The longest, regular polypeptide 3(10) helix at atomic resolution

A synthetic, terminally blocked homodecapeptide from the C alpha, alpha-dimethylated glycyl residue alpha-aminoisobutyric acid has been analyzed by single-crystal X-ray diffraction and the structure refined to R = 0.073. The compound crystallizes as a perfect 3(10) helix, stabilized by eight consecutive intramolecular N-H . . . O = C hydrogen bonds. This is the first observation at atomic resolution of a regular polypeptide 3(10) helix as long as three complete turns.

Replacement of the N alpha-blocking group in the formyl-methionyl tripeptide chemoattractant: an insight into the mode of binding at the receptor on rabbit neutrophils

The tripeptide N alpha-carbamoyl-L-methionyl-L-leucyl-L-phenylalanine methyl ester has been synthesized in solution by classical methods and fully characterized. This compound, prepared in order to obtain a deeper insight into the mode of binding at the formyl peptide chemotactic receptor, has been tested for its ability to induce granule enzyme secretion from rabbit peritoneal neutrophils and found to be a complete agonist. These results confirm the hypothesis that a proton on the N alpha-blocking group of the tripeptide forms a hydrogen bond with an acceptor in the binding site.

Critical main-chain length for conformational conversion from 3(10)-helix to alpha-helix in polypeptides

To assess the minimal peptide length required for the stabilization of the alpha-helix relative to the 3(10)-helix in Aib-rich peptides, we have solved the X-ray diffraction structures of the terminally blocked sequential hexa- and octapeptides with the general formula-(Aib-L-Ala)n-(n = 3 and 4, respectively). The hexapeptide molecules are completely 3(10)-helical with four 1----4 intramolecular N-H . . . O = C H-bonds. On the other hand, the octapeptide molecules are essentially alpha-helical with four 1----5 H-bonds; however, the helix is elongated at the N-terminus, with two 1----4 H-bonds, giving these molecules a mixed alpha/3(10)-helical character. In both compounds the right-handed screw sense of the helix is dictated by the presence of the Ala residues of L-configuration. This study represents the first experimental proof for a 3(10)----alpha-helix conversion in the crystal state induced by peptide backbone lengthening only.

N alpha-formylated and tert-butyloxycarbonylated Phe-(Leu-Phe)n and (Leu-Phe)n peptides as agonists and antagonists of the chemotactic formylpeptide receptor of the rabbit peritoneal neutrophil

The various diastereomers of the N alpha-formylated(CHO) and tert-butyloxycarbonylated (t-Boc) Phe-(Leu-Phe)n and (Leu-Phe)n methyl esters, where n = 1-2 and 1-3, respectively, have been newly synthesized and their physical properties described. The CHO-blocked peptides are all able to release beta-glucosaminidase from rabbit peritoneal neutrophils in a concentration-dependent manner. There is a strong effect of primary structure and of chirality on their biology activity; lengthening the peptide chain distinctly increases activity in each series and within a series the activity decreases in the order: all-L greater than D-L much greater than all-D. Of the t-Boc protected synthetic precursors, the all-L isomers have definite but weak agonist activity; the agonist activity of the other isomers is equivocal or not detectable. All the t-Boc peptides, however, are capable of acting as weak, specific antagonists. There is a dependence of antagonist activity on primary structure, but this is variable and contingent on the nature of the peptide. Similarly, an effect of chirality on antagonist activity, although present, also depends on the structure of the peptide. In the one instance directly tested, t-Boc-L-Phe-(D-Leu-L-Phe)2-OMe (OMe, methoxy) was found to be distinctly less active than the corresponding free acid.

Theoretical and experimental conformational analysis of two diastereomeric "Val"-statine derivatives

The preferred conformations and self-association modes of the two diastereomeric N-acetyl, methylamides of 3-hydroxy, 4-amino, 5-methylhexanoic acid ("Val"-statine) with (S,S) and (R,S) configurations at the 3-hydroxy and 4-amino carbon atoms, respectively, were determined in solution as well as in the crystal state by infrared absorption, 1H nuclear magnetic resonance and x-ray diffraction. A corollary conformational energy computation study was also carried out. In the crystal state intramolecular H-bonds are absent in both structures. However, the change in chirality of the carbon atom carrying the hydroxy group and the presence of a co-crystallized water molecule in the (S,S) isomer induce partially different backbone and "Val" side chain conformations and divergent intermolecular H-bonding schemes in the two isomers. A marked propensity to self-aggregate is seen in solvents of low polarity. The two isomers, however, are largely solvated in solvents of high polarity. Conformational energy computations indicate that in vacuo both diastereomers exhibit a significant flexibility and the conformers presenting absolute minima are not stabilized by any intramolecular bonding.

