Crystal and molecular structure of hexan-2,5-dione bis(4-phenyl-thiosemicarbazonato) nickel(II), (C20H22N6S2Ni): a model study of the enhancement of the antibacterial activity of a tetradentate N,S donor ligand on metal complexation

The crystal structure of the nickel(II) complex (C₂₀H₂₂N₆S₂Ni) of the N₂S₂ ligand hexan-2,5-dionebis(4-phenyl-thiosemicarbazone) has been solved using diffractometric data. The complex, exhibiting greater antibacterial activity than the free ligand, crystallizes in the space group C2 with a = 17.414(1) Å, b = 8.485(1) Å, c = 15.129(3) Å, β = 104.09(3)°, Z = 4, d(obsd) = 1.425 g cm^?3, d(calc) = 1.438 g cm^?3 and μ(Mo-K_ga) = 10.978 cm^?1. The structure has been refined by full-matrix least squares to a final R = 0.033 and R_w = 0.041 using 1743 reflections with I ≥ 3σ(I) out of 2049 unique reflections measured (2° ≤ gq ≤ 27°). The hydrogens were either located or placed in their calculated positions. The nickel(II) ion lies in the tetrahedrally distorted square planar ligand field of the tetradentate ligand forming two five membered and one seven membered chelate rings. It is observed that the lack of conjugation in the seven membered chelate rings of the present complex and of similar complexes leads to dissymmetry in the ring geometry. The metal ion is coordinatively unsaturated and available for additional coordination in its axial directions.     

Activation mechanisms of PIP5K isozymes by the small GTPase ARF6

The membrane phospholipid, phosphatidylinositol 4-phosphate 5-kinase (PIP5K), plays important roles in a wide variety of signal transduction systems through its product phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂). To date, three mammalian PIP5K isozymes, α, β and γ have been identified and each isozyme plays specific physiological functions. In this study, we refer to human PIP5Kα, β, and γ as PIP5K_A, _B, and _C, respectively. In the present study, we demonstrated that PIP5K_B, but not PIP5K_C, was directly activated by ARF6 in vitro, and provide a working hypothesis for the novel activation mechanism of PIP5K_C by ARF6 in the cell. In an in vitro system, PIP5K_B, which was expressed in and purified from mammalian cells, was significantly activated by ARF6. Although, in contrast, PIP5K_C was not activated by ARF6 under the same conditions, its kinase core domain (KCD) fragment, which is highly conserved among PIP5K isozymes, preserved the lipid kinase activity and was activated by ARF6. In addition, it was found that ARF6 bound to the KCD and that the N-terminal domain deletion mutant of PIP5K_C was activated by ARF6. These results suggest that the N-terminal domain of PIP5K_C masks its KCD, thereby preventing the interaction of PIP5K_C with and its activation by ARF6. Under certain physiological setting(s), the conformation of PIP5K_C would be changed to unmasked open form, and thereby PIP5K_C would be able to interact with and activated by ARF6.     

abeo‐Taxane Diterpenoids from the Taiwanese Yew Taxus sumatrana

Chemical investigation of the acetonic extract of the leaves and twigs of Taxus sumatrana (Taxaceae) led to the isolation of four new taxane diterpene esters, taiwantaxins A–D ( 1 – 4 , resp.). Their structures were determined primarily on the basis of 1D‐ and 2D‐NMR techniques as well as MS. Compound 1 is a rearranged taxane diterpenoid possessing an opened oxetane ring moiety containing C(4), C(5), and C(20). The metabolites 2 and 3 belong to a 5/6/6 taxene system having a rare five‐membered γ‐lactone ring comprising C(8), C(9), C(10), and C(19). Compound 4 is an example of a taxane diterpene containing a 6/8/6 ring system with a tetrahydrofuran ring comprising C(2), C(3), C(4), and C(20). The 11(15→1)abeo‐taxane diterpenoids, taiwantaxins A–C ( 1 – 3 , resp.) are lacking an O‐bearing functionality at either C(13) or C(14). Compound 2 showed significant cytotoxic activity against human PC‐3 tumor cells.     

