Ring Torsion Angles Research Articles

Crystallographic and oxygen-17 NMR studies of nitro group torsion angles in a series of 4-alkylaminonitroquinolines designed as hypoxia-selective cytotoxins

Nitro group torsion angles have been determined by 17O NMR spectroscopy for a series of 4-(alkyl-amino)nitroquinolines and their ortho-methyl-substituted analogues. Crystal structures were determined for two pairs of compounds, to further evaluate the validity of Boykin's equation. The crystallographic torsion angles were used to calculate a modified version of the equation, relating 17O chemical shift values (δ) and nitro group torsion angles (θ), applicable to N-heterocyclic systems, as follows: θ= 1.18(±0.13)δ– 661. This equation was then used to compute nitro group torsion angles for the nitroquinolines. Unhindered nitro groups were close to coplanar with the aromatic ring as expected, while addition of one ortho methyl group increased the torsion angle to ca. 30°. The 5-nitro derivative had a nitro group torsion angle of ca. 80°, due to peri interactions with the 4-aminoalkyl sidechain. The 8-nitroquinoline derivative is the first example of a nitroaromatic with a peri aromatic nitrogen substituent, and the torsion angle of 70–78°(measured by both NMR spectroscopy and X-ray crystallography) indicates the substantial steric effect of the nitrogen lone pair. Addition of a 7-methyl group in the other nitro ortho position of this compound results in the nitro group being virtually at right angles to the ring (torsion angle 86°). A comparison was made between the measured torsion angles and those calculated using the AM1 and PM3 methods. The former underestimates the nitro torsion angles in these systems, while the PM3 method significantly overestimates them. Overall, no simple relationship exists in the nitroquinolines between nitro group torsion angles and the reduction potentials of the compounds.

Ring-torsional solitons in polyaniline.

Topological defects involving distortions of the phenyl ring torsion angles in the leucoemeraldine base form of polyaniline are studied. A ring-torsional order parameter is introduced; two solitonlike defects of this order parameter are found, one of which has been discussed previously. These solitons are anticipated to exhibit a conventional spin-charge relationship, unlike the bond-alternation solitons in trans-polyacetylene. The creation energies of these solitons are estimated, and possible experimental signatures and relevance of neutral and charged solitons are discussed.

Ring-torsional polarons in polyaniline and polyparaphenylene sulfide

The role of the dihedral angle of the backbone phenyl rings in determining the electronic structure and charge storage mechanism in polyaniline (PAN) and polyparaphenylene sulfide (PPS) is reviewed. The sensitivity of the average ring torsion angle to the effects of steric repulsion and electron delocalization is manifest in the effects of derivatization and temperature on the optical spectra of these materials. It is shown that charge is self-trapped into localized defect states in part via changes in ring angle, leading to novel ring-torsional polarons which induce only one defect level into the energy gap and possess a very large kinetic mass (50 – 500 m e ). Evidence for the existence of these defects in PAN and PPS is discussed.

Torsional Defects and Photoinduced Charge Transfer in Ring-Containing Polymers

Abstract The importance of ring torsion angle for photoinduced charge generation and transfer in phenyl-ring-containing polymers such as polyaniline and polyparaphenylene sulfide is discussed. The magnitude of the ground state ring torsion angle in these materials is determined by the competition between electron delocalization, favoring planar rings, and the steric repulsion between rings, favoring rings twisted far out of the plane. Upon photoinduced charge transfer, the hole formed upon removal of an electron from the valence band becomes self-localized into a polaronic state in part through changes in ring torsion angle; the effective mass of this defect is estimated to be at least an order of magnitude greater than that of bond-alternation defects. The existence of these ring-torsional defects is borne out by the experimental observation of large effective masses, long relaxation times, and weak photoconductivity for photoexcited defects, as well as by x-ray structural evidence of the dependence of t...

