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1.
MAMATA S. PRACHAND M. M. DHINGRA ANIL SARAN EVANS COUTINHO J. BODI H. SÜLI-VARGHA K. MEDZIHARDSZKY 《Chemical biology & drug design》1998,51(4):251-265
Solution conformation of cyclo(Gly1-His2-Phe3-Arg4-Trp5-Gly6) and its d -Phe analog corresponding to the message sequence [Gly-α-MSH5-10] of α-MSH has been studied by 1D and 2D proton magnetic resonance spectroscopy in dimethyl sulfoxide (DMSO)-d6 solution and in a DMSO-d6/H2O cryoprotective mixture. The NMR data for both the analogs in solution at 300 K cannot be interpreted based on a single ordered conformation, as evidenced by the broadening of only -NH resonances as well as the temperature coefficients of the amide protons. An analysis of the nuclear Overhauser effect (NOE) cross-peaks in conjunction with temperature coefficient data indicates an equilibrium of multiple conformers with a substantial population of particular conformational states at least in the d -analog. The molecular dynamics simulations without and with NOE constraints also reveal numerous low-energy conformers with two γ-turns, a γ-turn and a β-turn, two β-turns, etc. for both the analogs. The observed NMR spectra can be rationalized by a dynamic equilibrium of conformers characterized by a γ-bend at Gly6, two γ-bends at Phe3 and Gly6 and a conformer with a single β-turn and a γ-bend for the l -Phe analog. On the other hand, a conformation with two fused β-turns around the two tetrads His2-d -Phe3-Arg4-Trp5 and Trp5-Gly6-Gly1-His2 dominates the equilibrium mixture for the d -Phe analog. For the d -Phe analog, the experimentally observed average conformation is corroborated by molecular dynamics simulations as well as by studies in cryoprotective solvent. 相似文献
2.
A systematic conformational build-up procedure was performed for the LHRH molecule, pGlu1-His2-Trp3-Ser4-Tyr5-Gly6-Leu7-Arg8-Pro9-Gly10-NH2. The results showed a very high flexibility of the LHRH backbone, with 300 conformers being regarded as having low energy. At the same time, the conformational flexibility of LHRH differs among the fragments of the molecule. The most rigid fragments of LHRH are the Ser4-Tyr5-Gly6-Leu7 and Tyr5-Gly6-Leu7-Arg8 central tetrapeptides, the latter possessing only eight different types of low-energy backbone conformers. These eight conformer types belong to different kinds of chain reversals which are stabilized by different systems of intramolecular hydrogen bonds. Some of them resemble the β-II′ turn, which was derived as the LHRH structure from energy calculations by others. The results obtained are in good agreement with the experimental data on LHRH flexibility in solution. 相似文献
3.
The hexapeptide [cyclo(Leu1Ψ(CH2NH2)Leu2-Gln3-Trp4-Phe5-Gly6)] +1 is a potent antagonist of neurokinin A activity in tissues of hamster urinary bladder. The solution conformation of this cyclic hexapeptide has been characterized by the combined use of two dimensional nuclear magnetic resonance spectroscopy and restrained molecular dynamics. The proton spectrum of the peptide was fully assigned by the sequential assignment procedure. Interproton distances were derived from crosspeak volumes in two dimensional Nuclear Overhauser Effect spectra, and dihedral angles were calculated from appropriate coupling constants. Temperature coefficients of the amide protons were determined. Restrained molecular dynamics simulations were carried out using the backbone interproton distances as constraints. During 210 ps of restrained molecular dynamics the peptide interconverted among three closely related families of conformations. These interconversions occurred at picosecond timescales under the simulation conditions. 相似文献
4.
S. LAVIELLE G. CHASSAING A. BRUNISSEN M. RODRIGUEZ J. MARTINEZ O. CONVERT A. CARRUETTE C. GARRET F. PETITET M. SAFFROY Y. TORRENS J.-C. BEAUJOUAN J. GLOWINSKI 《Chemical biology & drug design》1993,42(3):270-277
Previous studies with Substance P (SP) antagonists (GR 71251, [d Pro9, Pro10, Trp11]SP and d Pro9, MeLeu10, Trp11]SP) have suggested the existence in the guinea-pig ileum (GPI) of two distinct tachykinin receptors associated with the contractile responses of [Pro9]SP and septide. In addition [Apa9-10]SP, a glycine-substituted analogue of SP with a carba bond between residues 9 and 10, [Gly9-ψ(CH2-CH2)-Gly10SP = [Apa9-10]SP, was shown to belong to the ‘septide family’ (low affinity for NK-1 specific binding sites and high potency in the GPI). In order to establish the importance of the isopropyl side-chain in position 10, the binding potencies and activities of [Gly9-ψ(CH2-CH2)-Gly10]SP, [Ala10]SP, [Gly9-ψ(CH2-CH2)-Leu10]SP and [Gly9-ψ(CH2-CH2)-d Leu10]SP were compared. Conformational behaviour of active peptides with a carba bond was analyzed by NMR and modelisation studies. This study with agonists demonstrated that undecapeptides substituted in position 10 in the SP sequence also enabled discrimination of NK-1 receptors from receptors responsible for the spasmogenic activities of peptides belonging to the ‘septide family’. [Gly9-ψ(CH2-CH2)- Leu10]SP is ahighly potent NK-1 agonist, [Gly9-ψ(CH2-CH2)-Gly10]SP acts on the septide-sensitive receptor, and [Ala10]SP is a mixed agonist. 相似文献
5.
