Showing papers on "Conformational isomerism published in 1993"

An efficient algorithm for searching low-energy conformers of cyclic and acyclic molecules

Abstract: A set of strategies for exhaustively finding low-energy conformers of cyclic, acyclic or alicyclic molecules is presented. Starting from any conformation, local perturbation is systematically applied to all the flexible portions of the molecule in question to produce candidates of new conformation. The perturbations consist of flapping and/or flipping for endocyclic bonds and stepwise rotation for acyclic bonds. The conformations they produced are believed to lie close to the initial geometry in the conformational space. The global energy minimum (GEM) structure of the starting domain of conformational space can be quickly reached by always choosing the most stable of the conformers produced in the last perturbation cycle as the next initial structure. Once GEM of the domain is reached, the local perturbations direct the search gradually to higher and higher energy regions while exhaustively finding all the low-energy conformers therein. The variable search-limit strategy allows one to use unstable conformers as the initial structure for perturbation to ensure the exhaustiveness of the search in the low-energy region. By further increasing the search-limit, new domains of conformational space may be found. A program CONFLEX3 containing several additional strategies for improved performance has been tested for n-alkanes up to decane and cycloalkanes up to cyclododecane.

Coexisting stable conformations of gaseous protein ions.

Abstract: For further insight into the role of solvent in protein conformer stabilization, the structural and dynamic properties of protein ions in vacuo have been probed by hydrogen-deuterium exchange in a Fourier-transform mass spectrometer. Multiply charged ions generated by electrospray ionization of five proteins show exchange reactions with 2H2O at 10(-7) torr (1 torr = 133.3 Pa) exhibiting pseudo-first-order kinetics. Gas-phase compactness of the S-S cross-linked RNase A relative to denatured S-derivatized RNase A is indicated by exchange of 35 and 135 hydrogen atoms, respectively. For pure cytochrome c ions, the existence of at least three distinct gaseous conformers is indicated by the substantially different values--52, 113, and 74--of reactive H atoms; the observation of these same values for ions of a number--2, 7, and 5, respectively--of different charge states indicates conformational insensitivity to coulombic forces. For each of these conformers, the compactness in vacuo indicated by these values corresponds directly to that of a known conformer structure in the solution from which the conformer ions are produced by electrospray. S-derivatized RNase A ions also exist as at least two gaseous conformers exchanging 50-140 H atoms. Gaseous conformer ions are isometrically stable for hours; removal of solvent greatly increases conformational rigidity. More specific ion-molecule reactions could provide further details of conformer structures.

Hydrogen/Deuterium Exchange Electrospray Ionization Mass Spectrometry: A Method for Probing Protein Conformational Changes in Solution

Abstract: Hydrogen/deuterium exchange electrospray ionization mass spectrometry is demonstrated to be effective in probing changes in the conformation of proteins in solution. The method is based on the mass spectrometric measurement of the extent of hydrogen/deuterium exchange that occurs in different protein conformers over defined periods of time. Results are presented in which the method is used to probe conformational changes in (a) bovine ubiquitin induced by the addition of methanol to aqueous acidic solutions and (b) chicken egg lysozyme caused by the reduction of the intramolecular disulfide cross-linkages.

Quantum Chemical Conformational Analysis of Glucose in Aqueous Solution

Abstract: We have calculated the aqueous solvation effects on the anomeric and conformational equilibria of D-glucopyranose using a quantum chemical solvation model based on a continuum treatment of dielectric polarization and solvent accessible surface area. The solvation model puts the relative ordering of the hydroxymethyl conformers in linewith the experimentally determined ordering of populations. Our calculations indicatethat the anomeric equilibrium is controlled primarily by effects that are also present in the gas-phase potential energy function, that the gauche/trans O-C(6)-C(5)-O hydroxymethyl conformational equilibrium is dominated by favorable solute-solvent hydrogen bonding interactions, and that other rotameric equilibria are controlled mainly by dielectric polarization of the solvent

Disulfide bond isomerization in BPTI and BPTI(G36S): an NMR study of correlated mobility in proteins.

