Porphyrins with multiple crown ether voids: novel systems for cation complexation studies

Porphyrins appended with crown ether, benzo-15-crown-5, at the methine positions have been synthesized and characterized. The fully and partially substituted porphyrins and their metallo (Co, Cu, and Zn) derivatives describe one or more ether cavities in the periphery that are capable of recognizing spherical cations. The ability of these macrocycles to complex cations (Na+, K+, Mg2+, Ca2+, Ba2', and NH4+) is investigated by use of visible, 'H NMR, ESR, and emission spectral studies. The tetrasubstituted crown porphyrin (TCP) exhibits very high selectivity for K+. The cations (K', Ba2+, and NH4+) that require two crown ether cavities for complexation promote dimerization of the porphyrins. The ESR study of the cation-induced porphyrin dimers reveals axial symmetry with the porphyrin planes separated by -4.2 A. This distance increases from the fully substituted to partially substituted porphyrins. The cations (K', Ba2+, and NH4') quench efficiently the fluorescence of the free base porphyrins and their metallo derivatives. The quenching process is attributed to the steric geometry of the dimers.

Relative Stability of closo-closo, closo-nido, and nido-nido Macropolyhedral Boranes: The Role of Orbital Compatibility

The concept of orbital compatibility is used to explain the relative energies of different macropolyhedral structural patterns such as closo-closo, closo-nido, and nido-nido. A large polyhedral borane condenses preferentially with a smaller polyhedron owing to orbital compatibility. Calculations carried out at the B3LYP/6-31G* level show that the macropolyhedron closo(12)-closo(6) is the most preferred structural pattern among the face-sharing closo-closo systems. The relative stabilities of four-shared-atom closo-closo, three-shared-atom closo-closo, three-shared-atom closo-nido, edge-sharing closo-nido, and edge-sharing nido-nido structures are in accordance with the difference in the number of vertices of the individual polyhedra of the macropolyhedra. When the difference in the number of vertices of the individual polyhedra is large, the stability of the macropolyhedra is also large. Calculations further show that the orbital compatibility plays an important role in deciding the stability of the macropolyhedral boranes with more than two polyhedral units. The dependence of the orbital compatibility on the relative stability of the macropolyhedron varies with other factors such as inherent stability of the individual polyhedron and steric factors.

Titanium Complexes for the Polymerisation of Ethene and Caprolactone

The literature part of the thesis mainly reviews the results of the use of titanium catalysts for ethene and caprolactone polymerisation. The behaviour of titanium catalysts bearing phenoxy-imino ligands has been the focus of more detailed investigations in ethene polymerisation. Reasons for the production of multimodal polyethene for a range of catalysts are also given. The experimental part of the thesis is divided into two sections based on the monomers used in the polymerisations: Part A (ethene) and part B (caprolactone). Part A: Titanium(IV) complexes bearing phenoxy-imino ligands are known to possess high ethene polymerisation activities after MAO activation. Depending on the ligand, the activities of the catalysts in polymerisation can vary between 1 and 44000 kgPE/(mol*cat*h*bar). Depending on the polymerisation temperature and the electronic and steric properties of the catalyst ligands, low to high molar mass values and uni- and multimodal polydispersity values can been observed. In order to discover the reasons for these differences, 22 titanium(IV) complexes containing differently substituted phenoxy-imino derivatives as di- and tetradentate ligands were synthesised with high yields and used as homogeneous catalysts in ethene polymerisations. Computational methods were used to predict the geometry of the synthesised complexes and their configuration after activation. Based on the results obtained, the geometry of the catalyst together with the ligand substituents seem to play a major role in defining the catalytic activity. Novel titanium(IV) complexes bearing malonate ligands were also synthesised. Malonates are considered to be suitable ligand pre-cursors since they can be produced by the simple reaction of any primary or secondary alcohol with malonylchloride, and thus they are easily modifiable. After treatment with MAO these complexes had polymerisation activities between 10 and 50 kgPE/(mol*cat*h*bar) and surprisingly low polydispersity values when compared with similar types of catalysts bearing the O?O chelate ligand. Part B: One of the synthesis routes in the preparation of the above mentioned phenoxy-imino titanium dichloride complexes involved the use of Ti(NMe2)4 with a range of salicylaldimine type compounds. On reaction, these two compounds formed an intermediate product selectively and quantitatively which was active in the ring-opening polymerisation of caprolactone. Several mono-anionic alcoholates were also combined with Ti(NMe2)4 in different molar ratios and used as catalysts. Full conversion of the monomer was achieved within 15 minutes with catalysts having a co-ordination number of 4 while after 22 hours full conversion was achieved with catalysts having a co-ordination number of 6.