PIETRO AMODEO GABRIELLA SAVIANO GIANFRANCO BORIN ANDREA CALDERAN PAOLO RUZZA TEODORICO TANCREDI 《Chemical biology & drug design》1998,51(3):180-187
To investigate the conformational flexibility of metal-complexed cyclodecapeptides, we synthesized and studied two antamanide analogs, in which the phenylalanine residue in position 6 or 9 of the sequence was substituted by Gly. Previous conformational studies on antamanide suggested that these backbone regions are affected by conformational variation. The NMR conformational study showed a high degree of flexibility for the two analogs. With sodium ions, on the other hand, [Gly9]. -antamanide was able to form a fairly stable equimolar complex, whereas [Gly6]. -antamanide showed a conformational heterogeneity, with one prevailing conformer. For the [Gly9]. -antamanide analog, the whole NMR data, combined with extensive theoretical calculations, were consistent with the presence of 1) two (β-turns of type I, centered on Gly9-Phe10 and Ala4-Phe5, respectively; 2) a central cavity with a six-carbonyl oxygen cage, optimal for a Na+ hexacoordination; 3) strongly H-bonded amide protons for residues 1 and 6, both involved in the formation of the two type I β-turns, which, however, exhibited some fluctuations during the molecular dynamics simulations. For the [Gly6]. -antamanide-Na+ complex the prevailing conformer was consistent with a more open structure, with the partial solvent exposure of all the amide protons; that is, the Gly residue in position 6 increases the flexibility of this critical site more than does the Gly in position 9. These data in some way parallel the results of the cytotoxicity tests on B16-F10 transformed cells for the two analogs: [Gly9]. -antamanide is cytotoxic after 48 h exposure, whereas [Gly6]. -antamanide is almost inactive. On the contrary, both analogs are practically inactive in vivo against phalloidin. 相似文献
6.
HORST KESSLER ARMIN GEYER HANS MATTER MATTHIAS KOCK 《Chemical biology & drug design》1992,40(1):25-40
One carbonyl oxygen of the cyclic hexapeptide cycle(-Gly1-Pro2-Phe3-Val4-Phe5-Phe6-) (A) can be selectively exchanged with sulphur using Yokoyama's reagent. Surprisingly it was not the C=O of Gly1 but that of Phe5 which was substituted and cyclo(-Gly1-Pro62-Phe3-Va14-Phe5ψ[CS-NH]Phe6-) (B)was obtained. Thionation results in a conformational change of the peptide backbone although the C=O of Phe5 and the corresponding C=S are not involved in internal hydrogen bonds. Two isomers in slow exchange, containing a CIS Gly1-Pro2 bond in a βVIa-turn (minor) and a trans Gly-Pro bond in a βII′-turn (major), were analyzed by restrained molecular dynamics in vacuo and in DMSO as well as using time dependent distance constraints. It is impossible to fit all experimental data to a static structure of each isomer. Interpreting the conflicting NOES, local segment flexibility is found. MD simulations lead to a dynamic model for each structure with evidence of an equilibrium between a βI- and βII-turn about the Val4-Phe5 amide bond in both the cis and trans isomers. Additionally proton relaxation rates in the rotating frame (R1p) were measured to verify the assumption of this fast βI/βII equilibrium within each isomer. Significant contributions to R1p-rates from intramolecular motions were found for both isomers. Therefore it is possible to distinguish between at least four conformers interconverting on different time scales based on NMR data and MD refinement. This work shows that thionation is a useful modification of peptides for conformation-activity investigations. 相似文献
7.
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
8.