Abstract: Two conformational isomers were observed in the 1H nuclear magnetic resonance (NMR) spectra of the basic pancreatic trypsin inhibitor (BPTI) and of a mutant protein with Gly 36 replaced by Ser, BPTI(G36S). The less abundant isomer differs from the major conformation by different chirality of the Cys 14-Cys 38 disulfide bond. In BPTI, the population of the minor conformer increases from about 1.5% at 4 degrees C to 8% at 68 degrees C. In BPTI(G36S), the population of the minor conformation is about 15% of the total protein, so that a detailed structural study was technically feasible; a trend toward increasing population of the minor conformer at higher temperatures was observed also for this mutant protein. The activation parameters for the exchange between the two conformations were measured in the temperature range 4-68 degrees C, using uniformly 15N-enriched protein samples. Below room temperature the exchange rate of the disulfide flip follows an Arrhenius-type temperature dependence, with negative activation entropy in both proteins. At higher temperatures the exchange rates are governed by a different set of activation parameters, which are similar to those for the ring flips of Tyr 35 about the C beta-C gamma bond. Although the equilibrium enthalpy and entropy were found to be largely temperature independent, the activation entropy changes sign and is positive at higher temperatures. These results suggest that, above room temperature, the disulfide flips are coupled to the same protein structure fluctuations as the ring flips of Tyr 35.

Syntheses and optical resolution of calix[4]arenes with molecular asymmetry. Systematic classification of all possible chiral isomers derivable from calix[4]arene

Abstract: All possible chiral isomers that can be derived from calix[4]arene by modification of the OH groups were systematically classified. Molecular asymmetry can be generated not only by different substituents but also by conformational isomerism. The numbers of chiral isomers are 24 for tetra-O-substituted calix[4]arenes, 10 for tri-O-substituted calix[4]arenes, 3 for di-O-substituted calix[4]arenes, and 0 for mono-O-substituted calix[4]arenes. This implies that calix[4]arene is a useful building block for the design of novel asymmetric ring structures. In order to demonstrate asymmetry in these chiral calix[4]arenes, we synthesized several tetra-, tri-, and d-O-substituted calix[4]arenes by using the metal template method, the stepwise synthesis method, the protection-deprotection method, etc., which were developed in efforts directed toward regioselectivite O-alkylation and control of conformational isomerism in calix[4]arenes. Finally, were succeded in complete optical resolution of chiral calix[4]arenes by an HPLC method using a chiral-packed column or by the formation of diastereoisomers with the (-) menthoxyacetyl group. This article thus contains all of the molecular design concepts, syntheses, and optical resolutions of chiral calix[4]arenes

Synthesis and conformational properties of calix[6]arenes bridged on the lower rim: self-anchored rotaxanes

Abstract: The 1,4-di-p-tolyl ether of p-tert-butylcalix[6]arene can be converted to transannularly bridged esters by treatment with diacid chlorides ranging from succinoyl to suberoyl and including terephthaloyl. The parent compounds p-tert-butylcalx[6]arene and p-H-calix[6]arene can also be transannularly bridged by treatment with bis-halomethylaryl compounds by using KOSiMe 3 as the base. Methylation of the resulting calix[6]arenes containing 1,4-CH 2 ArCH 2 bridging moieties produces the corresponding tetramethyl ethers. When Ar is anthrylene the conformation of the starting material is retained, but when Ar is phenylene the conformation changes; when Ar is duryl a mixture of conformers is obtained. 1 H NMR analysis and molecular modeling studies suggest that, when Ar is phenyl or durylene, the system is capable of undergoing a conformational transformation in which the bridging moiety becomes threaded through the annulus of the macrocyclic ring to produce a self-anchored rotaxane. Employing a new scheme for denotingf calixarene conformations, this is designed as a (u,u,u,u,u,u) to (u,u,d,d,d,u) transformation. The apparent driving force for the conformational change is the greater spatial sepearation of the OMe groups in the (u,u,d,d,d,u) conformation as compared with the (u,u,u,u,u,u) conformation, resulting in a more favorable electrostatic energy contribution to the total energy of the former