Ascotoxin (Decumbin), a Metabolite of Ascochyta imperfecta PECK.

From the mycelium of Ascochyta imperfecta decumbin, C₁₆H₂₄O₄, mp 203°C, was obtained in one percent yield.

The absolute structure of decumbin was presented as [II] by the following evidences: The configuration about C₄ was determined as (S) by the benzoate rule on the tetrahydromonoketone (21). The hydroxyl at C₇ is α, because tetrahydrodecumbin (23) showed no intramolecular hydrogen bond, while its C₇ epimer (24) did. Ring juncture was determined by ORD of a five membered ketone (16). Two double bonds were found to be trans from IR data. The stereochemistry of decumbin monoepoxide (7), tetrahydropyrans (12 and 13) was also studied. Plant tests of the twenty derivatives of decumbin on lucerne and rape revealed that the growth inhibition activity has close relation with the presence of double bond in the thirteen membered lactone ring.     

π Delocalization in iridathiabenzenes

Fenske-Hall calculations were carried out for (PEt₃)₃Ir(C₇H₉) (1), [(PEt₃)₃Ir(C₆H₈S)]⁺ (2), [(S-t-but)(PEt₃)₂]Ir(C₆H₈S) (3), and [(S-t-but)(PMet₃)₃]Ir(C₆H₈S) (4) in order to compare the degree of π delocalization in the metallathiacycle rings of (2) and (3). In comparison to (1), a true iridabenzene and (4), an iridathiacyclobutadiene, the π ring systems in (2) and (3) are considerably more localized than the π system in (1) but are not totally localized. Strong metal-sulfur bonding in (2) disrupts the π ring system and results in some localization of the ring π bonds. The introduction of the donor thiolate ligand in (3) disrupts the ring of π system even more by destabilizing the metal orbitals used for metal-sulfur interactions. This weakens the metal-sulfur interaction seen in (2) and leads to even more localization of the ring π system in (3).     

Evidence for stereospecific collision complexes of cyclo(tri-L-prolyl) with benzene

NMR spectra of cyclo(tri-L -prolyl), c-(P)₃, show large shifts of the H_α resonance on adding C₆D₆ to a solution of c-(P)₃ in CD₂Cl₂. CPK models and observed coupling constants indicate a rigid c-(P)₃ conformation, independent of solvent composition, suggesting that these shifts result from formation of stereospecific C₆D₆–c-(P)₃ collision complexes in which the c-(P)₃ H_α lie near the face of the aromatic ring. The temperature dependence of the H_α shifts and the solvent dependent shifts observed on adding toluene-d₈ or nitrobenzene-d₅ to the c-(P)₃ solution suggest that preferred C₆D₆–c-(P)₃ orientations result from attractive interactions between the electron-rich aromatic ring and the electropositive H_α's and/or δ⁺ nitrogen atoms in the peptide backbone. Reports of such interactions in increasingly diverse peptide model systems suggest that they may play a role in stabilizing protein structures.     

Characterization of a composite with enhanced attraction to savannah tsetse flies from constituents or analogues of tsetse refractory waterbuck (Kobus defassa) body odor

