113Cd NMR Research Articles

Synthesis, NMR ( 13C, 111Cd, 199Hg) spectra and structures of (Ph 4As) 2[Hg 2Cl 4(SC(O)Ph) 2] and (Ph 4As) 2[CdCl 4Hg(SC(O)Ph) 2

Combination of Ph 4AsCl · H 2O, Hg(SC(O)Ph) 2 and HgCl 2 or CdCl-· 2.5H 2O in the required stoichiometric ratio gives (Ph 4As) 2[MCl 4Hg(SC(O)Ph) 2], M = Hg ( 1) or Cd ( 2). Crystals of 1 were also isolated in an unsuccessful attempt to prepare the complex (Ph 4As)[Na{Hg(SC(O)Ph) 3} 2]. X-ray crystallographic analyses of 1 and 2 show both to contain a Hg(II) bonded to two thiobenzoate ligands with a near-linear SHgS skeleton. The SHgS angle is 174.9(2)° in 1 and 175.4(1)° in 2. In each, the Hg(SC(O)Ph) 2 moiety is involved in weak interactions with two chlorine atoms from [MCl 4] 2−. Mercury-199 and 111Cd NMR spectral studies indicate that the Hg(SC(O)Ph) 2 remains intact in solution, and that scrambling or transfer of ligands between the linear and tetrahedral metal centres does not occur. Carbon-13 and 199Hg NM R data are reported for Hg(SC(O)Ph) 2.

Mechanistic Aspects of the Copolymerization of CO2 and Epoxides by Soluble Zinc Bis(phenoxide) Catalysts as Revealed by Their Cadmium Analogues

A family of monomeric cadmium(II) phenoxides of the general formula Cd(O-2,6-R2C6H3)2(base)2-3 [R = Ph, tBu, Me; base = THF, THT, pyridine, propylene carbonate] has been synthesized. Determination of the solid-state structures of all complexes have uncovered a variety of geometries ranging from the expected distorted tetrahedral to square planar to trigonal bipyramidal, depending upon the identity of the phenoxide substituents and of the base ligands. By utilizing multinuclear NMR techniques available to cadmium, particularly 113Cd NMR, it was possible to discover the identities of these complexes in solution. Reactivities of these complexes with the small molecules CO2, COS, and CS2 were also carried out. While CO2 showed no reactivity with the sterically encumbered Cd(II) bis(phenoxides), insertion was observed spectroscopically to occur for COS and CS2. The product of CS2 insertion was isolated and characterized and found to be a dimeric xanthate complex, [Cd(S2CO-2,6-Ph2C6H3)]2[μ-O-2,6-Ph2C6H3]2. Thes...

Orientational ordering in some nematogens deviating from the classical rod-shape

Several new laterally substituted liquid crystalline compounds have been synthesized. They have the same main core which contains four rings (two aromatic, two alicyclic) with two lateral substituents introduced on the same side of one of the inner rings. One of the substituents is a 4-X-benzyloxy group (X=CH3,CN,Cl) and the other is a hexyloxy chain. The presence of the lateral aromatic substituent makes these compounds deviate markedly from the classical rod-shape. However, a wide enantiotropic nematic phase is present for all the compounds. The order parameters of the chain and the para -disubstituted aromatic rings were obtained by using a 2D 13C NMR technique with variable angle spinning. The temperature dependence of the order parameters was estimated using 13C chemical shifts with slow spinning of the sample parallel to the magnetic field. The results indicate that the two lateral substituents are more or less folded back along the mesogenic core. Thus, the flexible lateral chain is found to be roughly aligned with the molecular long axis, whereas the para axis of the less flexible aromatic branch makes a considerable angle with the molecular long axis imposed by the core, substantially increasing the mean width of the molecule. The core ordering does not seem to be influenced by the type and position of the substituents. The folding back of the lateral chain and the substantial tilt of the lateral aromatic branch with respect to the core main axis are confirmed by the X-ray structure of a parent compound.