The cyclopentapeptide, ·L·Val1-L· Pro2-Gly3-L· Val4-Gly5, was synthesized and its conformational characterization was carried out using n.m.r. and theoretical energy calculations. The n.m.r. studies indicated the existence of a cis Val1-Pro2 peptide bond in water and a very strong intramolecular H-bond between the Val1 NH and Gly3 C=O groups. This H-bond forms a β-turn (type II) placing Val4 Gly5 residues within the turn. Two minimum energy conformations were derived, one of which agrees very well with the solution conformation. 相似文献
9.
The structure of cyclic antagonists of luteinizing hormone-releasing hormone (LHRH), Ac-D-Phe(p-Cl)1-D-Phe(p-C1)2-Trp3-Ser4-c(Asp5-D-Arg6-Leu7-Lys8)-Pro9-D-Ala10-NH2 ( I ), Ac-D-Phe(p-Cl)l-D-Phe(p-Cl)2-D-Trp3-Ser4-c(Glu5-Arg6-Leu7-Lys8)-Pro9-D-Ala10-NH2 ( II ) and their linear analogues, Ac-D-Phe(p-Cl)1-Phe(p-C1)2-Trp3-Ser4-Asp5-D-Arg6-Leu7-Lys8-Pro9-D-Ala10-NH2 ( III ) and Ac-D-Phe(p-Cl)1-D-Phe(p-C1)2-Trp3-Ser4-Glu5-D-Arg6-Leu7-Lys8-Pro9-D-Ala10-NH2 ( IV ), have been studied by NMR spectroscopy. The cyclic peptides I and II are more potent antagonists than the corresponding linear peptides in an in vivo assay. All the peptides show propensity of an unusual type II′β-turn involving residues 3–6. Cyclic analogues also show some additional structure around residues 7 and 8 which is absent in the linear peptides. This additional structure in the cyclic peptides may be due to a minor conformation with a β-turn between residues 5 and 8. © Munksgaurd 1995. 相似文献
10.
Synthesis, proton magnetic resonance and carbon-13 magnetic resonance characterizations, including complete assignments, are reported for the polyhexapeptide of elastin, HCO-Val-(Ala1-Pro2-Gly3-Val4-Gly5-Val6)18-OMe. Temperature dependence of peptide NH chemical shifts and solvent dependence of peptide C-O chemical shifts have been determined in several solvents and have been interpreted in terms of four hydrogen bonded rings for each repeat of the polyhexapeptide. The more stable hydrogen bonded ring is a β-turn involving Ala1 C-O…HN·Val4 More dynamic hydrogen bonds are an 11-atom hydrogen bonded ring Gly3 NH · O-C Gly5, a 7-atom hydrogen bonded ring (a γ-turn) Gly3 C-O … HN · Gly5 and a 23-atom hydrogen bonded ring Val61NH … O-C Val6(l+l). This set of hydrogen bonds results in a right-handed β-spiral structure with slightly more than two repeats (approximately 2.2) per turn of spiral. The β-spiral structure is briefly discussed relative to data on the elastic fiber. 相似文献
11.
Synthesis and conformational analysis of the S-glycosylated cyclic hexapeptide cyclo(-d -Pro1-Phe2-Cys3(tetra-O-acetyl-β-d -galactopyranosyl)-Trp4-Lys(Z)5-Phe6-) I was carried out to examine the influence of a saccharide residue in position i of a standard β-turn on the formation of reverse turns and on the biological activity. Synthesis was carried out in the liquid phase employing a galactosylated cysteine building block. The cyclization reagents DPPA/NaHCO3 avoided high dilution conditions. Spectroscopic data were extracted from homo- and heteronuclear 2D-NMR techniques (TOCSY, NOESY, HMQC, HMQC-TOCSY, HMBCS-270). For structural refinement restrained molecular dynamics (MD) simulations in vacuo and with explicit DMSO as solvent were performed. Finally, simulations in DMSO without experimental restraints provided insight in stability and dynamics of the structural model. A comparison of the S-glycosylated Cys3 peptide with the analogous Thr3 peptide exhibits a similar overall conformation of the hexapeptide [βII’d -Pro-Phe and another β-turn about Trp4-Lys5(Z)]. However, the latter shows a distinct dynamic flip βI, βII in the glycopeptide, whereas the Thr-analogue only populates βI. This influence is attributed to a βI stabilizing effect of a hydrogen bridge of Thr-O, in position i to the NH of the amino acid in position i+ 2, which is lacking in the glycosylated compound. 相似文献
12.
