Orientation Of Guest Research Articles

Influence of Dab2 and Pro3 configuration of [Leu]-enkephalins on the interactions with β-cyclodextrin studied by fluorescence spectroscopy, microcalorimetry and 1H NMR spectroscopy

Natural enkephalins and their analogues are very important as potential therapeutic agents (analgetics). Herein we describe the influence of Dab and Pro chirality of cyclic [Leu]enkephalins (X1-c[Dab2-Pro3-βNal(2)4-Leu5], where X = Tyr or Phe) on the binding constant with β-cyclodextrin and spatial and mutual orientation of guest and host molecules. The formation of complexes is enthalpy driven for all cyclic [Leu]enkephalins studied as well as for Nal and AcNalNH2. Moreover, change of Dab residue configuration has a greater influence on changes of the binding constant of cyclic enkephalin with β-CD than change of Pro chirality has. Also, the replacement of Tyr1 residue by Phe1 substantially changes the peptide chain conformation. An analysis of 2D NMR spectra reveals that, apart from inclusion complex formed by penetration of cyclodextrin cavity from wider and narrow rims by Nal, Tyr or Phe or Leu residue, a side and/or bottom association complexes are formed.

Density functional calculations on meloxicam–β-cyclodextrin inclusion complexes

The geometries of the cyclodextrin (CD) inclusion complexes with various tautomeric forms of meloxicam in gas phase were determined by DFT calculation (B3LYP/6-31G (d,p)). The interaction energies were estimated including basis set superposition error (BSSE) correction. Two orientations of the meloxicam guest were considered: the benzene ring located near the narrow rim and at the wider rim of the β-cyclodextrin, respectively. The calculations show that in all cases the molecules are located inside the CD cavity. The preferred complexation orientation is that one, in which the benzene ring of meloxicam is located near the wider rim with the secondary hydroxyl groups of the CD. The stabilization energies for the encapsulation of the meloxicam guest molecules show an overall affinity ranking for the meloxicam guest molecule in the following order: anionic (deprotonated) form > zwitterionic form ∼ enolic form > cationic (protonated) form. A comparison of the enolic and zwitterionic neutral forms shows, that the zwitterionic structure is better stabilized upon complexation due to the geometry of two extra hydrogen bonds between host and guest.

Study on dipolar orientation and relaxation characteristics of guest–host polymers affected by corona poling parameters

The dipolar orientation and relaxation characteristics of guest–host polymers poled by corona poling have been studied in detail. The mechanisms of dipolar orientation affected by poling parameters (voltage, temperature, time and cooling velocity) in polymers have been analyzed by UV–Vis absorption spectra and microscope. The results show that with increasing poling voltage, the orientation order parameter increases to maximum and then drops. Also the same trend of orientation order parameter has been obtained with increasing poling temperature. While the orientation order parameter increases to saturation with increasing poling time. With the biexponential model analyzing, it is shown that the relaxation of dipolar orientation can be slowed by slowing the cooling velocity during the poling process.

Dipolar guest orientation in polymer co-crystals and macroscopic films

The crystal structures of δ-clathrates of syndiotactic polystyrene (s-PS) with highly polar guest molecules (nitrobenzene, NB and p-nitroaniline, NA) have been defined, on the basis of X-ray diffraction and FTIR linear dichroism analyses. The s-PS/NB co-crystal presents a structure very similar to those of the δ-clathrates already described in the literature, i.e. monoclinic and with the guest molecular plane nearly perpendicular to the host helical axes. The s-PS/NA co-crystal presents a definitely different structure, i.e. triclinic and with guest molecular planes inclined with respect to the host helical axes. The definition of the co-crystalline structures has allowed to establish that the a⊥c// uniplanar orientation of the co-crystalline phase leads the guest dipoles roughly perpendicular to the film plane, i.e. to a dipole orientation suitable for poling processes. Films presenting a high degree of a⊥c// uniplanar orientation of the s-PS/NA co-crystalline phase have been obtained and characterized by X-ray diffraction measurements.