Savannah tsetse flies avoid flying toward tsetse fly-refractory waterbuck (Kobus defassa) mediated by a repellent blend of volatile compounds in their body odor comprised of δ-octalactone, geranyl acetone, phenols (guaiacol and carvacrol), and homologues of carboxylic acids (C₅-C₁₀) and 2-alkanones (C₈-C₁₃). However, although the blends of carboxylic acids and that of 2-alkanones contributed incrementally to the repellency of the waterbuck odor to savannah tsetse flies, some waterbuck constituents (particularly, nonanoic acid and 2-nonanone) showed significant attractive properties. In another study, increasing the ring size of δ-octalactone from six to seven membered ring changed the activity of the resulting molecule (ε-nonalactone) on the savannah tsetse flies from repellency to attraction. In the present study, we first compared the effect of blending ε-nonalactone, nonanoic acid and 2-nonanone in 1:1 binary and 1:1:1 ternary combination on responses of Glossina pallidipes and Glossina morsitans morsitans tsetse flies in a two-choice wind tunnel. The compounds showed clear synergistic effects in the blends, with the ternary blend demonstrating higher attraction than the binary blends and individual compounds. Our follow up laboratory comparisons of tsetse fly responses to ternary combinations with different relative proportions of the three components showed that the blend in 1:3:2 proportion was most attractive relative to fermented cow urine (FCU) to both tsetse species. In our field experiments at Shimba Hills game reserve in Kenya, where G. pallidipes are dominant, the pattern of tsetse catches we obtained with different proportions of the three compounds were similar to those we observed in the laboratory. Interestingly, the three-component blend in 1:3:2 proportion when released at optimized rate of 13.71mg/h was 235% more attractive to G. pallidipes than a combination of POCA (3-n-Propylphenol, 1-Octen-3-ol, 4-Cresol, and Acetone) and fermented cattle urine (FCU). This constitutes a novel finding with potential for downstream deployment in bait technologies for more effective control of G. pallidipes, G. m. morsitans, and perhaps other savannah tsetse fly species, in ‘pull’ and ‘pull-push’ tactics.     

Pierisformotoxins A – D,Polyesterified Grayanane Diterpenoids from Pieris formosa and Their cAMP‐Decreasing Activities

Four highly acylated diterpenoids, designated as pierisformotoxins A–D ( 1 – 4 , resp.), along with 26 known compounds, were isolated from the flowers of Pieris formosa. Among them, pierisformotoxins A and B ( 1 and 2 , resp.) were new highly acylated grayanane diterpenoids, of which the five‐membered ring A has undergone an oxidative cleavage between C(3) and C(4), followed by lactonization, to give rise to a five‐membered lactone ring between C(3) and C(5), differing from the previously reported grayanane diterpenoids with a 5/7/6/5 ring system. Results of the cAMP‐regulation‐activity assay showed that pierisformotoxin C ( 3 ) at 10 μM (inhibitory ratio (IR): 10.1%) or 2 μM (9.8%), and pierisformotoxin B ( 2 ) at 50 μM (13.9%) significantly decreased the cAMP level in N1E‐115 neuroblastoma cells (p<0.05).     

Syntheses and single crystal structures of chiral tetrahedral clusters containing osmium

New chiral tetrahedral clusters (μ₃-S)OsCoMo(CO)₈C₅H₄C(O)R(R = H 2, CH₃ 3, C₆H₄C(O)OCH₃ 4) were synthesized by the reaction of the precursor (μ₃-S)OsCo₂(CO)₉ 1 with the functionally substituted metal exchange reagents [Mo(CO)₃(η⁵-C₅H₄)C(O)R]⁻ (R = H, CH₃, C₆H₄C(O)OCH₃). Then clusters 2, 3 and 4 were treated with 2,4-dinitrophenylhydrazine to obtain clusters (μ₃-S)OsCoMo(CO)₈C₅H₄CNNHC₆H₃(NO₂)₂R (R = H 5, CH₃ 6, C₆H₄C(O)OCH₃ 7), respectively. All the clusters were characterized by Element Analysis, IR and ¹H NMR. The structures of clusters 3 and 4 were established by X-ray single crystal diffraction. Interestingly, carbonyl group on the cyclopentadienyl ligand and cyclopentadienyl ring are not in the same plane, in cluster 3, torsion angle C₂₇-C₂₈-C₂₉-O₁₈ is −176.1(9), but in cluster 4, torsion angle C₁₂-C₁₃-C₁₄-O₉ is 167.5(17), which shows that carbonyl function group on the cyclopentadienyl ligand offsets the cyclopentadienyl ring more markedly than that in cluster 3. It showed that both conjugated effects and space hinder of phenyl ring in the cluster 4 are important factors to decide atoms positioning in three-dimensional structure of the clusters.     