HIN-WING YEUNG DONALD YAMASHIRO LIANG-FU TSENG WEN-CHANG CHANG CHOH HAO LI 《Chemical biology & drug design》1981,17(2):235-238
Four analogs of the opioid peptide human β-endorphin (βh-EP) have been synthesized: [d -Lys9,Phe27,Gly31]-βh-EP, [d -Phe18,Phe27,Gly31)-βh-EP, [d -Thr2,d -Lys9,Phe27,Gly31]-βh-EP, and [d -Thr2,d -Phe18,Phe27,Gly31]-βh-EP. All are practically indistinguishable from βh-EP in the guinea pig ileum assay. All show diminished analgesic potency in the mouse tail-flick assay. 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.
ROBERT E. LONDON PAUL G. SCHMIDT RAYMOND J. VAVREK JOHN M. STEWART 《Chemical biology & drug design》1982,19(4):334-342
Substitution of Pro residues with AIB (α-aminoisobutyric acid) residues in peptides provides a means of evaluating the presence of cis' proline conformations both in solution and, using bioassay data, in a receptor complex. 1 H n.m.r. has been used to probe the DMSO solution conformation of all seven of the possible AIB/Pro isomers of bradykinin. AIB substitution for Pro2 and/or Pro3 appears to stabilize a type III β-turn involving the N-terminal residues, but not an incipient 310 helix suggested by model peptides. These substitutions are correlated with low biological potencies, suggesting that such conformational features may be incompatible with receptor complexation. Alternatively, AIB7 -bradykinin analogs exhibit a variety of long range shift perturbations relative to bradykinin. The data suggests that bradykinin can adopt several folded conformations, including β-turns involving both Ser6-Pro7-Phe8-Arg9 and Phe5-Ser6-Pro7-Phe8. The relatively high biological activities of the AIB7-BK suggest that the complexed form of the peptide is characterized by a cis' Pro7 conformation. 相似文献
15.
HIROO YONEZAWA KOUJI OKAMOTO MAKOTO KANEDA NAOTOMO TOMINAGA NOBUO IZUMIYA 《Chemical biology & drug design》1983,22(5):573-581
The antibiotic gramicidin S (GS) has the structure of cyclo (-l -Val1-L-Orn2-l -Leu3-d -Phe4-l -Pro5-L-Val1′-l -Orn2′-l -Leu3′-d -Phe4′-l -Pro5′-) and is basic in character. Five GS analogs including [Gly1,1′]-GS and the neutral [l -Hnv2,2′]-GS (Hnv represents δ-hydroxynorvaline) were synthesized by the solid-phase method to evaluate the role of l -Val1,1′ and l -Orn2,2′ residues in GS. The hybrid analogs ([Gly1]-GS and [l -Hnv2]-GS) and [d -Tyr4,4′]-GS showed high antibacterial activities, whereas [Gly1,1′]-GS and [l -Hnv2,2′]-GS possessed no activity. Inhibitory effects by these analogs for the adsorption of 14C-labeled GS on cells of bacteria sensitive to GS were determined. The structure-activity relationship of GS is discussed on the basis of the results on these GS analogs. 相似文献
16.
[Gly8]βhEP(1–27)NH2 and [l -Leu8]βhEP(1–27)NH2, two analogs of human β-endorphin, were synthesized by both all-stepwise solid phase synthesis and peptide segment coupling. For the peptide segment coupling method, two thiocarboxyl peptides, Msc-[Gly8]βhEP(1–8)SH and Msc-[l -Leu8]βhEP(1–8)SH, were synthesized by standard solid phase method on 4-[α-(Boc-Gly-S)benzyl]phenoxyacetamidomethy-resin and 4-[α-(Boc-l -Leu-S)benzyl]phenoxyacetamidomethy-resin. These two thiocarboxyl peptides were coupled to H-[Lys(Cit)9, 19, 24]-βhEP(9–27)NH2 [Gly8]βhEP(1–27)NH, and [l -Leu8]βhEP(1–27)NH2 were obtained after removal of Msc groups and citraconyl groups from products of the segment coupling reaction. The yields of both [Gly8]βhEP(1–27)NH2 and [l -Leu8]βhEP(1–27)NH2 in the segment coupling reaction were approximately 18%. Less than 1 % of racemization of Leu-8 occurred during coupling of Msc-[l -Leu8]βhEP(1–8)SH to H-[Lys(Cit)9, 19, 24]-βh EP(9–27)NH2. Results of amino acid composition analysis, analysis by reverse phase high pressure liquid chromatography and receptor binding activity assays of the analogs showed that peptide analogs prepared by segment coupling method and those prepared by all-stepwise solid phase synthesis were identical. Results of receptor binding activity assays suggested that the molecular charge properties of β-endorphin(1–27) and its analogs influenced the receptor binding activity. 相似文献
17.