Structural versatility of peptides containing C alpha, alpha-dialkylated glycines: conformational energy computations, i.r. absorption and 1H n.m.r. analysis of 1-aminocyclopropane-1-carboxylic acid homopeptides

Conformational energy computations on the 1-aminocyclopropane-1-carboxylic acid mono-, di-, and tripeptide amides, Ac-(Ac3c)n-NHMe (n = 1-3), indicate that this C alpha, alpha-dialkylated, cyclic alpha-amino acid residue is conformally restricted and that type-I(I') beta-bends and distorted 3(10)-helices are particularly stable conformations for the di- and tripeptide amides, respectively. The results of the theoretical analysis are in agreement with those obtained in an i.r. absorption and 1H n.m.r. investigation in chloroform solution of Ac3c-rich tri- and tetrapeptide esters. A comparison is also made with the conclusions extracted from our previous work on peptides rich in Aib (alpha-aminoisobutyric acid), Ac5c (1-aminocyclopentane-1-carboxylic acid), and Ac6c (1-aminocyclohexane-1-carboxylic acid).

Structural versatility of peptides containing C alpha, alpha-dialkylated glycines. An X-ray diffraction study of six 1-aminocyclopropane-1-carboxylic acid rich peptides

The molecular and crystal structures of six fully blocked, Ac3c-rich peptides to the tetramer level were determined by X-ray diffraction. The peptides are Fmoc-(Ac3c)2-OMe-CH3OH, Ac-(Ac3c)2-OMe, t-Boc-Ac3c-L-Phe-OMe, pBrBz-(Ac3c)3-OMe.H2O, Z-Gly-Ac3c-Gly-OTmb.(CH3)2CO, and t-Boc-(Ac3c)4-OMe.2H2O. Type-I (I') beta-bends and distorted 3(10)-helices were found to be typical of the tri- and tetrapeptides, respectively. In the dipeptides, too short to form beta-bend conformations, other less common structural features may be observed. The average geometry of the cyclopropyl moiety of the Ac3c residue is asymmetric and the N-C alpha-C' bond angle is significantly expanded from the regular tetrahedral value. A comparison with the structural preferences of other extensively investigated C alpha, alpha-dialylated alpha-amino acids is made and the implications for the use of the Ac3c residue in conformational design are examined.

Psi [CH2NH] backbone-modified peptides: first unequivocal observation of a C7 structure in a linear peptide

We have elucidated the X-ray diffraction structures of the psi[CH2NH] backbone-modified analogs of Z-Pro-Leu-Gly-NH2 and t-Boc-Pro-Leu-Gly-NH2 (N alpha-protected derivatives of the tripeptide amide representing the C-terminal tail of oxytocin) with the "reduced peptide bond" located at the Pro-Leu sequence. The comparative results of these pseudopeptides show that conformational properties are similar (i.e., C7 structure at the Pro), whereas the unmodified peptides diverge substantially (i.e., t-Boc-Pro-Leu-Gly-NH2 and H-Pro-Leu-Gly-NH2 each show type-II beta-bend at the Leu-Gly; and Z-Pro-Leu-Gly-NH2 shows an open folded structure). The results for t-Boc-Pro psi[CH2NH]Leu-Gly-NH2 represent the first unequivocal proof for the existence of a C7 structure in a linear peptide.

Conformationally restricted formyl methionyl tripeptide chemoattractants: a three-dimensional structure-activity study of analogs incorporating a C alpha,alpha-dialkylated glycine at position 2

The conformationally restricted CHO-L-Met-Xxx-L-Phe-OY (where Xxx = Aib, Ac3c, Ac5c, Ac6c, and Ac7c; Y = H, Me) tripeptides, analogs of the chemoattractant CHO-L-Met-L-Leu-L-Phe-OH, have been synthesized in solution by classical methods and fully characterized. Compounds were compared to determine the combined effect of backbone conformational preferences and side-chain bulkiness on the relation of three-dimensional structure to biological activity. Each peptide was tested for its ability to induce granule enzyme secretion from rabbit peritoneal polymorphonuclear leukocytes. In parallel, a conformational analysis on the CHO-blocked peptide and their tertbutyloxycarbonylated synthetic precursors was performed in the crystal state and in solution using X-ray diffraction, infrared absorption, and 1H nuclear magnetic resonance. The biological and conformational data are discussed in relation to the proposed model of the chemotactic peptide receptor of rabbit neutrophils.

Crystallographic characterization of conformation of 1-aminocyclopropane-1-carboxylic acid residue (Ac3c) in simple derivatives and peptides