Guest Orientation in Uniplanar-Axial Polymer Host Films and in Co-Crystal Unit-Cell, Determined by Angular Distributions of Polarized Guest Fluorescence

Syndiotactic polystyrene (sPS) films with uniplanar-axial orientation of their co-crystalline phase with a fluorescent guest (naphthalene, NP) have been prepared and the orientation of their crystalline phase has been fully characterized by X-ray diffraction measurements. The prepared films have been examined in detail using Nishijima's method, where angular distribution of NP polarized fluorescence intensities was measured at each setting film angle by the rotation of films around the excitation light beam. This polarized fluorescence method turns out to be quite effective for monitoring the orientation of fluorescent guest molecules in the films. In particular, in semicrystalline films presenting uniplanar-axial orientation of their sPS/NP clathrate phase, NP molecules were found to exhibit a high three-dimensional orientational order all over the films. The experimental data have also allowed to determine the orientation of the NP guest molecule with respect the axes of the co-crystal unit-cell: in satisfactory agreement with molecular modeling results, angles formed by the short and the long axes of NP with the c axis are not far from 80° while the angle between the short axis of NP and the a axis is nearly 105°.

Surface and interlayer structure of vermiculite intercalated with methyl viologen

Molecular modeling using empirical force field revealed the differences between the surface and interlayer arrangement of the dye guest molecules in vermiculite intercalated with the divalent methyl viologen cation (MV(2+)). Conformation and anchoring of MV(2+) cations on the silicate layer in the interlayer space of vermiculite host structure is different from that on the crystal surface. A preferential position has been found for the anchoring of guests on the silicate layer. Anyway the arrangement of guests in the interlayer space as well as on the crystal surface exhibits a high degree of disorder due to a certain flexibility in guest molecules arrangement and first of all due to the presence of water molecules in the interlayer space. The presence of water disturbs not only the regularity in guest positions and orientations but also in conformation of guest molecules in the interlayer space of the host structure.

Influence of Leu5 configuration on the equilibrium constants of complexes of [Leu]-enkephalin with β-cyclodextrin studied by fluorescence spectroscopy, microcalorimetry and 1H NMR spectroscopy

Natural enkephalins and their analogues are very important as potential therapeutic agents (analgetics). In this paper we describe the influence of Leu chirality of cyclic [Leu]enkephalins on the binding constant with β-cyclodextrin and spatial and mutual orientation of guest and host molecules. The formation of complexes is enthalpy driven for both cyclic [Leu]enkephalins. Moreover, d-configuration of Leu residue causes an increase of the binding constant of cyclic enkephalin compared to l-analogue. An analysis of 2D NMR spectra reveals that, apart from inclusion complex formed by penetration of cyclodextrin cavity from wider and narrow rims by Trp or Leu residue, a side and/or bottom association complexes are formed.

Tossing and Turning: Guests in the Flexible Frameworks of Metal(III) Dicarboxylates

Single crystals of Ga(OH)(C(8)H(4)O(4)).0.74C(8)H(6)O(4) (2) and Ga(OH,F)(C(8)H(4)O(4)).0.74C(8)H(6)O(4) (3) were obtained under hydrothermal conditions. The structures of 2 and 3 have the same topological framework as the previously reported aluminum 1,4-benzenedicarboxylate (BDC), Al(OH)(C(8)H(4)O(4)).0.7C(8)H(6)O(4) (1). The frameworks are built by interconnecting M-OH-M chains (M = Al, Ga) with BDC anions to form large diamond-shaped one-dimensional channels filled with additional H(2)BDC guest molecules occupying disordered positions in the channels. Upon removal of H(2)BDC, other guest molecules such as H(2)O and pyridine can be inserted. In this work, we present a study of the intercalation of aromatic guests (BDC and pyridine) into frameworks of 1-3 by liquid and vapor diffusion into the empty channels of 1 and by single-crystal-to-single-crystal solvothermal guest exchange for 2 and 3. In the case of Al(OH)BDC and Ga(OH,F)BDC, two interconvertible, guest-concentration-dependent phases with different orientations of the pyridine guests have been observed, while only one pyridine orientation is found in Ga(OH)BDC.