NMR Coupling versus NMR Chemical Shift Information in Metallobiochemistry. High-Resolution One-Bond 1H−13C Coupling Constants Obtained by a Sensitive Reverse Detection Method

Imidazole rings are involved in acid/base chemistry, catalysis, H-bonding, and metal complexation throughout biochemistry; these rings are frequently targets for anticancer drugs and carcinogens. However, interpreting the changes in (13)C NMR shifts of these rings is often difficult. We explore the use of high-resolution one-bond (1)H-(13)C coupling constants ((1)J(CH)) for the identification of electronic changes within imidazole rings of samples containing (13)C in natural abundance. The reverse detection method used, called J-coupled heteronuclear multiple quantum coherence (JHMQC) spectroscopy, employs a modified HMQC pulse sequence. The method was evaluated with B(12) models of the type Me(3)BzmCo(DH)(2)(R or X), where Me(3)Bzm = 1,5,6-trimethylbenzimidazole and DH = the monoanion of dimethylglyoxime. (1)J(CH) values of Me(3)BzmCo(DH)(2)CH(3) obtained from both JHMQC and standard coupled 1D (13)C NMR spectra led to similar values, but the JHMQC method gave better resolution and much higher signal-to-noise ratios. The (1)J(CH) values for the endocyclic carbons and the N-methyl group of the Me(3)Bzm in five models fell between those of the free and protonated Me(3)Bzm ligands. Thus, donation of electron density from Me(3)Bzm to the Co center typically increases (1)J(CH) values with respect to the free ligand. Values of (1)J(CH) for several (13)C NMR signals correlated with both EP, a reported measure of electron-donating ability of R or X, and Co-N bond lengths from X-ray structures. For the assessment of electronic properties of the metal center, the (1)J(CH) values appear to be more reliable parameters than the traditionally used (13)C shifts, especially for C's close to the metal. Moreover, (1)J(CH) values for the (13)C signals for Co-(13)C were observed in several models; the (13)C signals for these carbons attached to the quadrupolar cobalt are too broad for (1)J(CH) determination by the traditional 1D method. The JHMQC method developed here is thus very versatile and can provide information on any type of molecule showing resolved CH signals.

Unprecedented Polycrystal Structure of a New Cadmium Thiolate Containing an Unusually Highly Charged [ClCd8{SCH(CH2CH2)2N(H)Me}16]15+ Core

A new cadmium thiolate with overall composition [ClCd8{SCH(CH2CH2)2N(H)Me}16](ClO4)15·16[SCH(CH2CH2)2N(H)Me]·32H2O has been obtained in a high ionic strength aqueous medium. The solid is a polycrystal consisting of equally oriented microcrystals of formula [ClCd8{SCH(CH2CH2)2N(H)Me}16](ClO4)15·16H2O held together by uncoordinated aminothiolate ligands and water molecules occupying the intercrystal spaces. The composition and structure of the microcrystals is shown by X-ray, XPS, and 113Cd NMR data. The uncoordinated aminothiolate ligands in zwitterionic form and water molecules are essential for the initial formation but not for the continued stability of the microcrystals as their chemical extraction leads to a new microcrystalline powder with essentially the same powder X-ray diffraction pattern. The existence of intercrystal spaces is confirmed by electron and atomic force microscopy techniques, SEM and AFM. The [ClCd8{SCH(CH2CH2)2N(H)Me}16]15+ cage can be described in terms of metal and ligand concent...

Using Cadmium-113 NMR Spectrometry To Study Metal Complexation by Natural Organic Matter

Because of its relatively high natural abundance and the broad range of chemical shifts (900 ppm), 113Cd is a potentially excellent istope for NMR studies of metal binding sites of natural organic matter (NOM). This paper presents a 113Cd NMR study of the effects of pH and, to a lesser extent, Cd/C molar ratio on the complexation of cadmium ion by Suwannee River NOM. Spectra are presented for solutions ranging in pH from 3.6 to 9.0 and ranging in Cd/C ratio from 0.0013 to 0.0068. At acidic pH, a single somewhat sharp peak is observed, but more complex spectra are obtained at alkaline pH. These changes indicate that Cd−NOM exchange rates are much faster at acidic pH, where the concentration of free Cd2+ is greatest. Under nearly all the conditions of Cd/C ratio and pH that were used, Cd2+ is primarily coordinated by O donor atoms, probably in carboxyl groups. At high pH, some evidence of Cd2+ coordination by N donor atoms is observed. No evidence of Cd2+ ions coordinated by S donor atoms was observed in the NMR spectra, even at Cd/C ratios that were significantly lower than the S/C ratio of the NOM.