Titanium(III) alkoxides. A new synthetic route and solid state properties (CP/MAS 13C NMR,X-ray and IR)

Ti(OR)₃ compounds (R=C₂H₅, C₄H₉ⁿ, C₆H₅) were prepared by reduction of titanium tetralkoxides with organosilicon compounds containing SiH bonds. The reaction mechanism probably involves a four membered cyclic intermediate. The tervalent alkoxides have been characterized by elemental analyses, X-ray powder diffraction, infrared spectroscopy and solid-state magic angle sample spinning ¹³C NMR.The compounds are polymeric owing to the presence of alkoxide bridges. They are diamagnetic, insoluble materials which decompose on melting. Previously reported results are critically discussed and compared with the experimental findings from both infrared and NMR spectroscopy.     

Biosynthesis of Carotenoids in the Chloroplasts of Algae and Higher Plants

Physiological, biochemical, and genetic aspects of carotenoid biosynthesis in the chloroplast membranes of green algae and higher plants are discussed starting from the earliest stages of biosynthesis of key C₅-isoprene units. The latter are synthesized either from acetate (C₂) to mevalonic acid (C₆) or from glucose (C₆) by forming glyceraldehyde 3-phosphate (C₃) and pyruvate decarboxylation product (C₂) through intermediate compounds to isopentenyl diphosphate (C₅). In all organisms, the further carotenoid synthesis from isopentenyl diphosphate and its isomer dimethylallyl diphosphate (C₅) proceeds through their transformation into geranyl diphosphate (C₁₀), farnesyl diphosphate (C₁₅), geranylgeranyl diphosphate (C₂₀) and phytoene (C₄₀). Phytoene desaturation (dehydrogenation) to carotene, neurosporene, and lycopene, and all steps of their cyclization to , and carotenes are discussed in detail. The synthesis of xanthophylls in chloroplasts is presented as the sequential formation of hydroxy-, epoxy- and oxo- groups. Genetic control of biosynthesis, as well as the localization and functional role of carotenoids in the chloroplast membranes of plants and algae are briefly discussed.     

Occurrence of Melibiose‐Containing Glycosphingolipids in a Sample of a Sponge‐Coral Association (Desmapsamma anchorata/Carijoa riisei)

In our research on biologically active compounds from Vietnamese marine invertebrates, rare melibiose‐containing glycosphingolipids were found in a sample of a sponge‐coral association (Desmapsamma anchorata/Carijoa riisei). Melibiosylceramides were analyzed as constituents of some multi‐component RP‐HPLC fractions, and the structures of 14 new ( 1b , 3b , 4a – 4c , 6a – 6c , 8b , 9a , 9b , 10b , 11a , 11b ) and five known ( 2b , 5a – 5c , 7b ) natural compounds were elucidated using NMR, mass spectrometry, optical rotation, and chemical transformations. These α‐d ‐Galp‐(1→6)‐β‐d ‐Glcp‐(1 1)‐ceramides (presumably sponge‐derived compounds) were shown to contain phytosphingosine‐type n‐t17:0 ( 1 ), (6E)‐n‐t17:1 ( 2 ), i‐t17:0 ( 3 ), n‐t18:0 ( 4 ), (6E)‐n‐t18:1 ( 5 ), i‐t18:0 ( 6 ), (6E)‐i‐t18:1 ( 7 ), i‐t19:0 ( 8 ), (6E)‐i‐t19:1 ( 9 ), ai‐t19:0 ( 10 ), and (6E)‐ai‐t19:1 ( 11 ) backbones N‐acylated with saturated straight‐chain (2R)‐2‐hydroxy C₂₁ ( a ), C₂₂ ( b ), and C₂₃ ( c ) acids. Characteristic trends in the fragmentations of the terminal parts of tetraacetylated normal‐chain and iso‐ and anteiso‐branched sphingoid bases were observed using GC/MS. The total sum of melibiosylceramides and compound 5b caused a reduction in colony formation of human melanoma cells.     