Synthesis and Ion Selectivity of Conformers Derived from Hexahomotrioxacalix[3]arene

Abstract: The reaction of 7,15,23-tri-tert-butyl-25,26,27-trihydroxy-2,3,10,11,18,19-hexahomo-3,11,19- trioxacalix-[3]arene (1H 3 ) with ethyl bromoacetate yielded the cone conformer (cone-1Es 3 ) as a minor product and the partial-cone conformer (partial-cone-1Es 3 ) as a major product. Interconversion between these two conformers did not take place, indicating that the OCH 2 CO 2 Et group is bulky enough to inhibit the oxygen-through-the-annulus rotation. The conformer distribution was affected by the metal cation present in the base, suggesting the importance of the metal template effect. When NaH was used in THF; the partial-cone/cone ratio was 1.6-3.8; when Cs 2 CO 3 was used in acetone, partial-cone-1Es 3 was produced quantitatively. Two-phase solvent extraction established that cone-1Es 3 shows Na + selectivity, whereas partial-cone-1Es 3 shows K + selectivity. Cone-1Es 3 showed high affinity toward RNH 3 + cations, indicating that the structure of the structure of cone-1Es 3 with C 3 symmetry is very suitable for the binding of RNH 3 + cations with C 3 symmetry

Comprehensive conformational analysis of the four- to twelve-membered ring cycloalkanes: identification of the complete set of interconversion pathways on the MM2 potential energy hypersurface

Abstract: Comprehensive conformational analysis of the 4- to 12-membered ring hydrocarbon series cyclobutane through cyclododecane has been carried out. The complete set of minima and saddle points has been located on the MM2 potential energy hypersurface using systematic and Monte Carlo conformational search techniques associated with a combination of Newton-type energy minimization methods. The complete network of conformational interconversions has also been identified.It has been found that the different modes of conformational interconversion associated with the lowest barriers between minimum energy conformers display fascinating patterns of well-coordinateds atomic movement

Orientation of the saccharide chains of glycolipids at the membrane surface : conformational analysis of the glucose-ceramide and the glucose-glyceride linkages using molecular mechanics (MM3)

Abstract: Preferred conformations of the saccharide-ceramide linkage of glucosylceramides with different ceramide structures (normal and hydroxy fatty acids) were investigated by molecular mechanics (MM3) calculations and compared with conformational features obtained for glucosylglycerolipids (diacyl and dialkyl analogues). Relaxed energy map calculations with MM3 were performed for the three bonds (C1'-O1-C1-C2, torsion angles phi, psi, and theta 1) of the glucose-ceramide/diglyceride linkage at different values of the dielectric constant. For the phi torsion of the glycosidic C1'-O1 bond the calculations show a strict preference for the +sc range whereas the psi/theta 1 energy surface is dependent on the structure of the lipid moiety as well as on the dielectric constant (epsilon). Calculations performed on glucosylceramide with normal and hydroxy fatty acids at epsilon = 4 (bilayer subsurface conditions) show three dominating conformers (psi/theta 1 = ap/-sc, -sc/ap, and ap/ap). The ap/-sc conformer, which represents the global energy minimum, is stabilized by polar interactions involving the amide group. The +sc rotamer of theta 1 is unfavored in sphingolipids due to a Hassel-Ottar effect involving the sphingosine O3 and O1 oxygen atoms. Comparative calculations on glycosylglycerolipid analogues (ester and ether derivatives) show a distinct preference for the ap rotamer of theta 1. An evaluation of the steric hindrance imposed by the surrounding membrane surface shows that in a bilayer arrangement the range of possible conformations for the saccharide-lipid linkage is considerably reduced. The significance of preferred conformations of the saccharide-ceramide linkage for the presentation and recognition of the saccharide chains of glycosphingolipids at the membrane surface is discussed.

