Correction: Chrysomycins A-C, antileukemic naphthocoumarins from Streptomyces sporoverrucosus.

[This corrects the article DOI: 10.1039/C3RA42884B.].

Pregabalin peptides: conformational comparison of γ3- and γ4-substituted γ-amino acids in αγααα pentapeptides.

Gamma-aminobutyric acid (GABA, gammaAbu), an unsubstituted gamma-amino acid, is an important inhibitory neurotransmitter in the mammalian brain. The role of GABA in the treatment of epilepsy has triggered a great deal of interest in substituted gamma-amino acids, which may serve as GABA analogs, acting as inhibitors of GABA aminotransferase. Pregabalin (Pgn), a well-known antiepileptic drug, is also a beta-substituted gamma3-amino acid. Pregabalin and gamma4Leu, an isomer of the pregabalin (Pgn) residue, both carrying the same isobutyryl group in the side chain, were introduced in the present study to have a comparison of their respective conformational differences as well as their role in influencing the overall conformation of the peptides, they are inserted in. Two alpha-gamma-alpha-alpha-alpha hybrid pentapeptides were designed that contain Aib-Pgn and Aib-gamma4Leu segments at the N terminus. The study provides a detailed analysis of the conformational properties and non-covalent interactions observed in the crystal structures of two polymorphs of the pentapeptide monohydrate, Boc-Aib-(S)Pgn-Leu-Phe-Val-OMe (C38H63N5O8·H2O) and the isomeric pentapeptide, Boc-Aib-gamma4(R)Leu-Leu-Phe-Val-OMe (C38H63N5O8), obtained from single crystal X-ray diffraction experiments.

Crystallographic characterization of the α,γ C12 helix in hybrid peptide sequences.

The solid-state conformations of two αγ hybrid peptides Boc-[Aib-γ(4) (R)Ile]4 -OMe 1 and Boc-[Aib-γ(4) (R)Ile]5 -OMe 2 are described. Peptides 1 and 2 adopt C12 -helical conformations in crystals. The structure of octapeptide 1 is stabilized by six intramolecular 4 → 1 hydrogen bonds, forming 12 atom C12 motifs. The structure of peptide 2 reveals the formation of eight successive C12 hydrogen-bonded turns. Average backbone dihedral angles for αγ C12 helices are peptide 1, Aib; φ (°) = -57.2 ± 0.8, ψ (°) = -44.5 ± 4.7; γ(4) (R)Ile; φ (°) = -127.3 ± 7.3, θ1 (°) = 58.5 ± 12.1, θ2 (°) = 67.6 ± 10.1, ψ (°) = -126.2 ± 16.1; peptide 2, Aib; φ (°) = -58.8 ± 5.1, ψ (°) = -40.3 ± 5.5; ψ(4) (R)Ile; φ (°) = -123.9 ± 2.7, θ1 (°) = 53.3 θ 4.9, θ 2 (°) = 61.2 ± 1.6, ψ (°) = -121.8 ± 5.1. The tendency of γ(4) -substituted residues to adopt gauche-gauche conformations about the C(α) -C(ß) and C(ß) -C(γ) bonds facilitates helical folding. The αγ C12 helix is a backbone expanded analog of α peptide 310 helix. The hydrogen bond parameters for α peptide 310 and α-helices are compared with those for αγ hybrid C12 helix. Copyright © 2016 European Peptide Society and John Wiley & Sons.

Ribbon structure stabilized by C10 and C12 turns in αγ hybrid peptide.

The present study describes the synthesis and crystallographic analysis of αγ hybrid peptides, Boc-Gpn-L-Pro-NHMe (1), Boc-Aib-Gpn-L-Pro-NHMe (2), and Boc-L-Pro-Aib-Gpn-L-Pro-NHMe (3). Peptides 1 and 2 adopt expanded 12-membered (C12 ) helical turn over γα segment. Peptide 3 promotes the ribbon structure stabilized by type II ß-turn (C10 ) followed by the expanded C12 helical γα turn. Both right-handed and left-handed helical conformations for Aib residue are observed in peptides 2 and 3, respectively.