Unusual C19-diterpenoid alkaloids from Aconitum vilmorinianum var. patentipilum

Three new C₁₉-diterpenoid alkaloids vilmotenitines A-C (1-3), together with seven known ones, vilmorine D (4), talatizidine (5), isotalatizidine (6), vilmorrianine B (7), vilmorrianine D (8), talatizamine (9), 8-deacetyl-yunaconitine (10), were isolated from Aconitum vilmorinianum var. patentipilum. Vilmotenitines A (1) and B (2) are the second natural occurrences of C₁₉-diterpenoid alkaloids with a unique rearranged six-membered B ring formed by the C₍₈₎–C₍₁₀₎ linkage.     

Synthesis, crystal structure and esterification of a novel iron(III) 18-metallacrown-6 complex

The trianionic heptadentate ligand, (Z)-3-(5′-chlorosalicylhydrazinocarbonyl) propenoic acid, has been synthesized and reacted with FeCl₃·6H₂O, to produce the complex [Fe^III₆(C₁₂H₈N₂O₅Cl)₆(H₂O)₄(CH₃OH)₂]·8H₂O·4CH₃OH. In the self-assembly process the ligand was esterified and transferred into (Z)-methyl 3-(5′-chlorosalicylhydrazinocarbonyl) propenoate. In the crystal structure, the neutral Fe(III) complex contain a 18-membered metallacrown ring consisting of six Fe(III) and six trianionic ligands. The 18-membered metallacrown ring is formed by the succession of six structural moieties of the type [Fe(III)-N-N]. Due to the meridional coordination of the ligands to the Fe³⁺ ions, the ligands enforce the stereochemistry of the Fe³⁺ ions as a propeller configuration with alternating Λ/Δ forms. The metallacrown can be treated with SnCl₂ or Zn powder to obtain purified ester.     

Cometabolism of Methyl tertiary Butyl Ether and Gaseous n-Alkanes by Pseudomonas mendocina KR-1 Grown on C5 to C8 n-Alkanes

Pseudomonas mendocina KR-1 grew well on toluene, n-alkanes (C₅ to C₈), and 1° alcohols (C₂ to C₈) but not on other aromatics, gaseous n-alkanes (C₁ to C₄), isoalkanes (C₄ to C₆), 2° alcohols (C₃ to C₈), methyl tertiary butyl ether (MTBE), or tertiary butyl alcohol (TBA). Cells grown under carbon-limited conditions on n-alkanes in the presence of MTBE (42 μmol) oxidized up to 94% of the added MTBE to TBA. Less than 3% of the added MTBE was oxidized to TBA when cells were grown on either 1° alcohols, toluene, or dextrose in the presence of MTBE. Concentrated n-pentane-grown cells oxidized MTBE to TBA without a lag phase and without generating tertiary butyl formate (TBF) as an intermediate. Neither TBF nor TBA was consumed by n-pentane-grown cells, while formaldehyde, the expected C₁ product of MTBE dealkylation, was rapidly consumed. Similar K_s values for MTBE were observed for cells grown on C₅ to C₈ n-alkanes (12.95 ± 2.04 mM), suggesting that the same enzyme oxidizes MTBE in cells grown on each n-alkane. All growth-supporting n-alkanes (C₅ to C₈) inhibited MTBE oxidation by resting n-pentane-grown cells. Propane (K_i = 53 μM) and n-butane (K_i = 16 μM) also inhibited MTBE oxidation, and both gases were also consumed by cells during growth on n-pentane. Cultures grown on C₅ to C₈ n-alkanes also exhibited up to twofold-higher levels of growth in the presence of propane or n-butane, whereas no growth stimulation was observed with methane, ethane, MTBE, TBA, or formaldehyde. The results are discussed in terms of their impacts on our understanding of MTBE biodegradation and cometabolism.     