How Organizational Standards and Coworker Support Improve Restaurant Service

A model examining the relationship between restaurant employees' reactions to their work environment and their jobs as service providers and guest satisfaction was tested among twenty-five restaurants from a casual dining restaurant chain. In the model, the relationship between guest service employees' work-related perceptions and attitudes are connected to guests' reported satisfaction. Results show that employees' perceptions of the presence of organizational standards for service delivery were strongly related to their perceptions of receiving adequate support from coworkers and supervisors to perform their jobs. Employees' perceived support from coworkers was significantly related to service providers' guest orientation (commitment to their guests), while perceived support from supervisors proved to be a weak influence on guest orientation. Ultimately, service providers' guest orientation was strongly related to guests' satisfaction with their service experience in the restaurant.

Origin and Model for Cooperative Host–Guest Interaction Affecting the Orientation of Guests in Oriented Nematic Hosts

It has been observed for the binary systems of oriented nematic hosts and nonmesomorphic guests (substituted anthraquinones) that the orientational orders of amino-substituted anthraquinones ascend with increasing guest concentrations. A new quantitative and experimental procedure has been proposed for obtaining the host–guest isotropic dispersion, isotropic electrostatic, anisotropic dispersion, and anisotropic electrostatic interaction energies. Among these four host–guest interaction energies, the host–guest isotropic dispersion interaction energy alone is demonstrated to be well correlated with the concentration dependence of the orientational order of the guest. A model is also proposed for the cooperative and isotropic host–guest interaction.

Molecular Recognition in Cyclodextrin Complexes of Amino Acid Derivatives: The Effects of Kinetic Energy on the Molecular Recognition of a Pseudopeptide in a Nonconstraining Host Environment as Revealed by a Temperature-Dependent Crystallographic Study

The crystal structure of a triclinic 2:2 inclusion complex of beta-cyclodextrin with N-acetyl-L-phenylalanine methyl ester has been determined at several temperatures between 298 and 20 K to further study molecular recognition using solid-state supramolecular beta-cyclodextrin complexes. The study reveals kinetic energy dependent changes in guest molecule conformations, orientations, and positions in the binding pocket presented by the crystal lattice. Accompanying these changes are observable differences in guest-guest interactions and hydrogen-bonding interactions in the binding pocket that involve guest molecules, water of hydration molecules, and beta-cyclodextrin molecules. On the basis of the differences observed in the crystal structures, we present a solid-state example of a system that displays the properties of both classical and quantum chemical models. At higher temperatures, the structure conforms to a classical mechanical model with dynamic disorder. At lower temperatures, the observations conform to examples in which there is static disorder representative of models in which quantum states differing in conformation, position, and orientation of components in the crystal structure are occupied. Ab initio theoretical calculations on the different guest molecule conformations have been carried out. Superpositions of theoretical electrostatic surface potential diagrams on the observed molecular positions in the complexes provide confidence that the deconvolution of the guest molecule disorder is acceptable. Temperature-dependent solid-state magic angle spinning deuteron NMR measurements provide evidence for large-amplitude, diffusive motion on a microsecond time scale in the complex.

A Clear-Cut Experimental Method to Discriminate between In-Plane and Out-of-Plane Molecular Transition Moments

A new method to orient organic molecules, based on their absorption as guest of the crystalline nanoporous delta phase of uniaxially stretched syndiotactic polystyrene films, is presented. This molecular alignment method not only allows high degrees of guest orientation to be attained but also orients the planar guest molecules with their smallest cross section nearly parallel to the stretching direction (B) rather than perpendicular (A, as usual for absorption in amorphous polymeric phases). As a consequence, in-plane and out-of-plane transition moment vectors maximize their absorption intensities for light polarization nearly perpendicular and parallel to the stretching direction, respectively. Hence, simple linear dichroism measurements by polarized spectra can allow an easy and clear-cut discrimination between in-plane and out-of-plane guest transition moment vectors.