Conformation and crystal structures of 1-amino-cyclo-hexa-neacetic acid (ß(3,3)Ac6c) in N-protected derivatives.

N-Protected derivatives of 1-amino-cyclo-hexa-neacetic acid (ß(3,3)-Ac6c), namely Valeroyl-ß(3,3)-Ac6c-OH [2-(1-pentanamidocyclohexyl)acetic acid, C13H23NO3], (I), Fmoc-ß(3,3)-Ac6c-OH [2-(1-{[(9H-fluoren-9-yloxy)carbonyl]amino}cyclohexyl)acetic acid, C23H25NO4], (II), and Pyr-ß(3,3)-Ac6c-OH {2-[1-(pyrazine-2-amido)cyclohexyl]acetic acid, C13H17N3O3}, (III), were synthesized and their conformational properties were determined by X-ray diffraction analysis. The backbone torsion angles (ϕ, θ) for ß(3,3)-Ac6c-OH are restricted to gauche conformations in all the derivatives, with a chair conformation of the cyclo-hexane ring. In the crystal structure of (I), the packing of mol-ecules shows both carb-oxy-lic acid R 2 (2)(8) O-H⋯O and centrosymmetric R (2) 2(14) N-H⋯O hydrogen-bonding inter-actions, giving rise to chains along the c-axis direction. In (II), centrosymmetric carb-oxy-lic acid R 2 (2)(8) O-H⋯O dimers are extended through N-H⋯O hydrogen bonds and together with inter-ring π-π inter-actions between Fmoc groups [ring centroid distance = 3.786 (2) Å], generate a layered structure lying parallel to (010). In the case of compound (III), carb-oxy-lic acid O-H⋯Npyrazine hydrogen bonds give rise to zigzag ribbon structures extending along the c-axis direction.

An unusual conformation of gabapentin (Gpn) in Pyr-Gpn-NH-NH-Pyr stabilized by weak interactions.

The crystal structure of N-[(1-{2-oxo-2-[2-(pyrazin-2-ylcarbonyl)hydrazin-1-yl]ethyl}cyclohexyl)methyl]pyrazine-2-carboxamide monohydrate (Pyr-Gpn-NN-NH-Pyr·H2O), C19H23N7O3·H2O, reveals an unusual trans-gauche (tg(-)) conformation for the gabapentin (Gpn) residue around the C(γ)-C(ß) (θ1) and C(ß)-C(α) (θ2) bonds. The molecular conformation is stabilized by intramolecular N-H...N hydrogen bonds and weak C-H...O interactions. The packing of the molecules in the crystal lattice shows a network of strong N-H...O and O-H...O hydrogen bonds together with weak C-H...O and π-π inteactions.

Conformation of a terminally protected ßγ hybrid dipeptide Boc-Ant-Gpn-OMe stabilized by C6/C7 hydrogen bonds.

The hybrid ßγ dipeptide, methyl 2-[1-({2-[(tert-butoxycarbonyl)amino]benzamido}methyl)cyclohexyl]acetate (Boc-Ant-Gpn-OMe), C22H32N2O5, adopts a folded conformation stabilized by intramolecular six- (C6) and seven-membered (C7) hydrogen-bonded rings, together with weak C-H...O and C-H...π interactions, resulting in a ribbon-like structure.

Cyanuric chloride/sodium borohydride: a new reagent combination for reductive opening of 4,6-benzylidene acetals of carbohydrates to primary alcohols.

In the first such example, NaBH4 in combination with cyanuric chloride (TCT) has been used to obtain 6-hydroxy-4-benzyl ether derivatives from 4,6-benzylidene acetals of carbohydrates. The nature of hydride donor determines the regioselectivity of acetal opening. High regioselectivity, scope for using a broad range of substrates, functional group tolerance, mild reaction conditions, easy handling process, inexpensive reagents and wide application mark the benefits of the newly developed reagent system.

C5 and C7 intramolecular hydrogen bonds stabilize the structure of N-pyrazinoyl-gabapentin (Pyr-Gpn-OH).