High resolution S1←S0 fluorescence excitation spectra of hydroquinone. Distinguishing the cis and trans rotamers by their nuclear spin statistical weights

Abstract: Hydroquinone (HYQ) has two rotational isomers, differing in the orientation of the two O–H bonds with respect to the benzene ring. In the S1←S0 fluorescence excitation spectrum of HYQ, two electronic origins, one for each rotamer, are present with a separation of 34.7 cm−1. Rotationally resolved spectra of both origins have been obtained and analyzed. This analysis reveals that the rovibronic lines in the two origins exhibit different alternating intensity patterns due to nuclear spin statistical weights, thereby providing a means for distinguishing the two rotamers. On this basis, we assign the origin at 33 500 cm−1 to the trans, C2h rotamer and the origin at 33 535 cm−1 to the cis, C2v rotamer of HYQ. Coordinate determinations of the hydroxy–hydrogen atoms of both rotamers, in both electronic states confirm this assignment.

Stereochemistry. 82. Conformation of fused five-membered heterocyclic rings derived from the intramolecular oxime olefin cycloaddition reaction

Abstract: Thermal intramolecular oxime olefin cycloaddition of α-allylamino aldoximes and ketoximes 4 led stereospecifically to formation of oxadiazabicyclo[3.3.0]octanes 6. The presence of heteroatoms in these bicyclic fused 5-membered rings permits for the first time an evaluation of the conformation of this system by means of NMR. We found that some substituents in 6 restrict the conformational mobility of these five-membered rings to the extent that only one conformer was detected at 20 o C. In other cases an equilibrium between two major conformers was revealed by NMR. Equilibrium measurements indicated a free energy of conversion of 13.2-13.4 kcal/mol, apparently a manifestation of the N-inversion in the isoxazolidine ring. NMR studies also showed that the NH-proton in isoxazolidines 6 prefers an axial orientation. Empirical force field data were adjusted for MM calculations in these bicyclic heterocycles. The MM2 force field with AMBER charge gave the best fit between calculated and experimental coupling constants

Planar-nonplanar conformational equilibrium in metal derivatives of octaethylporphyrin and meso-nitrooctaethylporphyrin

Abstract: The planar and nonplanar conformers of metal derivatives of 2,3,7,8,12,13,17,18-octaethylporphyrin (OEP) and 5-nitro-2,3,7,8,12,13,17,18-octaethylporphyrin (NO[sub 2]-OEP) are investigated using resonance Raman spectroscopy. The structural heterogeneity is assessed by analysis of the line shapes of the structure-sensitive Raman lines. First, heterogeneity in the conformation of the macrocycle has been detected in solutions of the nickel and cobalt derivatives of OEP, that is, both planar and nonplanar conformers are found to coexist at room temperature for these metal porphyrins but not for the Cu and Zn derivatives. The latter metals expand the porphyrin core, shifting the equilibrium entirely to the planar conformer. Second, we find that substitution with a single NO[sub 2] group at one of the methine-bridge carbons shifts this planar-nonplanar equilibrium substantially toward the nonplanar conformer. Thus, both crowding of the peripheral substituents and contracting of the porphyrin core (Ni(II) < Co(II) < Cu(II) < Zn(II)) displace the equilibrium toward the nonplanar conformer. Finally, the frequencies of several Raman lines correlate with structural parameters such as core size (obtained either from molecular mechanics calculations or from X-ray crystallographic studies). 30 refs., 6 figs., 3 tabs.

Sterically Congested Phosphite Ligands: Synthesis, crystallographic characterization, and observation of unprecedented eight-Bond 31P,31P coupling in the 31P-NMR spectra