Conformation of an eleven-membered lactone: structure of (E,13R,14S)-13-hydroxy-17-nor-8,9-secolabd-9(11)-en-8,14-olide

C19H3203, Mr=308.49, orthorhombic, P212121, a = 7-072 (3), b = 9389 (3), c = 27-888(11)A, v=1852(2)A 3, z=4, Ox = I. 11 Mg m - 3, ,t(Mo Ka) = 0.71069 )k, /z = 0.08 mm-1, F(000) = 680, T = 293 K, R = 0.050 for 1475 independent reflections with I> 2-5o'(/). The conformation of the l l-membered lactone ring is essentially the same as the calculated minimum- energy conformation of trans, trans-cycloundeca- 1,6-diene with differences between corresponding ring torsion angles of only 1-15 ° (mean 6°). This conformation is found in several cytochalasans and dolabellane diterpenoids but not in the pyrrolizidine alkaloids with l l-membered dilactone rings. The cyclohexane ring is distorted to alleviate 1,3-diaxial repulsion between methyl substituents (C(18)-C(20) = 3.291 (7) A); the smallest torsion angles (-47.4 (5), 48.1 (5) °) are at ring bonds between the methyl groups, the largest torsion angles (-55-6 (6), 56.4 (6) °) at bonds remote from these substituents. Introduction. Eleven-membered rings are features of several classes of natural products, e.g. the cyto- chalasans (Tamm, 1980), dolabellane diterpenoids (Matsuo, Kamio, Uohama, Yoshida, Connolly & Sim, 1988) and pyrrolizidine alkaloids (Robins, 1982). Examination of crystallographic results reveals two conformations for the trans, trans- cycloundeca-l,5-diene system in humulene deriva- tives (Khan, MacAlpine, Porte & Sim, 1983) and a number of dolabellane diterpenoids (Matsuo, Uohama, Yoshida, Nakayama, Hayashi, Connolly &

Role of ring torsion angle in polyaniline: Electronic structure and defect states.

The role of phenyl-ring torsion angle in determining the nature of the ground and charged-defect states of polyaniline is explored. The coupling of the transfer integral between nitrogen atom and phenyl-ring constituents of the polyaniline chain to the dihedral angle of the rings competes with the substantial steric repulsion between adjacent rings in determining the conformation of these systems. The ring conformation of the leucoemeraldine-base (LB) form of polyaniline is described by a novel ring-torsion-angle order parameter. The anharmonicity of the interring steric potential leads to a temperature-dependent mean order parameter, and thus to the prediction of thermochromic effects consistent with experiment. Furthermore, changes in optical spectra accompanying derivatization of the rings can be understood by the response of the substituted polymer to modifications of the steric potential. The existence of two degenerate ring-torsion-angle phases in LB implies that both polaronic and solitonic ring-angle-alternation defect states may be relevant in describing the charged states in polyaniline. As these defects involve substantial changes in ring torsion angle, they are expected to possess large kinetic mass, in agreement with photoinduced absorption experiments on polyaniline. The anticipated Peierls ground state of the oxidized pernigraniline-base form of polyaniline can be regarded in part as a ring-torsion-angle dimerized state; consequently, the charge states of this material are also expected to be massive defects in the ring-rotational order. The importance of ring rotations in other ring-containing electronic polymers, such as poly(paraphenylene sulfide), is discussed.

Bond‐Optimized ring closure for proline: Comparison of conformations and semiempirical energies with small molecule X‐ray structures

A method is described for generating proline ring structures by successive addition of atoms, wherein ring closure is achieved by optimizing the fit to known ring bond-angles and one closing bond-length ("bond-optimized ring closure"). Two ring torsion angles are fixed independently within broad, allowed ranges, and the remaining torsion angles are determined uniquely in most cases. The independent torsion angles are chosen as phi and chi 2, and ring closure is achieved without prohibitive strain through most of the ranges -130 degrees less than phi less than -20 degrees and -60 degrees less than chi 2 less than 60 degrees. Comparisons of predicted ring structures to 191 X-ray diffraction structures from the literature, starting with the known values of phi and chi 2, yielded root-mean-square deviations of 4.8 degrees in chi 1, 4.7 degrees in chi 3, 8.3 degrees in chi 4, and 0.3-2% in the ring bond angles and the N-C delta distance. Semiempirical energies were calculated for the optimized structures using three sets of energy parameters from the literature. The energy surfaces show broad minima coinciding with the torsion angle regions in which the highest concentrations of observed structures are found. Two of the sets of energy parameters produce double minima corresponding to the "up" and "down" puckered conformations.

Ring-rotational defects in polyaniline

The effects of electron-lattice coupling via ring rotation on the electronic structure of leucoemeraldine base (LB) and its charged defects are investigated. We suggest that in LB and other phenyl-ring-containing polymers the charged defect states, particularly hole polarons, are associated with localized distortions of the ground state ring torsion angles toward a planar conformation, in contrast with the bond-alternation defects present in conducting polymers. The relevance of these defects to recent photoinduced optical experiments is discussed.