Synthesis, characterization of a new open-framework gallium phosphite containing 5,6-fold coordinate gallium atoms

A new open-framework gallium phosphite Ga₅(OH)₂(HPO₃)₈(C₄N₃H₁₆) · 2H₂O (1) is synthesized solvothermally using diethylenetriamine (DETA) as template from a mixed solvent system in which ethylene glycol (EG) was used as the co-solvent. The as-synthesized product is characterized by single crystal X-ray diffraction, powder X-ray diffraction, IR spectroscopy, thermogravimetric analysis (TGA), ICP-AES and elemental analyses. The three-dimensional open-framework of compound 1 is constructed from two novel secondary building units (SBUs), Ga₂O₇(OH)(HPO₃) three-membered ring (3R) and Ga(HPO₃)₆ tooth-wheel type unit. These two novel secondary building units are first found in gallium phosphate/phosphite, which lead to form the one-dimensional 8-membered ring channels along the a-axis. Moreover, it is noted that compound 1 is the first gallium phosphite containing 5-fold coordinate gallium atom.     

Synthesis and characterization of two isomers of Os3(CO)10(C5H4N-2-C(H)NR) and the conversion to ortho-metallated HOs3(C5H3N-2-C(H)NR)(CO)9. Part III. X-ray Structure of HOs3(C5H3N-2-C(H)Ni-Pr)(CO)9

Os₃(CO)₁₀(MeCN)₂ reacts at room temperature in MeCN or toluene with R-Pyca2 to yield two isomers of Os₃(CO)₁₀(R-Pyca) that differ in the bonding of the R-Pyca ligand to the Os₃(CO)₁₀ unit. In all cases Os₃(CO)₁₀(R-Pyca(4e)) (isomer A; 4a: R = c-Pr, 4b: R = i-Pr, 4c: R = neo-Pent, 4d: R = t-Bu), containing a chelating 4e donating R-Pyca ligand and three OsS bonds, could be isolated. In the case of R = c-Pr and R = i-Pr Os₃(CO)₁₀(R-Pyca(6e)) (isomer B; 5a: R = c-Pr, 5b: R = i-Pr), in which only two OsS bonds are present and the R-Pyca ligand is bonded as a 6e donating ligand bridging two non-bonded Os atoms, could be isolated as a minor product.At 70 °C Os₃(CO)₁₀(R-Pyca(4e)) (4b and 4d) loses one carbonyl and the pyridine moiety of the R-Pyca ligand is ortho-metallated to form HOs₃(C₅H₃N-2-C(H)NR)(CO)₉ (6b: R = i-Pr and 6d: R = t-Bu). Under the same conditions Os₃(CO)₁₀(i-Pr-Pyca(6e)) (5b) reacts to Os₂(CO)₆(6e)) (7b) containing a bridging 6e donating ligands. The latter two reactions were followed with FT-IR spectroscopy in a high temperature IR cell.The structure of the complexes in solution have been studied by ¹H and ¹C NMR and IR spectroscopy. The stoichiometries of 4a and 5a were determined by FAB-mass spectrometry while an exact mass determination was carried out for 4a.The crystal structure of 6b has been determined. Crystal of 6b are monoclinic, space group P2₁/n, with a = 7.808(2),b = 17.613(3),c = 16.400(8)Å, β = 94.09(3)° and Z = 4. The structure was refined to R = 0.039. The molecule contains a triangular array of osmium atoms [Os(1)Os(2) = 2.898(2)Å, Os(1)Os(3) = 2.886(2)Åand Os(2)O(3) = 2.911(2)Å] and nine terminally bonded carbonyl ligands. The C₅H₃N-2-C(H)N-i-Pr ligand is chelate bonded to Os(2) with the pyridine and imine nitrogens atoms axially and equatorially coordinated respectively [Os(2)N(1) = 2.00(2)Åand Os(2)N(2) = 2.11(2)Å]. The i-Pr-Pyca ligand is ortho-metallated at C(1) and forms a four membered ring containing Os(2), Os(3), C(1) and N(1), the Os(3)C(1) distance being 2.12(2)Å. The hydride, which could not be located unequivocally from a difference Fourier map is proposed to bridge the Os(2)(3) bond on the basis of stereochemical considerations.     

