thus, the critical length for transition from the 310- to α-helix of Boc-(Ala-Aib)n-OMe is 8. © 1993 John Wiley & Sons, Inc.  相似文献   

13.
To obtain general rules of peptide design using α,β-dehydro-residues, a sequence with two consecutive ΔPhe-residues, Boc-L -Val-ΔPhe–ΔPhe- L -Ala-OCH3, was synthesized by azlactone method in solution phase. The peptide was crystallized from its solution in an acetone/water mixture (70:30) in space group P61 with a=b=14.912(3) Å, c= 25.548(5) Å, V=4912.0(6) Å3. The structure was determined by direct methods and refined by a full matrix least-squares procedure to an R value of 0.079 for 2891 observed [I?3σ(I)] reflections. The backbone torsion angles ?1=?54(1)°, ψ1= 129(1)°, ω1=?177(1)°, ?2 =57(1)°, ψ2=15(1)°, ω2 =?170(1)°, ?3=80(1)°, ψ3 =7(2)°, ω3=?177(1)°, ?4 =?108(1)° and ψT4=?34 (1)° suggest that the peptide adopts a folded conformation with two overlapping β-turns of types II and III′. These turns are stabilized by two intramolecular hydrogen bonds between the CO of the Boc group and the NH of ΔPhe3 and the CO of Val1 and the NH of Ala4. The torsion angles of ΔPhe2 and ΔPhe3 side chains are similar and indicate that the two ΔPhe residues are essentially planar. The folded molecules form head-to- tail intermolecular hydrogen bonds giving rise to continuous helical columns which run parallel to the c-axis. This structure established the formation of two β-turns of types II and III′ respectively for sequences containing two consecutive ΔPhe residues at (i+2) and (i+3) positions with a branched β-carbon residue at one end of the tetrapeptide.  相似文献   

14.
α,β-Dehydro amino acid residues are known to constrain the peptide backbone to the β-bend conformation. A pentapeptide containing only one α,β dehydrophenylalanine (ΔPhe) residue has been synthesized and crystallized, and its solid state conformation has been determined. The pentapeptide Boc-Leu-Phe-Ala-ΔPhe-Leu-OMe (C39H55N5O8, Mw = 721.9) was crystallized from aqueous methanol. Monoclinic space group was P21, a = 10.290(2)°, b = 17.149(2)°, c = 12.179(2) Å, β = 96.64(1)° with two molecules in the unit cell. The x-ray (Mo Kα, λ = 0.7107A) intensity data were collected using a CAD4 diffractometer. The crystal structure was determined by direct methods and refined using least-squares technique. R = 4.4% and Rw = 5.4% for 4403 reflections having |F0| ≥ 3σ(|F0|). All the peptide links are trans and the pentapeptide molecule assumes 310-helical conformation. The mean ?,ψ values, averaged over the first four residues, are ?64.4°, ?22.4° respectively. There are three 4 → 1 intramolecular hydrogen bonds, characteristic of 310,-helix. In the crystal, the peptide helices interact through two head-to-tail. N? H? O intermolecular hydrogen bonds. The peptide molecules related by 21, screw symmetry form a skewed assembly of helices. © 1995 John Wiley & Sons, Inc.  相似文献   

15.
Abstract

The total potential energy including nonbondedJuntorsional and hydrogen bond contributions has been computed for pustulan, a (1?6) linked β-D-glucan polysaccharide, as a function of rotational angles φ, ψ, and ω The (φ, ψ, ω)-space contains many local minima and at least three distinct deep minima. Two minima at (φ, ψ, ω)=(25°,190°,gg) and (φ, ψ, ω)=(65°,150°,gg) of almost equal energies have helical parameters (n=5.2, A=1.0Å) and (n=3.2, h= 1.5Å), respectively. A third minimum at (φ, ψ, ω)=(40°,70°gt) leads to an extended zig-zag structure (n=2.2, h=2.2Å). Energy maps obtained for gentiobiose, the disaccharide of pustulan, also reveal many local minima and the small energy differences among them indicate that gentiobiose is extremely flexible. Gentiodextrins, a family of cyclic molecules of (l?6)-β-D- glucose residues, were also studied. Conformations free from steric hindrance were found for cyclic molecules with three to six glucose residues.  相似文献   