Application of 113Cd NMR to metallothioneins.

Metallothioneins constitute a class of ubiquitously occurring low molecular mass proteins (6-7 kDa) possessing two cysteine thiolate-based metal clusters usually formed by the preferential binding of d10 metal ions such as ZnII and CdII. The three-dimensional solution structure of mammalian proteins has been determined by two-dimensional NMR spectroscopy of 113Cd7-metallothionein. The structure shows two protein domains encompassing the M3(CysS)9- and M4(CysS)11-cluster with each metal ion being tetrahedrally coordinated by thiolate ligands. The application of 113Cd NMR proved to be indispensable in the structural studies of metallothioneins. Thus, both homonuclear 113Cd decoupling studies and 113Cd-113Cd COSY of 113Cd7-metallothionein established the existence of two metal-thiolate clusters in this protein. The identification of sequence specific cysteine-cadmium coordinative bonds came from heteronuclear 113Cd-1H COSY experiments. Independently, the 113Cd NMR characterization of the intermediate metal-protein complexes, leading to the cluster structure in 113Cd7-metallothionein, revealed a stepwise cluster formation process with the Cd4(CysS)11-cluster being formed first. The recent demonstration of a Karplus-like dependence between the heteronuclear3 J(113Cd, 1H) coupling constants for the cysteine C beta protons and the H beta-C beta-S gamma -Cd dihedral angles should allow to derive the geometry of the CD-(S-Cys) centers in various metallothioneins and related metalloproteins. A possible application of 113Cd NMR to the study of metallothioneins in the environment is discussed.

Ligation properties of N-substituted imidazoles: synthesis, spectroscopic and structural investigation, and behaviour in solution of zinc(II) and cadmium(II) complexes

Novel zinc(II) and cadmium(II) derivatives of 1-methyl- (L′) and 1-benzyl-imidazole (L″) of the type 1:1 [(L)MX 2] (L = L″, M = Zn, X = 4-acetyl-1-phenyl-3-methylpyrazolonato (Q″), 4-benzoyl-1-phenyl-3-mrthylpyrazol-5-onato (Q′) or 4-benzoyl-1,3-dimethylpyrazol-5-onato (Q ∗); L= L″, M = Cd, X = Q′or Q ∗, 2:1 [(L) 2MX 2]·y(H 2O) (L = L′, M = Zn, X = Cl, Br, I, CH 3CO 2, Q′, Q″ or Q ∗; L = L′, M = Cd, X = I, CH 3CO 2, Q′ or Q ∗; L = L″, M = Zn, X = Cl, Br, I; L = L″, M = Cd, X = I or Q″), 3:1 [(L″) 3MX 2]·y(H 2O) (M = Zn, X = CF 3CO 2, NO 3 or BF 4; M = Cd, X = NO 3), 4:1 [(L′) 4Zn]X 2·y(H 2O) (X = NO 3 or BF 4), 5:1 [(L′) 5Zn]X 2·y(H 2O) (X = ClO 4 or CF 3CO 2) 6:1 [(L′) 6Zn]X 2·y(H 2O) (L = L′, M = Zn, X = CF 3SO 3; L = L′, M = Cd, X = NO 3, ClO 4 or BF 4; L = L″, M = Cd, X = ClO 4 or BF 4), 4:3 [(L″) 4(CdX 2) 3] (X = Cl or Br) and finally 5:3 [(L′) 5(CdBr 2) 3] have been prepared and characterized by IR and far-IR data, conductivity, 1H NMR and in some cases also with 113Cd NMR spectroscopy. The 4,7-dimethyl-1,10-phenanthroline (Me 2Phen) and 2,2′-bipyridine (Bipy) displace all the 1-methylimidazole donors from [(L′) 5Zn](ClO 4) 2·H 2O or [(L′) 6Cd(ClO 4) 2 diethyl ether suspension, yielding the trischelate complexes [(Me 2Phen) 3M](ClO 4) 2 and [(Bipy) 3M](ClO 4) 2 (M = Zn or Cd). On the other hand, 2-benzoylpyridine (Bzpy) and 4,7-diphenyl-1,10-phenanthroline (Ph 2Phen) react with [(L′) 5Zn](ClO 4) 2·H 2O yielding the mixed ligands complexes [(Bzpy) 2(L′) 2Zn](ClO 4) 2 and [(Ph 2Phen) 2(L′)Zn](ClO 4) 2 respectively. From the reaction of Ph 2Phen with [(L′) 6Cd(ClO 4) 2 the ionic compound [(Ph 2Phen) 3Cd](ClO 4) 2 is obtained. The structure of [(L″) 3Cd(NO 3) 2] was determined by X-ray single crystal diffraction: the Cd atom exhibited a distorted trigonal bipyramidal geometry, involving two O-monodentate nitrato groups in axial position and three 1-benzylimidazole ligand in the equatorial positions. 113Cd NMR solution data carried out on selected soluble pyrazolonato, carboxylato and nitrato derivatives indicated five- or six-coordinate N 3CdO 2, N 4CdO 2, N 2CdO 4 and NCdO 4 species in solution.