Polymerization of Ethene and Branched α-olefins with Group IV Metal Complexes Bearing Nitrogen- and Oxygen-Based Ligands

The commodity plastics that are used in our everyday lives are based on polyolefin resins and they find wide variety of applications in several areas. Most of the production is carried out in catalyzed low pressure processes. As a consequence polymerization of ethene and α-olefins has been one of the focus areas for catalyst research both in industry and academia. Enormous amount of effort have been dedicated to fine tune the processes and to obtain better control of the polymerization and to produce tailored polymer structures The literature review of the thesis concentrates on the use of Group IV metal complexes as catalysts for polymerization of ethene and branched α-olefins. More precisely the review is focused on the use of complexes bearing [O,O] and [O,N] type ligands which have gained considerable interest. Effects of the ligand framework as well as mechanical and fluxional behaviour of the complexes are discussed. The experimental part consists mainly of development of new Group IV metal complexes bearing [O,O] and [O,N] ligands and their use as catalysts precursors in ethene polymerization. Part of the experimental work deals with usage of high-throughput techniques in tailoring properties of new polymer materials which are synthesized using Group IV complexes as catalysts. It is known that the by changing the steric and electronic properties of the ligand framework it is possible to fine tune the catalyst and to gain control over the polymerization reaction. This is why in this thesis the complex structures were designed so that the ligand frameworks could be fairly easily modified. All together 14 complexes were synthesised and used as catalysts in ethene polymerizations. It was found that the ligand framework did have an impact within the studied catalyst families. The activities of the catalysts were affected by the changes in complex structure and also effects on the produced polymers were observed: molecular weights and molecular weight distributions were depended on the used catalyst structure. Some catalysts also produced bi- or multi-modal polymers. During last decade high-throughput techniques developed in pharmaceutical industries have been adopted into polyolefin research in order to speed-up and optimize the catalyst candidates. These methods can now be regarded as established method suitable for both academia and industry alike. These high-throughput techniques were used in tailoring poly(4-methyl-1-pentene) polymers which were synthesized using Group IV metal complexes as catalysts. This work done in this thesis represents the first successful example where the high-throughput synthesis techniques are combined with high-throughput mechanical testing techniques to speed-up the discovery process for new polymer materials.

Polarized Ethylenes: Structures of (l,3-Dimethyl-2-imidazolidinylidene)malononitrile and (l,3-Dimethyl-2-perhydropyrimidinylidene)malononitrile

Crystal structures of the title compounds, (I) and (II), have been determined by three-dimensional diffraction methods. Crystals of CsHIoN 4 (I) are monoclinic, space group P21/a with Z = 4, Mr= 162, a = 7.965 (1), b = 16.232 (2), c = 7.343 (1) A, fl = 113.54 (1) °, V = 890.7 A 3, D,n = 1.218, D x = 1.208 gcm -3, g(Cu Ka, 2 = 1.5418/~) = 6.47 em -1, F(000) = 344. The crystals of C9H12N4 (II) are orthorhombic, space group P21en, with Z = 4, Mr = 176, a = 7.983 (3), b = 8.075 (2), c = 14.652 (3) ./k, V = 44.43/~3, Dm= 1.219, D x = 1.237 g cm -3, #(Mo Ka, ). = 0.7107 ,/k) = 0.868 cm -1, F(000) = 376. Both structures were solved by direct methods and refined to R = 5.8% for (I) and 5.3 % for (II). The C-C double-bond distances are 1.407 (3) in (I) and 1.429 (6)/~ in (II), appreciably longer than normal. The steric and push-pull effects result in rotation about the C=C bond, the rotation angles being 20.2 (3) in (I) and 31.5 (6) o in (II).