Abstract: The synthesis and characterization of 2-{1-{3,5-bis(1,1-dimethylethyl)-2-{[2,4,8,10-tetrakis(1,1-dimethylethyl)dibenzo[d,f][1,3,2]dioxaphosphepin-6-yl]oxy}phenyl}ethyl}-4,6-bis(1,1-dimethylethyl)phenyl diphenyl phosphite (6) is described. In the 31P-NMR spectrum (1H-decoupled) of 6, an unprecedented eight-bond P,P coupling of J = 72.8 Hz is observed. In the X-ray crystal structure of 6, an intramolecular P–P distance of 3.67 A is found, which is within the sum of the van-der-Waals radii of the P-atoms. The observed intramolecular P–P distance suggests that a through-space coupling mechanism is operative. The solid-state conformation of 6 is compared to the conformation obtained by semi-empirical MO geometry optimizations (PM3 method). The calculated geometry suggests that the solid-state structure is near a true energy minimum, but that crystal-packing forces decrease the intramolecular P–P distance in the solid state. In the absence of crystal-packing forces, however, the collisional and vibrational energy available in solution may lead to the population of states with a shortened intramolecular P–P distance in 6. The proximity of the P-atoms in 6 is due to restricted conformational freedom resulting from steric congestion within the molecule. The free energy of activation (ΔG* = 10.2 and 10.8 kcal/mol for unequal populations of exchanging conformers) for ring inversion of the dibenzo[d,f][1,3,2]dioxaphosphepin ring in 6 is determined by variable-temperature 31P-NMR spectroscopy. Semi-empirical MO calculation on model compounds suggest that the structure of the transition state for ring inversion has the two aryl rings and O-atoms in a common plane, with the P-atom lying above this plane.

Warum Pentose‐ und nicht Hexose‐Nucleinsäuren??. Teil VI. ‘Homo‐DNS’: 1H‐, 13C‐, 31P‐ und 15N‐NMR‐spektroskopische Untersuchung von ddGlc(A‐A‐A‐A‐A‐T‐T‐T‐T‐T) in wässriger Lösung

Abstract: Why Pentose- and Not Hexose-Nucleid Acids? Part IV. ‘Homo-DNA’: 1H-, 13C-, 31P-, and 15N-NMR-Spectroscopic Investigation of ddGlc(A-A-A-A-A-T-T-T-T-T) in Aqueous Solution From a comprehensive NMR structure analysis, it is concluded that the ‘homo-DNA’ oligonucleotide ddGlc(A-A-A-A-A-T-T-T-T-T) in 3 mM D2O solution (100 mM NaCl, 50 mM phosphate buffer, pH 7.0, T = 50°) forms a duplex of C2-symmetry, with its self-complementary oligonucleotide strands in antiparallel orientation. The 2′,3′-dideoxy-β-D-glucopyranosyl rings are in their most stable chair conformation, with all three substituents equatorial and with the adenine as well as the thymine bases in the anti-conformation. The base pairing is of the Watson-Crick type; this pairing mode (as opposed to the reverse-Hoogsteen mode) was deduced from the observation of inter strand NOEs between the adenine protons HC(2) and the pyranose protons Hα–C(2′) of the sequentially succeeding thymidine nucleotides of the opposite strand, a correlation which discriminates between the Watson-Crick and the reverse-Hoogsteen pairing mode. The NOEs of the NH protons with either the adenine protons HC(2) or HC(8), that are normally used to identify the pairing mode in DNA duplexes, cannot be observed here, because the NH signals are very broad. This line broadening is primarily due to the fact that the exchange of the imino protons with the solvent is faster than for corresponding DNA duplexes. Computer-assisted modeling of the [ddGlc(A5-T5)]2 duplex with the program CONFOR [23], using the linear (idealized) homo-DNA single-strand conformation (α = −60°, β = 180°, γ = 60°, δ = 60°, ϵ = 180°, ζ = −60°, see [1] [3]) as the starting structure, resulted in two duplex models A and B (see Figs. 27–32, Scheme 9, and Table 4) which both contain quasi-linear double strands with the base-pairing axis inclined relative to the strand axes by ca. 60° and 45°, respectively, and with base-pair stacking distances of ca. 4.5 A. While neither of the two models, taken separately, can satisfy all of the NMR constraints, the NMR data can be rationalized by the assumption that the observed duplex structure represents a dynamic equilibrium among conformers which relate to models A and B as their limiting structure. The required rapid equilibrium appears feasible, since the models A and B are interconvertible by two complementary 120° counter rotations around the α-axis and the γ-axis, respectively, of the phosphodiester backbone. The models A and B correspond to the two types of linear (idealized) single-strand backbone conformation derived previously by qualitative conformational analysis without and with allowance for gauche-trans-phosphodiester conformations, respectively [1] [3]. Refinement of the models A and B with the use of the program AMBER [27] by energy minimization in a water bath and molecular-dynamics simulations (2 ps, 300° K) resulted in two dynamic structures (Figs. 33 and 34, Table 4). These have roughly the same energy, closely resemble the starting structures A and B, and satisfy - as an ensemble - all of the NMR constraints without violating any van der Waals distances by more than 0.2 A. Extensive fluctuations in base-pair distance and deviations from base-pair coplanarity, as well as the presence of water molecules in the cavities between some of the base pairs, were observed in both dynamic structures A and B, which, on the other hand, did not mutually interconvert within the short simulation time period used. These model properties, together with the conjectured equilibrium between the two structure types A and B, lead to the hypothesis of a homo-DNA duplex containing a ‘partially molten’ pairing core. This proposal could qualitatively account for a high rate of the NH exchange, as well as for part of the previously established [3] deficits in both enthalpic stabilization and entropic destabilization of homo-DNA duplexes relative to corresponding DNA duplexes. The phenomenon of the higher overall stability of homo-DNA duplexes vs. DNA duplexes (e.g, [ddGlc(A5-T5)]2, Tm = 59° vs. [d(A5-T5)]2, Tm = 33°, both at c ≈ 50 μM [3]) can then be seen as the result not only of a higher degree of conformational preorganization of the homo-DNA single strand toward the conformation of the duplex backbone [1] [3], but also of the entropic benefit of greater disorder in the central pairing zone of the homo-DNA duplex. This view of the structure of a homo-DNA duplex relates its characteristic properties to a central structural feature: the average base-pair distance in the models of homo-DNA is too large for regular base stacking (ca. 4.5 A vs. ca. 3.5 A in DNA). This difference in the distances between adjacent base pairs is a direct consequence of the quasi-linearity of the homo-DNA double strand as opposed to the right-handed twist of the helical DNA duplexes [1] [3], which is directly related to the specific conformational properties of pyranose rings as opposed to furanose rings [1]. Thus, the structural hypothesis derived from the NMR analysis of [ddGlc(A5-T5)]2 relates the conformational differences between homo-DNA and DNA directly to the sugar ring size, which is the essential constitutional difference between the two types of structure. The English footnotes to Figs. 1–34, Schemes 1–9, and Tables 1–4 provide an extension of this summary.