A theoretical study of the influence of noradrenaline ring fluorination on its adrenergic activity

An account is given of the differing α- and β-adrenergic activities of the ring fluorinated noradrenaline derivatives. By introducing a formyl group as a model for the adrenergic receptor it is found that the noradrenaline side chain hydroxyl function is oriented away from the phenyl ring. In the case of the ring fluorinated compounds a fluorine-oxygen electrostatic repulsion leads to a preferred phenyl conformation. It is suggested that the α-receptor conformation corresponds to a value of the ring torsion angle, τ, of about 130° whereas 310° is required for the torsion angle when bound to the β-receptor.

Crystal and molecular structure of 3,3-diphenyl-3-phosphoniabicyclo[3.2.1]oct-6-ene bromide monohydrate

The title compound crystallizes in the monoclinic space groupP21/n,a=13.429(3),b=18.248(6),c=14.497(4)A,β=90.61(2)°,Z=8, with two independent molecules in the asymmetric unit. The structure was solved by heavy-atom methods, and refined to a finalR value of 0.040. The phosphoranium ring adopts a chair conformation which is substantially flattened at the phosphorus end. The ring conformation and torsion angles are compared with similar compounds.

Conformational analysis of MBBA fluorinated analogues by 1H and 13C - NMR

1H- 13C -chemical shifts correlation analysis for MBBA and a series of its fluorinated analogues have been carried out. The azomethine proton chemical shift is shown to be sensitive to the aniline ring torsion angle and can be used for its approximate estimation.

A twist boat: Structure and conformation oftrans-2,4,6-tris(trichloromethyl)-1,3,5-trithian

Trans-2,4,6-tris(trichloromethyl)-1,3,5-trithian, (CCl3·CHS)3, has been prepared free from persistent impurification bycis-trans-2,4,6-tris(trichloromethyl)-1,3,5-oxadithian and its conformation determined by single crystal x-ray crystallography. The title compound is orthorhombic,Pbca, a=10.479(10),b=27.957(14),c=11.093(3) A,V=3250 A3,Z=8. The structure was solved by direct methods, from data collected at room temperature on an Enraf-Nonius CAD4 diffractometer, and refined by full-matrix least squares to a finalR value at 0.074 using 1186 reflections. The molecule exhibits a twist boat conformation with the −CCl3 groups occupying pseudoequatorial positions. The torsion angles in the trithian ring are successively 29(1), −79(1), 41(1), 41(1), −84(1), 39(1)°. Average values are: S-C 1.82(2), C(ring)-C(Cl3) 1.56(4), C-Cl 1.76(2) A, S-C-S 112.7(2)°. The C-S-C angles are 97.4(7), 101.4(7), 97.3(7)°.

The conformation and molecular structure of a 6,7-diazabicyclo[3.2.0] hept-6-ene derivative: a gas-phase electron diffraction study of perfluoro-(1,5-dimethyl-6,7-diazabicyclo[3.2.0] hept-6-ene)

The molecular structure of perfluoro-(1,5-dimethyl-6,7-diazabicyclo[3,2,0]help-6-ene) has been studied using gas-phase electron diffraction data collected on the Balzers KDG2 instrument at UMIST. The data are compatible with C 1 endo model (boat conformation) with −CF 3 groups in independent orientations. Apart from the −CF 3 groups the symmetry is C s. The preferred refinement gave CC distances in the range 1.51–1.57 Å with the longer ones including CCF 3. The ring flap angles are 109.5(9)° across the bridge bond and 143.7° for the five-membered ring. The latter has ring torsion angles of 0, ±36 and ±24° and ∠CCC angles in the range 98–107°, while in the planar four-membered ring ∠NNC = 96.3° and ∠NCC = 83.7°with CN = 1.53(1)° and NNN = 1.17(2)». The NCCF torsion angles for corresponding F atoms of the —CF 3 groups are + 76(1) and − 21(1)°. Other parameters include CF(mean) = 1.346(2)Å, ∠NCC(F 3) = 119.8(12)°, ∠CCF(CF 3) = 111.6(5)°, ∠FCF(CF 2) = 112.7(7)° and torsion angle NNCC(F 3) = ±121.4(3)°. The geometrical parameters are similar to those of related molecules, with the principal influence of fluorination probably being the short NN bond.