Studies of phosphazenes. 12. Reactions of N4P4Cl8 with dibenzylamine- isolation of an unusual "bicyclic" phosphazene, N4P4[N(CH2Ph)2]6(NCH2Ph)

Octachlorocyclotetraphosphazene, N4P4CIa, reacts with dibenzylamine to give the chloro(dibenzy1amino) derivatives, N4P,C18,[N(CH2Ph)2],,, n = 1, 2 (two isomers), and 4 (three isomers). Nongeminal structures have been assigned to these compounds on the basis of ‘H and jlP NMR spectra. The presence of at least two tris(dibenzylamin0) derivatives in some reaction mixtures is also inferred from NMR spectra. Steric effects become important at the tetrakis stage of chlorine replacement, and further substitution by dibenzylamine to give monocyclic tetrameric derivatives does not occur. A “bicyclic” phosphazene, N4P4[N(CH2Ph)2]6(NCHzPh)is, obtained from the reaction of N4P4Claw ith an excess of dibenzylamine in boiling methyl cyanide. The formation of this derivative and its spectroscopic data are discussed.

Oxidations of thio ketones by singlet and triplet oxygen

Oxidation of di-tert-butyl thioketone (1) and 2,2,4,4-tetramethylcyclobutylth ioketone (2) by singlet oxygen yields the corresponding sulfine and ketone; in the case of 1 the sulfine is the major product, whereas in 2 it is the ketone. 1,2,3-Dioxathietane has been suggested as the precursor for the ketones, and the zwitterionic/diradid peroxide is believed to be a common primary intermediate for both sulfine and ketone. Steric influence is felt both during primary interaction between singlet oxygen and thioketone and during the partitioning of the peroxide intermediate. Steric interaction is suggested as the reason for variations in the product distribution between 1 and 2. Singlet oxygen is also generated through energy transfer from the triplet state of thioketones. These excited states also directly react with oxygen to yield ketone.

Crystal structure and conformation of the cyclic dipeptide cyclo-(L-histidyl-L-aspartyl) trihydrate

The crystal structure of cyclo-(L-histidyl-L-aspartyl) trihydrate has been determined by x-ray diffraction techniques, and refined to a final R index of 0.056 for 1601 reflections. The molecule is in a folded conformation, with the imidazole ring facing the diketopiperazine ring. However, since the diketopiperazine ring is essentially planar, the interaction between the two rings is not as intimate as in those cyclic dipeptides in which the diketopiperazine ring is in a boat conformation with the side chain occupying an axial, or flagpole, site. Planarity of the diketopiperazine ring may be dictated by steric interactions between the imidazole ring and the aspartyl side chain. The molecule is a zwitterion, a proton having been transferred from the carboxyl group of the aspartyl side chain to the imidazole ring.

Dilute solution properties of methyl methacrylate�acrylonitrile copolymer (MA1)

This paper deals with studies on the dilute solution properties of methyl methacrylate�acrylonitrile copolymer of 0.289 mole fraction (mf) of acrylonitrile composition. Mark�Houwink parameters for this copolymer have been evaluated in acetonitrile (MeCN), 2-butanone (MEK), dimethylformamide (DMF) and γ-butyrolactone (γ-BL). The solvent power is found to be in the order of MEK < MeCN < DMF < γ-BL at 30°C. Herein, probably for the first time, the steric factor for the copolymer is found to be lower than that for the parent homopolymers and the excess interaction parameter, �AB is found to be negative. This probably suggests that the units are compatible to each other.