1H and 13C 2D NMR Studies on Substituted .DELTA.3-Pyrrolin 2-ones.

The 1H and 13C 2D NMR studies of pyrrolinones derivatives were performed using the method of proton detected 2D 1H, 13C correlation spectroscopy. HMQC and HMBC experiments were achieved in order to obtain the unambiguous assignment of the structures.

Tris(4-bromo-1 H-pyrazol-1-yl)borato derivatives of first-row transition and group 12 and 14 metals. X-ray crystal structure of [HB(4-Brpz) 3] 2 Cd. 113Cd solution NMR study of bis[poly(pyrazolyl)borato]cadmium complexes

A series of metal complexes M[HB(4-Brpz) 3] 2 [HB(4-Brpz) 3 = hydridotris(4-bromo-1 H-pyrazol-1-yl)borate and M = Mn, Fe, Co, Ni, Cu, Zn, Cd, Pb] were obtained from reaction of K[HB(4-Brpz) 3] and the appropriate metal salt in aqueous solution. These air-stable complexes are sparingly soluble in most common solvents; they have been investigated through elemental analysis, conductivity measurements, IR, UV-vis and, in the case of diamagnetic compounds, 1H and 13C NMR spectroscopy. The X-ray crystal structure of cadmium(II) bis[hydridotris(4-bromo-1 H-pyrazol-1-yl)borate] has been resolved. Six-coordinate cadmium is present in a slightly distorted octahedron with the axes not linear [angles 176.8(3), 177.2(3) and 178.7(3)°]. The ‘bite’ of the ligand, i.e. NCdN angle, lies in the range 81.6(2)–84.1(3)° (weighted average 82.4(4)°). The CdN distances vary from 2.298(8) to 2.329(8) Å (weighted average 2.317(5) Å). The geometry around the B atom is tetrahedral. The NBN angles range from 105.8(8) to 110.5(7)° (weighted average 108.5(7)°); the BN bond distances vary from 1.53(2) to 1.62(1) Å (weighted average 1.58(1) Å). We report and discuss also the 113Cd NMR solution data of several cadmium poly(pyrazol-1-yl)borates).