Conformational properties of monosubstituted cyclohexanes in their thiourea inclusion compounds and in solution: variable-temperature one-dimensional and two-dimensional carbon-13 NMR investigations

Abstract: Temperature-dependent changes in high-resolution solid state C-13 NMR spectra of thiourea inclusion compounds containing monosubstituted cyclohexane guest moleculeS (C6H11X: X = CH3, NH2, OH, Cl, Br, I) have demonstrated that a ''chair-chair'' ring inversion process occurs for these guest molecules inside the thiourea tunnel, and the energy barriers for this process have been determined. The relative populations of the axial and equatorial conformers of the guest molecules have been determined directly from the NMR spectra recorded (at low temperature) under slow exchange conditions. On this basis, the guest molecules studied can be divided into two classes: those with X = Cl, Br, and I have a predominance of the axial conformer (relative populations of equatorial conformer almost-equal-to 0.05-0.15), whereas those with X = CH3, NH2, and OH have a predominance of the equatorial conformer (relative populations of equatorial conformer almost-equal-to 0.82-0.97). In addition, conformational properties of these monosubstituted cyclohexanes in solution have been reinvestigated via variable-temperature C-13 NMR measurements. It is shown, from the reported results for methylcyclohexane, that two-dimensional exchange spectroscopy can successfully identify minor conformers with relative populations as low as 0.01 in solution and 0.03 in the solid state.