Structure and Dynamics of Coil-like Molecules by Residual Dipolar Couplings

Nuclear magnetic resonance (NMR) spectroscopy provides us with many means to study biological macromolecules in solution. Proteins in particular are the most intriguing targets for NMR studies. Protein functions are usually ascribed to specific three-dimensional structures but more recently tails, long loops and non-structural polypeptides have also been shown to be biologically active. Examples include prions, -synuclein, amylin and the NEF HIV-protein. However, conformational preferences in coil-like molecules are difficult to study by traditional methods. Residual dipolar couplings (RDCs) have opened up new opportunities; however their analysis is not trivial. Here we show how to interpret RDCs from these weakly structured molecules. The most notable residual dipolar couplings arise from steric obstruction effects. In dilute liquid crystalline media as well as in anisotropic gels polypeptides encounter nematogens. The shape of a polypeptide conformation limits the encounter with the nematogen. The most elongated conformations may come closest whereas the most compact remain furthest away. As a result there is slightly more room in the solution for the extended than for the compact conformations. This conformation-dependent concentration effect leads to a bias in the measured data. The measured values are not arithmetic averages but essentially weighted averages over conformations. The overall effect can be calculated for random flight chains and simulated for more realistic molecular models. Earlier there was an implicit thought that weakly structured or non-structural molecules would not yield to any observable residual dipolar couplings. However, in the pioneering study by Shortle and Ackerman RDCs were clearly observed. We repeated the study for urea-denatured protein at high temperature and also observed indisputably RDCs. This was very convincing to us but we could not possibly accept the proposed reason for the non-zero RDCs, namely that there would be some residual structure left in the protein that to our understanding was fully denatured. We proceeded to gain understanding via simulations and elementary experiments. In measurements we used simple homopolymers with only two labelled residues and we simulated the data to learn more about the origin of RDCs. We realized that RDCs depend on the position of the residue as well as on the length of the polypeptide. Investigations resulted in a theoretical model for RDCs from coil-like molecules. Later we extended the studies by molecular dynamics. Somewhat surprisingly the effects are small for non-structured molecules whereas the bias may be large for a small compact protein. All in all the work gave clear and unambiguous results on how to interpret RDCs as structural and dynamic parameters of weakly structured proteins.

Surface chemistry, dispersion behavior, and slip casting of Ti 3AlC2 suspensions

The surface chemistry and dispersion properties of aqueous Ti 3AlC2 suspension were studied in terms of hydrolysis, adsorption, electrokinetic, and rheological measurements. The Ti 3AlC2 particle had complex surface hydroxyl groups, such as ≡Ti-OH,=Al-OH, and -OTi-(OH)2, etc. The surface charging of the Ti3AlC2 particle and the ion environment of suspensions were governed by these surface groups, which thus strongly influenced the stability of Ti3AlC2 suspensions. PAA dispersant was added into the Ti3AlC2 suspension to depress the hydrolysis of the surface groups by the adsorption protection mechanism and to increase the stability of the suspension by the steric effect. Ti3AlC2 suspensions with 2.0 dwb% PAA had an excellent stability at pH=∼5 and presented the characteristics of Newtonian fluid. Based on the well-dispersed suspension, dense Ti3AlC2 materials were obtained by slip casting and after pressureless sintering. This work provides a feasible forming method for the engineering applications of MAX-phase ceramics, wherein complex shapes, large dimensions, or controlled microstructures are needed.

Electronic Structure and Bonding in Neutral and Dianionic Boradiphospholes: R ' BC2P2R2 (R=H, tBu, R '=H, Ph)

Classical and non-classical isomers of both neutral and dianionic BC2P2H3 species, which are isolobal to Cp+ and Cp-, are studied at both B3LYP/6-311++G(d,p) and G3B3 levels of theory. The global minimum structure given by B3LYP/6-311+ + G(d,p) for BC2P2H3 is based on a vinylcyclopropenyl-type structure, whereas BC2P2H32- has a planar aromatic cyclopentadienyl-ion-like structure. However, at the G3B3 level, there are three low-energy isomers for BC2P2H3: 1)tricyclopentane, 2) nido and 3) vinylcyclopropenyl-type structures, all within 1.7 kcal mol(-1) of each other. On the contrary, for the dianionic species the cyclic planar structure is still the minimum. In comparison to the isolobal Cp+ and HnCnP5-n+ isomers, BC2P2H3 shows a competition between pi-delocalised vinylcyclopropenyl- and cluster-type structures (nido and tricyclopentane). Substitution of H on C by tBu, and H on B by Ph, in BC2P2H3 increases the energy difference between the low-lying isomers, giving the lowest energy structure as a tricyclopentane type. Similar substitution in BC2P2H32- merely favours different positional isomers of the cyclic planar geometry, as observed in 1) isoelectronic neutral heterodiphospholes EtBu2C2P2 (E=S, Se, Te), 2) monoanionic heterophospholyl rings EtBu2C2P2 (E=P-, As-, Sb-) and 3) polyphospholyl rings anions tBu(5-n)C(n)P(5-n) (n=0-5). The principal factors that affect the stability of three-, four-, and five-membered ring and acyclic geometrical and positional isomers of neutral and dianionic BC2P2H3 isomers appear to be: 1) relative bond strengths, 2) availability of electrons for the empty 2p boron orbital and 3) steric effects of the tBu groups in the HBC(2)P(2)tBu(2) systems.