X-ray structure of the major adduct of the anticancer drug cisplatin with DNA: cis-[Pt(NH3)2(d(pGpG))

Crystals of the adduct of the anticancer drug cis-diamminedichloroplatinum(II), cis-DDP, with d(pGpG), its putative target on DNA in the cancer cell, have been obtained and used in an x-ray crystallographic study to elucidate the molecular structure to atomic resolution. Each of the four crystallographically independent cis-[Pt(NH3)2(d(pGpG))] molecules is comprised of a square-planar platinum atom bonded to two ammonia ligands and two N(7) atoms of guanosine nucleosides from the same chain. Base stacking of the two adjacent guanine rings is completely disrupted by coordination to the cis-(Pt(NH3)2)2+ unit. Comparison of the backbone and deoxyribose ring torsion angles with those found by previous (nuclear magnetic resonance spectroscopy) studies of this adduct in solution demonstrates that the solid state geometry is substantially the same as that in solution. The relevance of these results to the molecular mechanism of action of cis-DDP is discussed.

Conformational analysis of hydroxyproline and its protected derivatives by 1H NMR

From a computer simulation of the 270 MHz 1H NMR spectra of hydroxyproline (Hyp) and its protected derivatives, precise values of ring vicinal coupling constants were obtained. These couplings were related to ring torsional angles, using a Karplus type analysis. From the NMR analysis it was observed that the pyrrolidine ring possesses a unique and highly homogeneous conformation (C γ-exo form). Temperature dependence studies on protected dipeptides suggest that the pyrrolidine ring conformation is independent of backbone conformation. An unusual X-Hyp, β-turn was observed for Boc-Aib-Hyp-NHMe.

Bindungsabstände zwischen organylsubstituierten zinnatomen: I. Cyclo-hexastannane

The crystal structures of dodecaphenyl-cyclohexastannane (Ph 2Sn) 6 and dodecabenzyl-cyclohexastannane dimethylformamide (Bz 2Sn) 6·DMF have been determined. The molecules of (Ph 2Sn) 6 show, in contrast to those of (Ph 2Sn) 6·2toluene and (Bz 2Sn) 6·DMF, remarkable differences in the chair conformation indicated by the ring torsion angles and the bond angles. The tin—tin bonds of (Bz 2Sn) 6·DMF are somewhat longer than those of the phenyl compounds.

Structure of a mycotoxin produced by Fusarium nivale: 4-acetamido-2-buten-4-olide

C6H7NO3, M r = 141.13, monoclinic, P2~/c, a = 8.790 (4), b = 5.314 (2), c = 14.264 (3)A, fl = 96.47 (3) °, Z = 4, U = 662.03 A 3, D e = 1.415 Mg m -3, Cu K~, ~, = 1.5418 A, g = 0.994 mm-k Final R = 0.039 for 963 observed reflexions (I > 20(I)). Both o and L enantiomers appear in the structure. Bond lengths and angles are in agreement with expected values. The lactone ring is planar; mean value of the ring torsion angles is 0.7 (3) ° . The molecules are connected by hydrogen bonds N-H... O (3.000 (4) ,/k) forming infinite chains along b. Introduction. Toxic metabolites produced by the mould Fusarium nivale on tall fescue (Festuca arundinacea Schreb.) cause various maladies in cattle (Tookey, Yates, Ellis, Grove & Nichols, 1972). Among the three isolated toxins (a butenolide, an unknown toxin, and a sesquiterpenoid), the butenolide is the most abundant (ratios 87:8:3) (Yates, Tookey, Ellis & Burkhardt, 1968). The structure of the butenolide, assigned as 4-acetamido-2-buten-4-olide on the basis of the IR, UV and NMR spectra, is in agreement with the X-ray structure determination. Although the butenolide pos- sesses a chiral centre at the y-carbon, the lack of optical rotation between 600 and 290 nm (Yates et al., 1968) suggests isomerization or racemization during isolation and purification. An X-ray structure determination o(0) revealed the presence of both enantiomers in the c(1)

The molecular structure of trans- and cis-bicyclo-[4.3.0]nonane in the gas phase, studied by electron diffraction and molecular mechanics