Solid State NMR Method for the Determination of 3D Zeolite Framework/Sorbate Structures: 1H/29Si CP MAS NMR Study of the High-Loaded Form of p-Xylene in ZSM-5 and Determination of the Unknown Structure of the Low-Loaded Form

A general protocol is described for structure determinations of organic sorbate-zeolite complexes based on the selective, through-space, distance-dependent transfer of magnetization from protons in selectively deuterated organics to framework silicon nuclei. The method was developed using the known structure of the high-loaded ZSM-5/p-xylene complex containing p-xylene-d(6) or p-xylene-d(4). It was then applied to determine the unknown structure of the low-loaded ZSM-5/p-xylene complex using NMR alone. For the high-loaded complex improved data were obtained below 273 K, where slow motions and exchange processes of the p-xylene are eliminated. The general approach was validated by the exact agreement of the experimental (1)H-(29)Si CPMAS spectra obtained at a specific contact time and the complete 24-line spectra simulated using 1/T(CP) vs M(2) correlations from only the six clearly resolved resonances. For the low-loaded complex the (29)Si resonances were assigned at 267 K, and variable contact time CP experiments were carried out between 243 and 173 K using the same specifically deuterated p-xylenes. All possible locations and orientations of the p-xylene guests were sampled, and those solutions that gave acceptable linear 1/T(CP) vs M(2) correlations were selected. The optimum p-xylene location in this temperature range was determined to be in the channel intersection with the long molecular axis parallel to [0,1,0] (ring center fractional coordinates {-0.009, 0.250, 0.541}) with the ring plane oriented at an angle of 30 +/- 3 degrees about the crystallographic b axis. A subsequent single-crystal X-ray study confirmed this predicted structure.

Orientation and Microenvironment of Naphthalene Guest in the Host Nanoporous Phase of Syndiotactic Polystyrene

Syndiotactic polystyrene (s-PS) films including a fluorescent guest (naphthalene, NP) in their nanoporous δ crystalline phase have been prepared and characterized by X-ray diffraction, infrared linear dichroism, molecular modeling, and fluorescence depolarization techniques. A nearly perpendicular orientation of the fused rings of NP guest molecules with respect to the chain axis of the δ crystalline structure has been established by molecular mechanics and by evaluation of directions of transition moment vectors of infrared vibrational modes. The fluorescence depolarization is more efficient for NP guest molecules into the host phase than for NP molecules simply absorbed in the amorphous phase. This could be due to a more efficient resonance energy transfer between guest molecules, possibly associated with their ordered positioning and orientation into the host nanoporous polymeric crystallites.

Supramolecular recognition and structural elucidation of inclusion complexes of an achiral carbene precursor in beta- and permethylated beta-cyclodextrin.

[structure: see text] Inclusion of achiral carbene precursor endo-8-azibicyclo[3.2.1]octan-3-ol (1) in chiral beta-cyclodextrin (7-Cy) and tri-O-methyl-beta-cyclodextrin (TRIMEB) leads to 1:1 complexes 1@7-Cy and 1@TRIMEB, respectively. The combined methods of induced circular dichroism, NMR spectroscopy, and X-ray structure determination were employed for the first time for structural elucidation of the complexes in solution and the solid state. Significantly different orientations of 1 were observed. Compared with 1@7-Cy, 1@TRIMEB exhibits a different guest orientation and an association constant one-twentieth lower.

Factors influencing solvent adduct formation by calixarenes in the solid state.

Structural studies of seven very differently functionalised derivatives of calix[4]arene have been used to provide an analysis of the numerous factors which may influence solvent adduct formation by calixarenes. Evidence is presented that even where a solvent guest is included within the calixarene cavity, interactions solely within the cavity cannot be seen as the sole influences upon the guest position and orientation.