2-{1-[(Pyrazin-2-ylformamido)methyl]cyclohexyl}acetic acid (Pyr-Gpn-OH), C14H19N3O3, is an N-protected derivative of gabapentin (Gpn). The compound crystallizes in the triclinic space group P1 and the molecular conformation is stabilized by intramolecular five- (C5) and seven-membered (C7) hydrogen-bonded rings. The packing of the molecules reveals intermolecular O-H···O and C-H···N hydrogen bonds, together with π-π interactions.

2-(1-Amino-4-tert-butyl-cyclo-hex-yl)acetic acid (tBu-ß(3,3)-Ac6c) hemihydrate.

The title compound, C12H23NO2·0.5H2O, crystallized with two 2-(1-amino-4-tert-butylcyclohexyl)acetic acid mol-ecules, which are present as zwitterions, and one water mol-ecule in the asymmetric unit. The mol-ecular structure of each zwitterion is stabilized by an intra-molecular six-membered (C 6 ) N-H⋯O hydrogen bond. In the crystal, the two independent zwitterions are linked head-to-head by N-H⋯O hydrogen bonds. Further O-H⋯O and N-H⋯O hydrogen bonds link the zwitterions and the water molecules, forming sandwich-like layers, with a hydrophilic filling and a hydrophobic exterior, lying parallel to the ab plane.

Analysis of designed ß-hairpin peptides: molecular conformation and packing in crystals.

The crystal structures of several designed peptide hairpins have been determined in order to establish features of molecular conformations and modes of aggregation in the crystals. Hairpin formation has been induced using a centrally positioned (D)Pro-Xxx segment (Xxx = (L)Pro, Aib, Ac6c, Ala; Aib = α-aminoisobutyric acid; Ac6c = 1-aminocyclohexane-1-carboxylic acid). Structures of the peptides Boc-Leu-Phe-Val-(D)Pro-(L)Pro-Leu-Phe-Val-OMe (1), Boc-Leu-Tyr-Val-(D)Pro-(L)Pro-Leu-Phe-Val-OMe (2, polymorphic forms labeled as 2a and 2b), Boc-Leu-Val-Val-(D)Pro-(L)Pro-Leu-Val-Val-OMe (3), Boc-Leu-Phe-Val-(D)Pro-Aib-Leu-Phe-Val-OMe (4, polymorphic forms labeled as 4a and 4b), Boc-Leu-Phe-Val-(D)Pro-Ac6c-Leu-Phe-Val-OMe (5) and Boc-Leu-Phe-Val-(D)Pro-Ala-Leu-Phe-Val-OMe (6) are described. All the octapeptides adopt type II' ß-turn nucleated hairpins, stabilized by three or four cross-strand intramolecular hydrogen bonds. The angle of twist between the two antiparallel strands lies in the range of -9.8° to -26.7°. A detailed analysis of packing motifs in peptide hairpin crystals is presented, revealing three broad modes of association: parallel packing, antiparallel packing and orthogonal packing. An attempt to correlate aggregation modes in solution with observed packing motifs in crystals has been made by indexing of crystal faces in the case of three of the peptide hairpins. The observed modes of hairpin aggregation may be of relevance in modeling multiple modes of association, which may provide insights into the structure of insoluble polypeptide aggregates.

Exploring the synthetic potential of cell bound ß-glycosidase of Pichia etchellsii.

Enzymatic synthesis of oligosaccharides with absolute stereo-selectivity and regio-selectivity provides an economical alternative to classical chemical methods. Here we demonstrate, for the first time, that whole cells of P. etchellsii are highly efficient biocatalysts and can be used for oligosaccharide synthesis using p-nitrophenyl-ß-D-glucopyranoside, o-nitrophenyl-ß-D-glucopyranoside and p-nitrophenyl-ß-D-xylopyranoside as both donors and acceptors. Auto-condensation of p-nitrophenyl-ß-D-glucopyranoside and o-nitrophenyl-ß-D-glucopyranoside resulted in formation of ß-(1→6) linked disaccharide as major products in 4 and 12% yield respectively. By contrast, auto condensation of p-nitrophenyl-ß-D-xylopyranoside exclusively lead to formation of ß-(1→4) linked disaccharide in 24% yield.

Aromatic interactions in model peptide ß-hairpins: ring current effects on proton chemical shifts.