Chemospecific deuteration of prostaglandin silyl ethers

Complete chemical selectivity ( , chemospecificity) has been achieved in the homogeneous deuteration of C₅---C₆ and endocyclic C₁₀---C₁₁ prostaglandin double bonds without rearrangement or partial reduction of C₁₃---C₁₄ or C₈---C₁₂ double bonds. The homogeneous deuteration reaction utilizes protection of the C₁₃---C₁₄ double bond as the C₁₅ O-silyl ether and protection of the carboxyl group as the methyl ester prior to reduction under molecular deuterium with tris(triphenylphosphine)chlororhodium (I) (Wilkinson's catalyst) in 60:40 acetone:benzene at 25°C. The reaction has been used to prepare six specifically deuterated prostaglandins: 5,6-dideuterio-PGF_1α, 5,6-dideuterio-PGE₁, 5,6-dideuterio-PGB₁, 3,3,4,4,5,6-hexadeuterio-PGF_1α, 5,6,10,11-tetradeuterio-11-deoxy-PGE₁, and 10,11-dideuterio-11-deoxy-PGE₁.     

Cystine peptides: the intramolecular antiparallel beta-sheet conformation of a 20-membered cyclic peptide disulfide

A 20-membered cyclic peptide disulfide has been synthesized as a conformational model for disulfide loops of limited ring size. ¹H-nmr studies at 270 MHz establish the presence of three intramolecular hydrogen bonds involving the Leu, Val, and methylamide NH groups in CDCl₃. Evidence for peptide aggregation in CDCl₃ is also presented. A structural transition involving loosening of the hydrogen bond formed by the Val NH group is observed upon the measured addition of (CD₃)₂SO to CDCl₃. Hydrogen-bonding studies, together with unusually low field positions of the Cys(1) and Cys(6) C^αH resonances and high J values provide support for an intramolecular antiparallel β-sheet conformation, facilitated by a chain reversal at the Aib-Ala segment. Extensive nuclear Overhauser effect studies provide compelling evidence for the proposed conformation and also establish a type I′ β-turn at the Aib-Ala residues, the site of the chain reversal.     

Optical rotatory dispersion of mucopolysaccharides and mucopolysaccharide--dye complexes. II. Ultraviolet cotton effects in the amide transition bands

Ultraviolet optical rotatory dispersion curves of mucopolysaccharides exhibit particular Cotton effects in the spectral region of the n–π and π–π amide transitions. Two general patterns emerge: (1) enhancement of negative rotation and of the first negative Cotton effect (troughs 217–220 mμ) and (2) relative dominance of the positive Cotton effect in the π–π transition region (peak ～198 mμ). Groups (1) and (2) can be correlated with a structural difference in the linkages of the amino sugars: (1) occurs with polymers containing 3-1-linked glycosamino sugars and (2) with glycosamino moieties linked 4–1 by either α- or β-glycosidie bonds. Measurements of the circular dichroic absorption bands support the qualitative conclusions from optical rotation. All mucopolysaccharides exhibit a first, negative band centered at 208–211 mμ, while only those in group (2) show, in addition, a positive band centered at 189–192 mμ. A suggested unifying model considers that difference in kind and/or degree of preferred geometry of the amide groups obtains from two forms of secondary order: (1) having a linear hydrogen bond from the N (acceptor) to the (C₂)O—H of the preceding uronic acid and (2) having a linear hydrogen bond from the N (acceptor) to the (C₂ or ₃) O—H of the following sugar. The hydrogen bonds would have similar strength but opposite directions in two systems towards the nonreducing end (1) or towards the reducing end (2)], closing eight-membered rings