Theoretical studies of the 2- and 4-hydroxybenzoic acid with competing hydrogen bonds in the gas phase and aqueous solution

Abstract: MP2/6-31G * //6-31G * ab initio quantum chemical calculations in the gas phase and Monte Carlo simulations for isothermal-isobaric (NPT) ensembles in aqueous solution have been carried out for 2- and 4-hydroxybenzoic acid. After total geometry optimization of eight planar conformers for the 2-OH derivative and four conformers for the 4-OH derivative in the gas phase, the structure with the 2-OH phenolic hydroxyl with an intramolecular hydrogen bond to the carbonyl oxygen was found to be the most stable conformer. Normal frequency analyses were carried out using the 6-31G * optimized geometries, and thermal corrections were obtained at 298 K and 1 atm

Structures of two conformational polymorphs of the cholesterol-lowering drug probucol

Abstract: Two polymorphic forms of the drug probucol, (4,4′-[(1-Methylethylidene)-bis(thio)]bis-[2,6-bis (1, 1-dimethylethyl)phenol]), have been isolated and characterized by thermal analysis, X-ray powder diffraction and single crystal X-ray analyses. Form I, with onset melting point 125°C, is monoclinic, space groupP21 c witha=16.972(5),b=10.534(4),c=19.03(1)A,β=113.66(3)°,Z=4. Form II, with onset melting temperature 116°C, is monoclinic, space groupP21/n witha=11.226(2),b=15.981(2),c=18.800(3)A,β=104.04(1)°,Z=4. The probucol molecule adopts different conformations in the two polymorphs. In Form II, the C-S-C-S-C chain is extended and the molecular symmetry approximates C2v whereas in Form I, the two S-C-S-C torsion angles are approximately 80° and 165°. Molecular mechanics calculations show that the less symmetrical conformer of Form I is more stable than the conformer in Form II by approximately 26 kJ mol−1. Crystal packing in both polymorphs is determined by van der Waals interactions only. X-ray powder diffraction indicates that Form II converts to Form I on grinding.

Transduction of selective recognition by preorganized ionophores : K+ selectivity of the different 1,3-diethoxycalix[4]arene crown ether conformers

Abstract: Three different conformers of 1,3-diethoxy-p-tert-butylcalix[4]arene crown ethers have been used to study the effect of the ionophore preorganization on the potentiometric K+-selectivity. Selectivities were measured for chemically modified field effect transistors (CHEMFETs) and membrane ion-selective electrodes (ISEs) by two different methods. The ionophores show decreasing K+/Na+ selectivities in the order: partial cone > 1.3-alternate > cone. As a function of time the cone conformer maintained a constant selectivity whereas a continuous decrease of the selectivity values was observed for the other two conformers. This supports our prediction based on the association constants and our previous 1H NMR studies on conformational stability.

Tautomerism and stereochemistry of hypericin: Force field, NMR, and X-ray crystallographic investigations

Abstract: Stereochemistry and tautomerism of hypericin, pseudohypericin, and several of their partial structure models were investigated using an MM2 derived force field method. Besides the “propeller” type conformer, which was also found by the X-ray crystallographic study, the complicated energy hypersurface was shown to contain a novel “double-butterfly” conformer of similar stability. The upper limit interconversion barrier between these conformers and their enantiomers was found to be in the order of 115 kJ/mol.1H-NMR experiments suggested a lower limit interconversion barrier of at least 80 kJ/mol. From the ten tautomers possible in principle, the 7,14-species was derived to be the most stable one by at least 48 kJ/mol.

Liquid-crystalline polyethers based on conformational isomerism. 32. Effect of molecular weight on the phase behavior of linear and macrocyclic oligoethers and of linear polyethers based on 1-(4-hydroxy-4'-biphenylyl)-2-(4-hydroxyphenyl)butane and 1,10-dibromodecane

Abstract: The synthesis and phase behavior of the individual monomer, dimer, trimer, and tetramer, and of the polyethers of 1-(4-hydrory-4'-biphenylyl)-2-(4-hydroryphenyl)butane (TPB) with 1,10-dibromodecane (TPB-10) with Mn= 5200 to 57 200, Mm/Mn= 1.16 to 1.47 and with well-defined chain ends are described. The influence of various chain ends on the mesomorphic behavior of monomer and dimer was also discussed. There is a continuous increase of the nematic-isotropic (T ni ) and glass transition (T g ) temperatures over the entire range of molecular weights