Electronic and Spatial Structure Studies of Cadmium and Zinc Dialkyldithiocarbamate Molecules in Nonaqueous Solutions Used in the Processes Spray Pyrolysis

ABSTRACTCdK EXAFS and 113Cd NMR spectra, ZnK EXAFS and XANES spectra were measured for solutions of cadmium and zinc dialkyldithiocarbamates in organic solvents with different donor numbers: tributylphosphine, methylene chloride, benzene, dibutylsulfide, pyridine, dimethylsulfoxide and for some model compounds. The parameters of the local surroundings of the Cd and Zn atoms for complex forms in solutions were determined using EXAFS . Spatial structure models of the complex forms in a metal chelate - nonaqueous solvent system are suggested. It is established that coordination of nitrogen atoms by cadmium atoms in pyridine solutions is realized for both Cd[(C2H5)2NCS2 ]2 and Cd[(n−C4 H9)2NCS2]2. For the tributylphosphine solution of Cd[(n−C4H9) 2NCS2]2 and Zn[(n−C4H9)2NCS2]2, additional coordination of the phosphorus atoms of the solvent molecules by the metal atoms are established.

Structural Diversity in Monomeric Cadmium Phenoxides.

Three monomeric Cd(II) phenoxide complexes have been prepared by reacting Cd[N(SiMe(3))(2)](2) with 2 equiv of 2,6-disubstituted phenols bearing sterically bulky tert-butyl or phenyl groups. The strongly coordinating solvents THF, tetrahydrothiophene (THT), and pyridine used for these reactions were incorporated into the metal's coordination sphere, leading to complexes with a general formulation of Cd(O-2,6-R(2)C(6)H(3))(2)(solv)(2)(-)(3). The Cd complexes obtained have been characterized crystallographically and have been found to adopt differing solid-state geometries. The X-ray crystal structure of Cd(O-2,6-(t)BuC(6)H(3))(2)(THF)(2), 1, previously reported by Buhro, displayed unusual square-planar coordination of the metal center. Complex 2, Cd(O(t)BuC(6)H(3))(2)(THT)(2), has been found to take on the same square-planar geometry, even with the more strongly donating thioether ligand. Alternatively, complex 3, Cd(O-2,6-Ph(2)C(6)H(3))(2)(THF)(2), has been found to adopt distorted-tetrahedral geometry, quite similar to its zinc congener. The O(1)-Cd-O(2) angle between the phenoxide ligands in 3 is 150.1(2) degrees, and the angle between the ether ligands is 83.1(3) degrees. When the strongly basic solvent pyridine was used, a five-coordinate complex 4, Cd(O-2,6-(t)BuC(6)H(3))(2)(py)(3), was isolated. This complex 4 is best described as having trigonal bipyramidal geometry with the phenoxide ligands and one pyridine defining the equatorial plane and two axial pyridine ligands having an angle of 169.7(2) degrees. The angle between the phenoxide ligands in 4 is 156.1(2) degrees. These complexes 1-4 have also been characterized in noncoordinating solvent solutions by (1)H, (13)C, and (113)Cd NMR spectroscopy and have been found to contain labile donor ligands. Preliminary studies indicate that, in a noncoordinating solvent, a rapid equilibrium exists between species with and without coordinated donor solvent ligands.

Recombinant two-iron rubredoxin of Pseudomonas oleovorans: overexpression, purification and characterization by optical, CD and 113Cd NMR spectroscopies.

The gene (alk G) encoding the two-iron rubredoxin of Pseudomonas oleovorans was amplified from genomic DNA by PCR and subcloned into the expression vector pKK223-3. The vector directed the high-level production of rubredoxin in Escherichia coli. A simple three-step procedure was used to purify recombinant rubredoxin in the 1Fe form. 1Fe-rubredoxin was readily converted to the 2Fe, apoprotein and cadmium forms after precipitation with trichloroacetic acid and resolubilization in the presence or absence of ferrous ammonium sulphate or CdCl2 respectively. Recombinant 1Fe and 2Fe rubredoxins are redox-active and able to transfer electrons from reduced spinach ferredoxin reductase to cytochrome c. The absorption spectrum and dichroic features of the CD spectrum for the cadmium-substituted protein are similar to those reported for cadmium-substituted Desulfovibrio gigas rubredoxin [Henehan, Poutney, Zerbe and Vasak (1993) Protein Sci. 2, 1756-1764]. Difference absorption spectroscopy of cadmium-substituted rubredoxin revealed the presence of four Gaussian-resolved maxima at 207, 228, 241 and 280 nm; the 241 nm band is attributable, from Jorgensen's electronegativity theory, to a CysS-CdII charge-transfer excitation. The 113Cd NMR spectrum of the 113Cd-substituted rubredoxin contains two 113Cd resonances with chemical shifts located at 732.3 and 730 p.p.m. The broader linewidth and high frequency shift of the resonance at 730 p. p.m. indicates that the Cd2+ ion is undergoing chemical exchange and, consistent with the difference absorption spectra, is bound less tightly than the Cd2+ ion, giving rise to the chemical shift at 732.3 p.p.m.

