共查询到20条相似文献,搜索用时 46 毫秒
1.
EWA CISZAK GRZEGORZ PIETRZY
SKI BARBARA RZESZOTARSKA 《Chemical biology & drug design》1992,39(3):218-222
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) Å3 Z = 4, Dm= 1.17 g cm?3 and Dc=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. 相似文献
2.
MITSUNOBU DOI TOSHIMASA ISHIDA ALESSANDRA POLESE FERNANDO FORMAGGIO MARCO CRISMA CLAUDIO TONIOLO QUIRINUS B. BROXTERMAN JOHAN KAMPHUIS 《Chemical biology & drug design》1996,47(6):491-497
The molecular and crystal structures of the Cα-tetrasubstituted, δ-branched α-amino acid Cα-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 Cα-alkylated aromatic α-amino acids. © Munksgaard 1996. 相似文献
3.
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) Å3, 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. 相似文献
4.
G. VALLE M. CRISMA C. TONIOLO. S. POLINELLI W. H. J. BOESTEN H. E. SCHOEMAKER E. M. MEIJER J. KAMPHUIS 《Chemical biology & drug design》1991,37(6):521-527
The molecular and crystal structures of one derivative and three model peptides (to the pentapeptide level) of the chiral Cα,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. 相似文献
5.
T.P. SINGH PUNIT NARULA V.S. CHAUHAN ASHWANI K. SHARMA WINIFRIED HINRICHS 《Chemical biology & drug design》1989,33(2):167-172
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) Å3, 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 Cγ-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)°, χ22.1=– 112(2)° and χ22.2= 136(2)°. The crystal structure is stabilized by a network of hydrogen bonds and van der Waals interactions. 相似文献
6.
F. FORMAGGIO C. TONIOLO M. CRISMA G. VALLE B. KAPTEIN H.E. SCHOEMAKER J. KAMPHUIS B. DI BLASIO O. MAGLIO R. FATTORUSSO E. BENEDETTI A. SANTINI 《Chemical biology & drug design》1995,45(1):70-77
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 Nα-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. 相似文献
7.
R. BARDI A.M. PIAZZESI C. TONIOLO M. SUKUMAR P. ANTONY RAJ P. BALARAM 《Chemical biology & drug design》1985,25(6):628-639
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 (φAib = – 50.3°, ψAib = – 45.8°; φAcc6 = – 68.4°, ψAcc6 = – 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 (φAib = – 62.2°, ψAib = 143°) while the Acc6residue maintains a helical conformation (φAcc6 = 48°, ψAcc6= 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. 相似文献
8.
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. 相似文献
9.
C. PIETRZYASKI B. RZESZOTARSKA E. CISZAK M. LISOWSKI Z. KUBICA G. BOUSSARD 《Chemical biology & drug design》1996,48(4):347-356
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. 相似文献
10.
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. 相似文献
11.
SHAW-LIN HAN EVELYN R. STIMSON FREDERICK R. MAXFIELD HAROLD A. SCHERAGA 《Chemical biology & drug design》1981,17(3):297-315
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. 相似文献
12.
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) Å3 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. 相似文献
13.
S. ZERKOUT V. DUPONT A. AUBRY J. VIDAL A. COLLET A. VICHERAT M. MARRAUD 《Chemical biology & drug design》1994,44(4):378-387
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)Nx. 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. 相似文献
14.
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. 相似文献
15.
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 Oγ oxygen atom. The result is that the disposition of the Ser Cα–Cβ bond corresponds to the rotamer III (χ1? 60°). As shown by the crystal structure of 3 , the intra-Ser NH.Oγ 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 Cα–Cβ bond in peptides and protein. 相似文献
16.
ANDRE AUBRY GRZEGORZ PIETRZYNSKI BARBARA RZESZOTARSKA GUY BOUSSARD MICHEL MARRAUD 《Chemical biology & drug design》1991,37(1):39-45
The crystal structure of the tBuCO-d,l -Ala-Δz-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 dc= 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. 相似文献
17.
SUDHANAND PRASAD R. BALAJI RAO HAKAN BERGSTRAND BRITTA LUNDQUIST ELMER L. BECKER P. BALARAM 《Chemical biology & drug design》1996,48(4):312-318
Five stereochemically constrained analogs of the chemotactic tripeptide incorporating l-aminocycloalkane-l-carboxylic acid (Acnc) 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. 相似文献
18.
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. 相似文献
19.
Two isomeric, acyclic tetrapeptides containing a Z-dehydrophenylalanine residue (Δz-Phe) at position 2 or 3, Boc-Leu-Ala-Δz-Phe-Leu-OMe (1) and Boc-Leu-Δz-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 (CαjH ? Ni+1H and NiH ? Ni+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-Δz-Phe- Type II β-turn structure and a second species with Δz-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-Δz-Phe-Ala- segment and an almost completely extended conformation. 相似文献
20.
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 Å3, Z = 4 with a density of Dobs= 1.29 g-cm-3 and Dcalc= 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. 相似文献