16.
Ch. Pulla Rao  P. Balaram 《Biopolymers》1982,21(12):2461-2472
The pentapeptide Boc-Leu-Aib-Pro-Val-Aib-OMe, a fragment of alamethicin and suzukacillin, crystallizes in the space group P21, with a = 11.034 (2), b = 10.894 (2), c = 15.483 (2) Å, β = 104.80 (2)° and Z = 2. The crystal structure has been solved by direct methods and refined to an R value of 0.069. The peptide backbone folds into a right-handed 310-helical conformation, stabilized by two intramolecular 4 → 1 hydrogen bonds between the Leu(1) CO and Val(4) NH and Aib(2) CO and Aib(5) NH groups. The solid-state conformation is consistent with results of spectroscopic analysis in solution.  相似文献   

17.
The synthesis and X-ray diffraction analysis of two dipeptide taste ligands have been carried out as part of our study of the molecular basis of taste. The compounds L -aspartyl-D -α-methylphenylalanine methyl ester [L -Asp-D -(αMe)Phe-OMe] and L -aspartyl-D -alanyl-2,2,5,5-tetramethylcyclopentanyl ester [L -Asp-D -Ala-OTMCP] elicit bitter and sweet taste, respectively. The C-terminal residues of the two analogues adopt distinctly different conformations in the solid state. The aspartyl moiety assumes the same conformation found in other dipeptide taste ligands with the side-chain carboxylate and the amino groups formaing a zwitterionic ring with a conformation defined by ψ,χX1 = 157.7°, ?61.5° for L -Asp-D -Ala-OTMCP and 151.0°, ?68.8° for L -Asp-D -(αMe)Phe-OMe. In the second residue, a left-handed helical conformations is observed for the (αMe)Phe residue of L -Asp-D -(αMe)Phe-OMe with ?2 = 49.0° and ψ2 = 47.9°, while the Ala residue of L -Asp-D -Ala-OTMCP adopts a semi-exextended conformation characterized by dihedral angles ?2 = 62.8° and ψ2 = ?139.9°. The solid-state structure of the bitter L -Asp-D -(αMe)Phe-OMe is extended; while the crystal structure of the sweet L -Asp-D -OTMCP roughly adopts the typical L-shaped structure shown by other sweeteners. The data of L -Asp-D -(αMe)Phe-OMe are compared with those of its diastereoisomer L -Asp-L -(αMe)Phe-OMe. Conformational analysis of the two taste ligands in solution by NMR and computer simulations agrees well with our model for sweet and bitter tastes.  相似文献   

18.
The synthesis of the tetrapeptide benzyloxycarbonyl(α-aminoisobutyryl-L -prolyl)2-methyl ester (Z-(Aib-Pro)2-OMe) and an analysis of its conformation in solution and the solid state are reported. Stepwise synthesis using dicyclohexylcarbodiimide leads to racemization at Pro(2). Evidence for the presence of diastereomeric tetrapeptides is obtained from 270-MHz1H-nmr and 67.89-MHz 13C-nmr. The all-L tetrapeptide is obtained by fractional crystallization from ethyl acetate. The NH of Aib(3) is shown to be involved in an intramo-lecular hydrogen bond by variable-temperature 1H-nmr and the solvent dependence of NH chemical shifts. The results are consistent with a β-turn conformation with Aib(1) and Pro(2) at the corners stabilized by a 4 → 1 hydrogen bond. The molecule crystallizes in the space group P212121, with a = 8.839, b = 14.938, and c = 22.015 Å. The structure has been refined to an R value of 0.051. The peptide backbone is all-trans, and a 4 → 1 hydrogen bond, between the CO group of the urethane moiety and Aib(3) NH, is observed. Aib(1) and Pro(2) occupy the corner positions of a type I β-turn with ? = ?55.4°, Ψ = ?31.3° for Aib(1) and ? = ?71.6°, Ψ = ?38° for Pro(2). The tertiary amide unit linking Pro(2) and Aib(3) is significantly distorted from planarity (Δω = 14.3°).  相似文献   