MONO-AND DINUCLEAR COBALT(III) COMPLEXES WITH S-TYROSINE

In a direct synthesis, by oxidation of cobalt(II) to cobalt(III) by oxygen in the presence of the lithium salt of S-tyrosine, the following isomers of the tris(S-tyrosinato)cobalt(III) complex were obtained: Δ-(−)589-facial, Λ-(+)589- and Δ-(-)589-meridional, and (−)589-anti(N)-Δ-cis(N), cis(O)-Δ-cis(N), cis(O)-di-μ-hydroxo-terakis(S-tyrosinato)dico-balt(III). Their composition was established by microanalysis, and their structure by electronic absorption, CD and 13C NMR spectra. Finally, it was found that Δ-mer-tris(S-tyrosinato)cobalt(III) described earlier is a mixture of Δ and Δ diastereomers of the complex.

The helix-hinge-helix structural motif in human apolipoprotein A-I determined by NMR spectroscopy.

The conformation of a synthetic peptide of 46 residues from apoA-I was investigated by fluorescence, CD, and 2D NMR spectroscopies in lipid-mimetic environments. ApoA-I(142-187) is mainly unstructured in water but helical in SDS or dodecylphosphocholine (DPC), although the peptide only associates with DPC at approximately the critical micellar concentration. Solution structures of apoA-I(142-187) were determined by distance geometry calculations based on 450 (in DPC-d38) or 397 (in SDS-d25) NOE-derived distance restraints, respectively. Backbone RMSDs for superimposing the two helical regions 146-162 and 168-182 are 0.98 +/- 0.22 (2.38 +/- 0.20) and 1.99 +/- 0.42 (2.02 +/- 0.21) A in DPC (SDS), respectively. No interhelical NOE was found, suggesting that helix-helix interactions between the two helical domains in apoA-I(142-187) are unlikely. Similar average, curved helix-hinge-helix structures were found in both SDS and DPC micelles with the hydrophobic residues occupying the concave face, indicating that hydrophobic interactions dominate. Intermolecular NOESY experiments, performed in the presence of 50% protonated SDS, confirm that the two amphipathic helices and Y166 in the hinge all interact with the micelle. The involvement of Y166 in lipid binding is supported by fluorescence spectroscopy as well. On the basis of all the data above, we propose a model for the peptide-lipid complexes wherein the curved amphipathic helix-hinge-helix structural motif straddles the micelle. The peptide-aided signal assignment achieved for apoA-I(122-187) (66mer) and apoA-I suggests that such a structural motif is retained in the longer peptide and most likely in the intact protein.

4′-dehydroxycabenegrin A-I from roots of Harpalyce brasiliana

An ethanol extract from roots of Harpalyce brasiliana yielded betulinic acid and a prenylated pterocarpane, 3-hydroxy-4-(4-isopentenyl)-8(9)-methylenedioxi-6a R, 11a R-dihydro-6H-benzofuro[3,2-c] [1]benzopyran (4′-dehydroxy-cabenegrin A-I), very similar to the already known potent snake venom antidote, cabenegrin A-I. Structural determination, including the absolute stereochemistry, was accomplished by spectral analysis particularly CD and 2D NMR techniques. Chemical derivatization and comparison with literature data were also helpful for structural elucidation.

Conformational studies of the N-terminal lipid-associating domain of human apolipoprotein C-I by CD and 1H NMR spectroscopy.