Crystal structures of eight peptide ß-hairpins in the sequence Boc-Leu-Phe-Val-Xxx-Yyy-Leu-Phe-Val-OMe revealed that the Phe(2) and Phe(7) aromatic rings are in close spacial proximity, with the centroid-centroid distance (R(cen)) of 4.4-5.4 Å between the two phenyl rings. Proton NMR spectra in chloroform and methanol solution reveal a significant upfield shift of the Phe(7) C(δ,δ') H(2) protons (6.65-7.04 ppm). Specific assignments of the aromatic protons have been carried out in the peptide Boc-Leu-Phe-Val-(D)Pro-(L)Pro-Leu-Phe-Val-OMe (6). The anticipated ring current shifts have been estimated from the aromatic ring geometrics observed in crystals for all eight peptides. Only one of the C(δ,δ') H proton lies in the shielding zone with rapid ring flipping, resulting in averaging between the two extreme chemical shifts. An approximate estimate of the population of conformations, which resemble crystal state orientation, may be obtained. Key nuclear Overhauser effects (NOEs) between facing Phe side chains provide support for close similarity between the solid state and solution conformation. Temperature dependence of aromatic ring proton chemical shift and line widths for peptide 6 (Boc-Leu-Phe-Val-(D)Pro-(L)Pro-Leu-Phe-Val-OMe) and the control peptide Boc-Leu-Val-Val-(D)Pro-Gly-Leu-Phe-Val-OMe establish an enhanced barrier to ring flipping when the two Phe rings are in proximity. Modeling studies suggest that small, conformational adjustment about C(α)-C(ß) (χ(1) ) and C(ß)-C(γ) (χ(2) ) bonds of both the Phe residues may be required in order to permit unhindered, uncorrelated flipping of both the Phe rings. The maintenance of the specific aromatic ring orientation in organic solvents provides evidence for significant stabilizing interaction.

Helix and hairpin nucleation in short peptides using centrally positioned conformationally constrained dipeptide segments.

The effect of incorporation of a centrally positioned Ac(6)c-Xxx segment where Xxx = (L)Val/(D)Val into a host oligopeptide composed of l-amino acid residues has been investigated. Studies of four designed octapeptides Boc-Leu-Phe-Val-Ac(6)c-Xxx-Leu-Phe-Val-OMe (Xxx = (D)Val 1, (L)Val 2) Boc-Leu-Val-Val-Ac(6)c-Xxx-Leu-Val-Val-OMe (Xxx = (D)Val 3, (L)Val 4) are reported. Diagnostic nuclear Overhouse effects characteristic of hairpin conformations are observed for Xxx = (D)Val peptides (1 and 3) while continuous helical conformation characterized by sequential N(i)H ↔ N(i+1)H NOEs are favored for Xxx = (L)Val peptides (2 and 4) in methanol solutions. Temperature co-efficient of NH chemical shifts are in agreement with distinctly different conformational preferences upon changing the configuration of the residue at position 5. Crystal structures of peptides 2 and 4 (Xxx = (L)Val) establish helical conformations in the solid state, in agreement with the structures deduced from NMR data. The results support the design principle that centrally positioned type I ß-turns may be used to nucleate helices in short peptides, while type I'ß-turns can facilitate folding into ß-hairpins.

Characterization of bent helical conformations in polymorphic forms of a designed 18-residue peptide containing a central Gly-Pro segment.