Functional interaction of diphenols with polyphenol oxidase - Molecular determinants of substrate/inhibitor specificity SO FEBS JOURNAL

Polyphenol oxidase (PPO) catalyzes the oxidation of o-diphenols to their respective quinones. The quinones autopolymerize to form dark pigments, an undesired effect. PPO is therefore the target for the development of antibrowning and antimelanization agents. A series of phenolic compounds experimentally evaluated for their binding affinity and inhibition constants were computationally docked to the active site of catechol oxidase. Docking studies suggested two distinct modes of binding, dividing the docked ligands into two groups. Remarkably, the first group corresponds to ligands determined to be substrates and the second group corresponds to reversible inhibitors. Analyses of the complexes provide structural explanations for correlating subtle changes in the position and nature of the substitutions on o-diphenols to their functional properties as substrates and inhibitors. Higher reaction rates and binding are reckoned by additional interactions of the substrates with key residues that line the hydrophobic cavity. The docking results suggest that inhibition of oxidation stems from an interaction between the aromatic carboxylic acid group and the apical His 109 of the four coordinates of the trigonal pyramidal coordination polyhedron of CuA. The spatial orientation of the hydroxyl in relation to the carboxylic group either allows a perfect fit in the substrate cavity, leading to inhibition, or because of a steric clash flips the molecule vertically, facilitating oxidation. This is the first study to explain, at the molecular level, the determinants Of substrate and inhibitor specificity of a catechol oxidase, thereby providing a platform for the design of selective inhibitors useful to both the food and pharmaceutical industries.

Thermodynamic and kinetic studies on the mechanism of binding of methylumbelliferyl glycosides to jacalin

The binding of Artocarpus integrifolia lectin (jacalin) to 4-methylumbelliferyl (Meumb)-glycosides, Gal alpha Meumb, Gal beta Meumb, GalNAc alpha Meumb, GalNAc beta-Meumb, and Gal beta 3GalNAc beta Meumb was examined by extrinsic fluorescence quenching titration and stopped flow spectrofluorimetry. The binding was characterized by 100% quenching of fluorescence of Meumb-glycosides. Their association constants range from 2.0 x 10(4) to 1.58 x 10(6) M-1 at 15 degrees C. Entropic contribution is the major stabilizing force for avid binding of Meumb-glycosides indicating the existence of a hydrophobic site that is complementary to their methylumbelliferyl group. The second order association rate constants for interaction of these sugars with lectin at 15 degrees C vary from 8.8 x 10(5) to 3.24 x 10(6) M-1 S-1, at pH 7.2. The first order dissociation rate constants range from 2.30 to 43.0 S-1 at 15 degrees C. Despite the differences in their association rate constants, the overall values of association constants for these saccharides are determined by their dissociation rate constants. The second order rate constant for the association of Meumb-glycosides follows a pattern consistent with the magnitude of the activation energies involved therin. Activation parameters for association of all ligands illustrate that the origin of the barrier between binding of jacalin to Meumb-glycosides is entropic, and the enthalpic contribution is small. A correlation between these parameters and the structure of the ligands on the association rates underscores the importance of steric factors in determining protein saccharide recognitions.

Substituent effects on the addition of alcohols and anilines to organic isothiocyanates

The addition reactions of alcohols, ROH (R = CH3, C2H5 n-C3H7, i-C3H7 and t-C4H9), to p-bromophenylisothiocyanate show that the rates decrease in the order, CH3OH> C2H5OH> n-C3H7OH> i-C3H7OH> t-C4H9OH, although the basicities of the alcohols vary in the reverse order. The results indicate the greater importance of steric factors as compared with polar factors. Evidence is also presented for the formation of a complex between the isothiocyanate and the alcohol in the first stage of the addition reaction. In the addition of aniline to substituted phenylisothiocyanates the rate data give a satisfactory linear correlation with Hammett σ constants and the results clearly show that electron-withdrawing groups favour the addition reaction. The addition of aniline to alkyl isothiocyanates have been studied in order to find out the nature of alkyl group interaction in these derivatives. Kinetic studies on the addition of substituted anilines to phenylisothiocyanate show that the rate of reaction increases with the electron-donating ability of the substituents on the aniline as also the basicity of the aniline.