H. KESSLER H. MATTER G. GEMMECKER H.-J. DIEHL C. ISERNIA S. MRONGA 《Chemical biology & drug design》1994,43(1):47-61
Synthesis and conformational analysis of three cyclic hexapeptides cyclo(-Gly1-Pro2-Phe3-Val4-Xra5-Phe6), Xaa= Phe (I), D-Phe (II) and D-Pro (III), were carried out to examine the influence of proline on the formation of reverse turns and the dynamics of hydrophobic peptide regions. Assignment of all 1H and 13C resonances was achieved by homo- and heteronuclear 2D-NMR techniques (TOCSY, ROESY, HMQC, HMQC-TOCSY and HMBCS-270). The conformational analysis is based on interproton distances derived from ROESY spectra and homo- and heteronuclear coupling constants (E.COSY, HETLOC and HMBCS-270). For structural refinements restrained molecular dynamics (MD) simulations in vacuo and in DMSO were performed. Each peptide exhibits two conformations in DMSO solution due to cis-trans isomerism about the Gly-Pro peptide bond. Surprisingly the cis-Gly-Pro segment in the minor isomers is not involved in a βVI-turn, but forms a turn structure with cis-Gly-Pro in the i and i+ 1 positions. Although no stabilizing hydrogen bond is found in this turn, the φ and ψ-angles closely correspond to a βI-turn [Pro2:φ(i+ 1) -60°, ψ(i+ 1) -30° Phe3: φ(i+ 2) -100°, ψ(i+ 2) -50°]. Hence we call this structural element a pseudo-βI-turn. As expected, in the dominating all-trans isomers proline occupies the i+ 1 position of a standard βI-turn. Therefore, cis-trans isomerization of the Gly1-Pro2 amide bond only induces a local conformational rearrangement, with minor structural changes in other parts of the molecule. However, the geometry of the other regions is affected by the chirality of the i+ 1 amino acid for both isomers (βI for Phe5, βII′ for D-Phe5 or D-Prp5). 相似文献
18.
MOHMED K. ANWER DOUGLAS B. SHERMAN ARNO F. SPATOLA 《Chemical biology & drug design》1990,36(4):392-399
NMR and X-ray crystallographic studies have shown that cyclic pentapeptides of the general structure cyclo(D-Xxx-Pro-Gly-Pro-Gly) possess β- and γ-turn intramolecular hydrogen bonds. As part of our continuing series surveying the compatibility of various amide bond replacements on peptide structure, we have synthesized cyclo(D-Phe-Proψ [CH2S]Gly-Pro-Gly). The pseudopeptide was prepared by solid phase methods and cleaved from the resin by a new procedure involving phase transfer catalysis using K2CO3 and tetrabutylammonium hydrogen sulfate. Cyclization was carried out with the use of DPPA, HOBt, and DMAP to afford the product in 69% yield. The conformational behavior of the pseudopeptide was analyzed by 1H and 13C (1D and 2D) NMR techniques. The backbone modification replaced the amide bond that is involved in a γ-turn intramolecular hydrogen bond in the all-amide structure. In CDCl3, the pseudopeptide adopted the same all-trans conformation as its parent, although the remaining β-turn hydrogen bond was weaker according to Δδ/ΔTNH measurements. In DMSO-d6, the all-trans conformer and a second conformer were observed in a ratio of 55:45. These conformers, which slowly inter converted on the NMR time scale, could be separately assigned; peaks due to chemical exchange were readily distinguishable by the ROESY technique as reported earlier by others. 13C and ROESY experiments suggested the minor conformer contained one cis amide bond at the Gly1-Pro2 position. Thus, both the location and type of amide surrogate are important determinants affecting the compatibility of the replacement with a particular conformational feature. 相似文献
19.
The N-terminal tyrosine residue of Met-enkephalin could be readily incorporated without protection of its phenolic hydroxyl group. Furthermore, the HF-cleaved product contained fewer impurities than that derived from hydroxyl-protected material. Despite the presence of Tyr in the center of the chain, an LH-RH antagonist, [d -Phe2, d -Trp3, d -Phe6]-LH-RH, could also be made in normal yield by incorporation of free Boc-Tyr. Syntheses of the same model peptide without protection of the Ser residue and protection of the Arg residue as the guanidine HCl salt also gave excellent yields of analog. Finally, the LH-RH inhibitor and a highly active agonist, [d -Leu6, desGly-NH210]-LH-RH ethylamide, were synthesized without protection of Tyr, Ser and Arg, which enabled free peptides to be generated directly by ammonolysis and ethylaminolysis, respectively, without HF treatment. In all examples, no evidence emerged to suggest reaction of side-chains during synthesis. 相似文献
20.
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