An 18-residue sequence Boc-Aib-Val-Ala-Leu-Aib-Val-Ala-Leu-Gly-Pro-Val-Ala-Leu-Aib-Val-Ala-Leu-Aib-OMe (UK18) was designed to examine the effect of introducing a Gly-Pro segment into the middle of a potentially helical peptide. The crystal structures of two polymorphic forms yielded a view of the conformation of three independent molecules. Form 1 (space group P2(1)2(1)2(1,) a = 14.620Å; b = 26.506Å, c = 28.858Å, Z = 4) has one molecule in the asymmetric unit, with one cocrystallized water molecule. Form 2 (space group P2(1)2(1)2(1,) a = 9.696Å; b = 19.641Å, c = 114.31Å, Z = 8) has two molecules in the asymmetric unit with four cocrystallized water molecules. In Form 1, residues 1 to 18 adopt ϕ,ψ values that lie in the right-handed helical (α(R) ) region of the Ramachandran map. Two residues, Leu (8) (ϕ = -92.0°, ψ = -7.5°) and Leu (17) (ϕ = -94.7°, ψ = -1.7°) adopt conformations that deviate significantly from helical values. In Form 2, molecule A, residues 2 to 16 lie in the α(R) region of ϕ,ψ space, with Leu (8) (ϕ = -94.9°, ψ = -2.9°) deviating significantly from helical values. Aib (1) and Aib (18) adopt left-handed (α(L)) helical conformation. Significant distortion is observed at Leu (17) (ϕ = -121.3°, ψ = -31.3°). Molecule B, Form 2, adopts a right-handed helix over residues 1 to 17. In all three molecules, a distinct bend in the helix is observed, with the bend angle values varying from 40.8° to 58.9°.

Chain length effects on helix-hairpin distribution in short peptides with Aib-DAla and Aib-Aib segments.

The Aib-D Ala dipeptide segment has a tendency to form both type-I'/III' and type-I/III ß-turns. The occurrence of prime turns facilitates the formation of ß-hairpin conformations, while type-I/III turns can nucleate helix formation. The octapeptide Boc-Leu-Phe-Val-Aib-DAla-Leu-Phe-Val-OMe (1) has been previously shown to form a ß-hairpin in the crystalline state and in solution. The effects of sequence truncation have been examined using the model peptides Boc-Phe-Val-Aib-Xxx-Leu-Phe-NHMe (2, 6), Boc-Val-Aib-Xxx-Leu-NHMe (3, 7), and Boc-Aib-Xxx-NHMe (4, 8), where Xxx=DAla, Aib. For peptides with central Aib-Aib segments, Boc-Phe-Val-Aib-Aib-Leu-Phe-NHMe (6), Boc-Val-Aib-Aib-Leu-NHMe (7), and Boc-Aib-Aib-NHMe (8) helical conformations have been established by NMR studies in both hydrogen bonding (CD3OH) and non-hydrogen bonding (CDCl3) solvents. In contrast, the corresponding hexapeptide Boc-Phe-Val-Aib-DAla-Leu-Phe-Val-NHMe (2) favors helical conformations in CDCl3 and ß-hairpin conformations in CD3 OH. The ß-turn conformations (type-I'/III) stabilized by intramolecular 4→1 hydrogen bonds are observed for the peptide Boc-Aib-D Ala-NHMe (4) and Boc-Aib-Aib-NHMe (8) in crystals. The tetrapeptide Boc-Val-Aib-Aib-Leu-NHMe (7) adopts an incipient 3(10)-helical conformation stabilized by three 4→1 hydrogen bonds. The peptide Boc-Val-Aib-DAla-Leu-NHMe (3) adopts a novel α-turn conformation, stabilized by three intramolecular hydrogen bonds (two 4→1 and one 5→1). The Aib-DAla segment adopts a type-I' ß-turn conformation. The observation of an NOE between Val (1) NH↔HNCH3 (5) in CD3OH suggests, that the solid state conformation is maintained in methanol solutions.

Peptide hairpin nucleation with the obligatory Type I' beta-turn Aib-DPro segment.

The alpha-aminoisobutyric acid-D-proline (Aib-(D)Pro) dipeptide is an obligatory Type I' beta-turn forming segment that nucleates hairpin formation.

Helical conformations of hexapeptides containing N-terminus diproline segments.