A peptide comprising the N-terminal 38 residues of human apolipoprotein C-I (apoC-I(1-38)) was synthesized using solid-phase methods and its solution conformation studied by CD and 1H NMR spectroscopy. The CD data indicate that apoC-I(1-38) has a similar helical content (55%) in the presence of saturating amounts of SDS or egg yolk lysophosphatidylcholine. A structural ensemble of SDS-bound apoC-I(1-38) was calculated from 464 NOE-based distance restraints using distance geometry methods. ApoC-I(1-38) adopts a helical structure between residues V4 and K30 and an extended C-terminus from Q31 when associated with SDS. The region K12-G15 undergoes slow conformational exchange as indicated by above-average amide resonance linewidths, large temperature coefficients, and fast exchange (< 2 h) of backbone amide protons with deuterium. The mobility of K12-G15 is reflected in the poorly defined dihedral angles of K12 and E13 in the calculated ensemble of structures. The average structure of apoC-I(1-38) is curved toward its hydrophobic face with bends of 125 degrees, centered at K12/E13, and 150 degrees, centered at K21. This curvature appears to be driven by the interaction of two hydrophobic clusters, one formed by residues L8, L11, F14, and L18, and the other by L25, I26, and I29, with the amphiphile SDS. Based on our present structural definition of apoC-I(1-38) and the previously obtained structure of the fragment apoC-I(35-53), we propose the secondary structure of intact apolipoprotein C-I.

Solution structure of alpha t alpha, a helical hairpin peptide of de novo design.

alpha t alpha is a 38-residue peptide designed to adopt a helical hairpin conformation in solution (Fezoui Y, Weaver DL Osterhout JJ, 1995, Protein Sci 4:286-295). A previous study of the carboxylate form of alpha t alpha by CD and two-dimensional NMR indicated that the peptide was highly helical and that the helices associated in approximately the intended orientation (Fezoui Y, Weaver DL, Osterhout JJ, 1994, Proc Natl Acad Sci USA 91:3675-3679). Here, the solution structure of alpha t alpha as determined by two-dimensional NMR is reported. A total of 266 experimentally derived distance restraints and 20 dihedral angle restraints derived from J-couplings were used. One-hundred initial structures were generated by distance geometry and refined by dynamical simulated annealing. Twenty-three of the lowest-energy structures consistent with the experimental restraints were analyzed. The results presented here show that alpha t alpha is comprised of two associating helices connected by a turn region.

RKS-1778, a new mammalian cell-cycle inhibitor and a key intermediate of the [11]cytochalasin group.

In the course of screening for the mammalian cell-cycle inhibitors, we have isolated a new [11]cytochalasin, RKS-1778 (1), and epoxycytochalasin H (2) from a fungus, Phoma sp. SNF-1778. The structure of 1 was determined to be 21-acetoxy-18-hydroxy-10-phenyl-5,6,16,18- tetramethyl[11]cytochalasa-6,13,19-trien-1-one, one the basis of spectroscopic methods, including 1H- and 13C-2D NMR techniques, RKS-1778 (1) may be a precursor of 2 and the key direct product of a proposed biosynthetic intramolecular Diels-Alder reaction. Both 1 and 2 completely arrested the cell-cycle progression of tsFT210 cells in the M phase at concentrations of 2.1 and 2.0 microM, respectively.

New Insecticidal Rocaglamide Derivatives from Flowers of Aglaia odorata

Abstract Flowers of A. odorata yielded six insecticidal rocaglamide derivatives including four compounds which proved to be new natural products. Structure elucidation of the new com pounds, including establishment of their absolute configurations by CD spectroscopy and NMR is described. When incorporated into artificial diet all isolated rocaglamide derivatives exhibited strong insecticidal activity towards neonate larvae of the polyphagous pest insect Spodoptera littoralis with LC50s varying from 1.5-53.4 ppm (2.9-97.1 nᴍ) . Two of the isolated compounds which showed LC50s of 1.5 and 1.6 ppm (2.9 and 3.2 nᴍ ) respectively are similar with regard to their insecticidal activity to azadirachtin (LC50 0.9 ppm [1.3 nᴍ] ) which was included into the study as a positive control.