The structure and conformations of trans- and of cis-bicyclo[4.3.0]nonane have been studied in the gas phase. Molecular mechanics calculations applying the force field of Ermer and Lifson were used to obtain geometrical constraints, vibrational amplitudes and perpendicular vibrational corrections. The vibrational parameters were corrected for the large amplitude motion of the five-membered ring. The refinement for the trans-isomer confirms completely the predictions of the force field calculations. Although a stable solution could not be obtained for the cis-compound there is no contradiction between experiment and model calculations. The cyclohexane ring in both isomers is found to have a distorted chair conformation. In the cis-isomer it is flattened along the junction and more twisted in the other part. For the trans-compound the reverse is true. The following structural parameters r g, r α-structure) are put forward, (a) trans-compound: C 2-symmetry, r(C-C) av = 1.536 Å. Average bond angle and average torsion angle in the cyclohexane ring are 110.2° and 58.1°, respectively. The connection angle, defined as the angle between the planes bisecting C6-C1-C5-C9 and C2-C1-C5-C4, is 180°. (b) cis-compound: no symmetry, r(C-C) av = 1.536 Å. Average bond and torsion angles in the cyclohexane ring are 112.2° and 52.3°, respectively. The connection angle is 124.8°. A comparison is made with structures of related compounds.

Spectroscopic studies on the structure of poly(8‐bromoadenylic acid): Effect of glycosidic torsion angle on the conformation and flexibility in polyribonucleotides

AbstractThe helix–coil transition and conformational structure of poly(8‐bromoadenylic acid) [poly(8BrA)] have been investigated using 1H‐ and 13C‐nmr, CD, and ir spectroscopy. The results have been compared with the structure of the related 5′‐mono‐ and polynucleotides. The chemical shifts of H(2′), H(3′), C(2′), and C(3′) nmr signals show an interesting correlation with both the puckering of ribose ring and glycosidic bond torsion angle. Poly(8BrA) shows an upfield shift of the C(3′) signal and a downfield shift of the H(3′) signal compared to the chemical shifts in poly(A). These shifts are consistent with a C(3′) endo‐syn conformation for poly(8BrA). A similar effect has been reported previously and is also observed here on the C(2′) and H(2′) signals when the preferred conformation is C(2′)endo‐syn (e.g., in 5′‐8BrAMP). The chemical‐shift parameters thus act as a probe for studying syn ⇄ anti and N ⇄ S equilibria in solutions. The three‐bond 1H‐′13C coupling constants between H(1′) and C(8) and C(4) have been measured in poly(8BrA) and 5′‐8BrAMP and their structural implications have been discussed. The observed preference of a C(3′)endo‐syn conformation for poly(8BrA), coupled with other evidence, throws doubt on the validity of a correlation previously reported whereby a syn conformation is associated with a C(2′)endo ribose pucker. The backbone conformation of randomly coiled poly(8BrA) is very similar to the structures found in polyribonucleotides: poly(A) and poly(U). All three polymers show strong preferences for the backbone angles found in RNA helices. The CD spectrum of poly(8BrA) has a striking relationship to that of poly(A). The signs of all extrema are inverted, and the magnitudes are related by a constant factor. We suggest that these differences result from a change in the angle between coupled transition moment vectors in the two polymers. Infrared spectra of poly(8BrA) in H2O and D2O solution are reported for the frequency range below 1400 cm−1. The antisymmetric >PO stretching vibration is observed at an unusually low frequency in the helix (1214 cm−1). The symmetric >PO stretch occurs at ∼1095 cm−1 but is not resolved from a ring vibration near this frequency. A conformationally sensitive band, characteristic of helical RNA structures, is observed at 817 cm−1 and disappears when the helix is melted. This observation confirms the conclusion that ordered poly(8BrA) has a regular helical structure with an RNA backbone conformation. A stereochemical explanation is provided for the failure of poly(8BrA) (or other syn polymers) to form double helices with anti‐polyribonucleotides.

Synthesis, stereochemistry, and molecular and crystal structure of exo-3-phenyl-3-phosphabicyclo[3.2.1]oct-6-ene 3-oxide

The syntheses of both the title compound and the endo-3-phenyl isomer are described. The isomer assignments were made on the basis of n.m.r. studies and confirmed by a crystal-structure analysis of the exo-3-phenyl compound. The crystals of C13H15OP are triclinic, space group P, Z= 2, with unit cell dimensions a= 5.973(2), b= 10.496(2), c= 10.699(2)Å, α= 120.27(2), β= 104.12(2), and γ= 77.60(2)°. The structure was solved by the heavy-atom method from diffractometer data and refined to an R value of 0.030. The phosphorinan ring adopts a chair conformation which is substantially flattened at the phosphorus end. The ring conformation, bond lengths, and valence and torsional angles are compared with those in several bicyclic[3.2.1] and monocyclic systems reported in the literature.