The role of N-terminus diproline segments in facilitating helical folding in short peptides has been investigated in a set of model hexapeptides of the type Piv-Xxx-Yyy-Aib-Leu-Aib-Phe-OMe (Piv, pivaloyl). Nine sequences have been investigated with the following N-terminus dipeptide segments: (D)Pro-Ala (4) and Pro-PsiPro (5, Psi, pseudoproline), Ala-Ala (6), Ala-Pro (7), Pro-Ala (8), Aib-Ala (9), Ala-Aib (10). The analog sequences Piv-Pro-Pro-Ala-Leu-Aib-Phe-OMe (2) and Piv-Pro-Pro-Ala-Aib-Ala-Aib-OMe (3) have also been studied. Solid state conformations have been determined by X-ray crystallography for peptides 4, 6, and 8 and compared with the previously determined crystal structure of peptide 1 (Boc-Pro-Pro-Aib-Leu-Aib-Val-OMe); (Rai et al., JACS 2006, 128, 7916-7928). Peptides 1 and 6 adopt almost identical helical conformations with unfolding of the helix at the N-terminus Pro (1) residue. Peptide 4 reveals the anticipated (D)Pro-Ala type II' beta-turn, followed by a stretch of 3(10)-helix. Peptide 8 adopts a folded conformation stabilized by four successive 4-->1 intramolecular hydrogen bonds. Ala (2) adopts an alpha(L) conformation, resulting in a type II beta-turn conformation followed by a stretch of 3(10)-helix. Conformational properties in solution were probed using solvent perturbation of NH chemical shifts which permit delineation of hydrogen bonded NH groups and nuclear Overhauser effects (NOEs) between backbone protons, which are diagnostic of local residue conformations. The results suggest that, continuous helical conformations are indeed significantly populated for peptides 2 and 3. Comparison of the results for peptides 1 and 2, suggest that there is a significant influence of the residue that follows diproline segments in influencing backbone folding.

Hydrophobic peptide channels and encapsulated water wires.

Peptide nanotubes with filled and empty pores and close-packed structures are formed in closely related pentapeptides. Enantiomorphic sequences, Boc-(D)Pro-Aib-Xxx-Aib-Val-OMe (Xxx = Leu, 1; Val, 2; Ala, 3; Phe, 4) and Boc-Pro-Aib-(D)Xxx-Aib-(D)Val-OMe ((D)Xxx = (D)Leu, 5; (D)Val, 6; (D)Ala, 7; (D)Phe, 8), yield molecular structures with a very similar backbone conformation but varied packing patterns in crystals. Peptides 1, 2, 5, and 6 show tubular structures with the molecules self-assembling along the crystallographic six-fold axis (c-axis) and revealing a honeycomb arrangement laterally (ab plane). Two forms of entrapped water wires have been characterized in 2: 2a with d(O...O) = 2.6 A and 2b with d(O...O) = 3.5 A. The latter is observed in 6 (6a) also. A polymorphic form of 6 (6b), grown from a solution of methanol-water, was observed to crystallize in a monoclinic system as a close-packed structure. Single-file water wire arrangements encapsulated inside hydrophobic channels formed by peptide nanotubes could be established by modeling the published structures in the cases of a cyclic peptide and a dipeptide. In all the entrapped water wires, each water molecule is involved in a hydrogen bond with a previous and succeeding water molecule. The O-H group of the water not involved in any hydrogen bond does not seem to be involved in an energetically significant interaction with the nanotube interior, a general feature of the one-dimensional water wires encapsulated in hydrophobic environments. Water wires in hydrophobic channels are contrasted with the single-file arrangements in amphipathic channels formed by aquaporins.

Characterization of water wires inside hydrophobic tubular peptide structures.

The crystallographic observation of a hydrophobic, empty channel (diameter approximately 5.2 A) in the peptide Boc-(D)Pro-Aib-Leu-Aib-Val-OMe, prompted the investigation of the analog Boc-(D)Pro-Aib-Val-Aib-Val-OMe in which the side chain at position 3 was shortened, resulting in the structure of a channel (diameter approximately 7.5 A) containing a one-dimensional wire of water molecules. Crystallization in the space group P6(5) facilitates formation of a pore lined entirely by hydrocarbon side chains. Two forms of the entrapped water wires, with O...O separations of 3.5 and 2.6 A, are discussed. A lone hydrogen bond between the adjacent pairs of water molecules in the wire, with no strong interactions between the second water hydrogen and the hydrophobic walls of the channel, is a feature of the one-dimensional array. The structure provides the first crystallographic characterization of a water wire in a hydrophobic channel with implications in water and proton transport in membranes and carbon nanotubes.