Chlorinated Guest Orientation and Mobility in Clathrate Structures Formed with Syndiotactic Polystyrene

Syndiotactic polystyrene films uniaxially stretched at different draw ratios, and including different crystalline (δ and γ) phases as well as clathrate phases with 1,2-dichloropropane, 1,2-dichloroethane, and 1-chloropropane, have been studied by combined infrared linear dichroism and X-ray diffraction measurements. Information relative to the orientation and mobility of trans and gauche conformers of these chlorinated guests in their clathrate structures with syndiotactic polystyrene has been achieved by comparison between experimentally evaluated and calculated vibrational transition moment directions of conformationally sensitive infrared peaks of the guest molecules.

Guest binding and orientation within open nanoscale hosts.

The synthesis of three different nanoscale molecular hosts is reported. These cavitands each possess a highly preorganized cavity with an open portal (nearly 1 nm wide), by which guests can enter and egress the cavity. Additionally, these hosts are deep-functionalized with a crown of weakly acidic benzal C-H groups which can form a variety of noncovalent interactions with guest molecules residing within the cavity. Thirty-one guests were examined for their propensity to form complexes with the hosts. Guests that possess halogen atoms were the strongest binders, suggesting the formation of polydentate C-H triplebond X-R hydrogen bonds with the deep crown of benzal hydrogens. Exchange rates between the free and bound states were noted to be dependent on the size of the guest and the solvent used to study complexation. In general, stronger binding and slower exchange were noted for complexations carried out in DMSO with highly complementary guests. The orientation of each guest within the cavity was determined using either EXSY NMR spectroscopy or (1)H NMR shift data. Cumulatively these results showed that the principal factors directing orientation were interactions with the benzal groups and the type of solvent. Van't Hoff analyses of selected complexations were also carried out. As well as revealing that all complexations were entropically unfavorable, these experiments provided support for guest orientation determinations, and gave an estimation that the formation of a C-H triplebond I-R hydrogen bond releases between 1 and 1.5 kcal mol(-1).

A reexamination of the low-temperature crystal structure of the p-tert-butylcalix[4]arene-toluene inclusion compound. Differences in spatial averaging with Cu and Mo Kalpha radiation.

The low-temperature crystal structure of the p-tert-butylcalix[4]arene-toluene inclusion compound, C(44)H(56)O(4) x C(7)H(8), exhibits significant guest-induced distortion of the host calixarene molecule. This distortion persists long enough to establish a correlation in the orientation of guests in adjacent host molecules. Remarkably, the space group and unit cell that best describe the structure appear to depend on the wavelength of the incident radiation. This behaviour appears to arise from spatial averaging over the different scattering volumes required to establish the diffraction peaks.

Adjusting the binding thermodynamics, kinetics, and orientation of guests within large synthetic hydrophobic pockets.

Kinetic analysis of the host guest complexation of a large, open molecular basket and a highly complementary adamantoid guest reveals that for these types of systems a dissociative mechanism is in operation. Hence, the resident adamantyl guest must completely vacate the cavity before another guest molecule can move in to replace it. As a result of the rigid nature of the host, the energy barrier to this process is relatively high, about 16 kcal mol(-1) at room temperature. Modifying the cavity of the host by dangling either a methyl group or a hydroxyl group from the portal rim alters the thermodynamic binding profile of these hosts. (1)H NMR shift data analysis also reveals that these functional groups can adjust the orientation that monosubstituted guests adopt within the cavity. Additionally, (1)H NMR studies of the binding of (E)1,4-dibromoadamantane allow the observation of two energetically similar diastereomeric complexes. An examination of this guest binding to the three hosts reveals that the interchange between the isomers is much faster than the entry and egression rates, and that the functional groups at the rim of each cavity influence both the rates of reorientation and the equilibrium relating the isomers.