19.
The solid state conformations of cyclo[Gly–Proψ[CH2S]Gly–D –Phe–Pro] and cyclo[Gly–Proψ[CH2–(S)–SO]Gly–D –Phe–Pro] have been characterized by X-ray diffraction analysis. Crystals of the sulfide trihydrate are orthorhombic, P212121, with a = 10.156(3) Å, b = 11.704(3) Å, c = 21.913(4) Å, and Z = 4. Crystals of the sulfoxide are monoclinic, P21, with a = 10.662(1) Å, b = 8.552(3) Å, c = 12.947(2) Å, β = 94.28(2), and Z = 2. Unlike their all-amide parent, which adopts an all-trans backbone conformation and a type II β-turn encompassing Gly-Pro-Gly-D -Phe, both of these peptides contain a cis Gly1-Pro2 bond and form a novel turn structure, i.e., a type II′ β-turn consisting of Gly–D –Phe–Pro–Gly. The turn structure in each of these peptides is stabilized by an intramolecular H bond between the carbonyl oxygen of Gly1 and the amide proton of D -Phe4. In the cyclic sulfoxide, the sulfinyl group is not involved in H bonding despite its strong potential as a hydrogen-bond acceptor. The crystal structure made it possible to establish the absolute configuration of the sulfinyl group in this peptide. The two crystal structures also helped identify a type II′ β-turn in the DMSO-d6 solution conformers of these peptides. © 1993 John Wiley & Sons, Inc.  相似文献   

20.
The metastable state silk I structures of Bombyx mori silk fibroin in the solid state were studied on the basis of 15N- and 13C-nmr chemical shifts of Ala, Ser, and Gly residues. The 15N cross-polarization magic angle spinning (CP/MAS) nmr spectra of the precipitated fraction after chymotrypsin hydrolysis of B. mori silk fibroin with the silk I and silk II forms were measured to determine the 15N chemical shifts of Gly, Ala, and Ser residues. For comparison, 15N CP/MAS nmr chemical shifts of Ala were measured for [15N] Ala Philosamia cynthia ricini silk fibroin with antiparallel β-sheet and α-helix forms. The 13C CP/MAS nmr chemical shifts of Ala, Ser, and Gly residues of B. mori silk fibroin with the silk I and silk II forms, as well as 13C CP/MAS nmr chemical shifts of Ala residue of P. c. ricini silk fibroin with β-sheet and α-helix forms, are used for the examination of the silk I structure. Both silk I and α-helix peaks are shifted to a lower field than silk II (β-sheet) for the Cα carbons of the Ala residues, while both Cβ carbon peaks are shifted to higher field. However, the silk I peak of the 15N nucleus of the Ala residue is shifted to lower field than the silk II peak, but the α-helix peak is shifted to high field. Thus, the difference in the structure between the silk I and α-helix is reflected in a different manner between the 13C and 15N chemical shifts. The Cα and Cβ chemical shift contour plots for Ala and Ser residues, and the Cα plot for the Gly residue, were prepared from the Protein Data Bank data obtained for 12 proteins and used for discussing the silk I structure quantitatively from the conformation-dependent chemical shifts. The plots reported by Le and Oldfield for 15N chemical shifts were also used for the purpose. All these chemical shift data support Fossey's model (Ala: ϕ = −80°, φ = 150°, Gly: ϕ = −150°, φ = 80°) and do not support Lotz and Keith's model (Ala: ϕ = −104.6°, φ = 112.2°, Gly: ϕ = 79.8°, φ = 49.7° or Ala: ϕ = −124.5°, φ = 88.2°, Gly: ϕ = −49.8°, φ = −76.1°) as the silk I structure. © 1997 John Wiley & Sons, Inc.  相似文献   

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