The molecular and crystal structures of the C alpha,alpha-dialkylated alpha-amino acid residue 1-aminocyclopropane-1-carboxylic acid hemihydrate (H2+-Ac3c-O-.1/2 H2O) and nine derivatives and dipeptides have been determined by X-ray diffraction. The derivatives are pBrBz-Ac3c-OH, Piv-Ac3c-OH, Z-Ac3c-OH, the alpha-and beta-forms of t-Boc-Ac3c-OH, Z-Ac3c-OMe, and the 5(4H)-oxazolone from pBrBz-Ac3c-OH; the dipeptides are H-(Ac3c)-OMe and c(Ac3c)2. The values determined for the torsion angles about the N-C alpha (phi) and C alpha-C' (psi) bonds for the single Ac3c residue of Piv-Ac3c-OH, the alpha- and beta-forms of t-Boc-Ac3-OH and Z-Ac3c-OMe, and the C-terminal Ac3c residue of H-(Ac3c)2-OMe correspond to folded conformations in the "bridge" region of the Ramachandran map. The structures of pBrBz-Ac3c-OH and Z-Ac3c-OH, however, are unusual in having a semi-extended conformation for the phi, psi angles. The N-terminal Ac3c residue of H-(Ac3c)2-OMe adopts a novel type of C5 conformation, characterized inter alia by an (amino) N. . .H-N (peptide) intramolecular hydrogen bond. While the acyl N alpha-blocking groups form trans amides (pBrBz-Ac3c-OH and Piv-Ac3c-OH), the urethane groups may adopt either the trans [Z-Ac3c-OH and t-Boc-Ac3c-OH (alpha-form)] or the cis amide conformations [t-Boc-Ac3c-OH(beta-form) and Z-Ac3c-OMe]. The five- and six-membered rings of the 5(4H)-oxazolone and the 2,5-dioxopiperazine, respectively, are planar. The four independent molecules in the asymmetric unit of the free alpha-amino acid are zwitterionic.

Crystal-state structural analysis of two gamma-lactam-restricted analogs of Pro-Leu-Gly-NH2

The crystal structures of two analogs of Pro-Leu-Gly-NH2 (1), containing a gamma-lactam conformational constraint in place of the -Leu-Gly- sequences, are described. The highly biologically active (S,R)-diastereomer 2a is semi-extended at the C-terminus, with the N-terminal Pro residue in the unusual "C5" conformation [psi 1 = -0.8(15) degrees] stabilized by a (peptide)N-H...N(amino) intramolecular H-bond [the N(3)...N(4) separation is 2.687(11)A]. Conversely, the N,N'-isopropylidene aminal trihydrate of the (S,S)-diastereomer 2b, compound 3, adopts a beta-bend conformation at the C-terminus, as already reported for 1. However, the backbone torsion angles [phi 2 = 57.4(4), psi 2 = -129.9(3) degrees; psi 3 = -92.3(4), phi 3 = 6.4(5) degrees] lie close to the values expected for the corner residues of an ideal type-II' beta-bend. A weak intramolecular 4----1 H-bond is seen between the Gly carboxyamide anti-NH and Pro C = O groups. In the newly formed 2,2,3,4-tetraalkyl-5-oxo-imidazolidin-1-yl moiety the psi 1 torsion angle is 12.9(4) degrees and the intramolecular N(3)...N(4) separation is 2.321(4)A.

Structure of (3S)-3-tert-butyloxycarbonylamino-2-piperidone

C10H18N2O3, Mr = 214.27, monoclinic, P2(1), a = 15.515 (2), b = 6.730 (1), c = 12.541 (2) A, beta = 113.6 (2) degrees, V = 1200.0 A3, Z = 4, Dx = 1.186 g cm-3, lambda(Mo K alpha) = 0.71069 A, mu = 0.54 cm-1, F(000) = 464, T = 295 K. The final R value for 1611 observed (3139 unique) reflections is 0.055. In both the independent molecules A and B of the asymmetric unit of the title compound, the conformation of the urethane moiety is trans. The lactam group of molecule A is non-planar, the C(9)-N(2)-C(10)-C(6) torsion angle being 12.4 (14) degrees. One main difference between molecules A and B is in the value of the phi[C(5)-N(1)-C(6)-C(10)] torsion angle [52.3 (11) degrees for molecule A while -86.5 (10) degrees for molecule B] as a consequence of a rotation of the ring relative to the tert-butyloxycarbonylamino substituent. A second major difference is the delta-lactam ring conformation which is approximate half-chair for molecule A while boat for molecule B.

Linear oligopeptides. 188.Crystallographic characterization of the conformation of the 1-aminocyclopentane-1-carboxylic acid residue in simple derivatives

The crystal structures of six derivatives of the Cα,α-dialkylated α-aminoacid residue 1-aminocyclopentane-1-carboxylic acid (Acc5) have been determined. These are Z-Acc5-OH, Z-Acc5-OH acetone hemi-solvate, Fmoc-Acc5-OH, mClAc-Acc5-OH, pBrBz-Acc5-OH, and the 5(4H)-oxazolone from pBrBz-Acc5-OH. The values determined for the torsion angles about the [Formula: see text] and Cα—CO (ψ) bonds correspond to folded, potentially α/310-helical conformations for the Acc5 residue of Z-Acc5-OH, Z-Acc5-OH acetone hemi-solvate, mClAc-Acc5-OH, and pBrBz-Acc5-OH. The structure of Fmoc-Acc5-OH, however, is unusual in having a semi-extended conformation for the [Formula: see text], ψ angles, accompanied by a cis arrangement of the amide group of the secondary urethane moiety. The dependence of the N—Cα—C′ bond angle on the ψ torsion angle and the asymmetry at the Cα atom are relevant features of the Acc5 residues. The position of the cyclopentyl ring relative to the peptide chain and its puckering parameters for the six compounds are also discussed.