Influence of ozonized kraft lignin on the crystallization of CaCO3

Results are reviewed from a study examining how structural modifications introduced by ozonization enhance the influence of kraft lignin on the crystallization of CaCO(3). Ozone treatment of kraft lignin in an aqueous environment is shown to increase its carboxylic acid and overall oxygen content and reduce its molecular weight. Calcium concentration and temperature were monitored in heated supersaturated solutions containing ozonized kraft lignins to gauge their influence on CaCO(3) crystallization processes. The presence of kraft lignin raises the temperature necessary to induce crystallization. This effect is shown to level off at relatively low lignin concentrations and be dependent on the extent of ozone treatment the kraft lignin has undergone. A linear correlation is found between crystallization temperatures and the carboxylic acid content of ozonized lignin samples indicating the introduction of these functional groups plays an important role in enhancing its inhibitory effect. Scanning electron microscopy images of crystals grown in the presence of kraft lignins show significant morphological modifications. These are consistent with specific or pseudo specific interactions between the lignin and crystal faces of calcite to inhibit growth parallel to its c axis. The influence over crystal morphology demonstrated by modified kraft lignin increases with increasing ozonization. Also presented here are crystallization temperature data for a range of kraft lignin ultrafiltration fractions, which indicate that the optimal (nominal) molecular weight of kraft lignin for inhibiting the crystallization of CaCO(3) lies between 5000 and 10000.     

Collagen stability: insights from NMR spectroscopic and hybrid density functional computational investigations of the effect of electronegative substituents on prolyl ring conformations

Collagen-like peptides of the type (Pro-Pro-Gly)(10) fold into stable triple helices. An electron-withdrawing substituent at the H(gamma)(3) ring position of the second proline residue stabilizes these triple helices. The aim of this study was to reveal the structural and energetic origins of this effect. The approach was to obtain experimental NMR data on model systems and to use these results to validate computational chemical analyses of these systems. The most striking effects of an electron-withdrawing substituent are on the ring pucker of the substituted proline (Pro(i)) and on the trans/cis ratio of the Xaa(i-1)-Pro(i) peptide bond. NMR experiments demonstrated that N-acetylproline methyl ester (AcProOMe) exists in both the C(gamma)-endo and C(gamma)-exo conformations (with the endo conformation slightly preferred), N-acetyl-4(R)-fluoroproline methyl ester (Ac-4R-FlpOMe) exists almost exclusively in the C(gamma)-exo conformation, and N-acetyl-4(S)-fluoroproline methyl ester (Ac-4S-FlpOMe) exists almost exclusively in the C(gamma)-endo conformation. In dioxane, the K(trans/cis) values for AcProOMe, Ac-4R-FlpOMe, and Ac-4S-FlpOMe are 3.0, 4.0, and 1.2, respectively. Density functional theory (DFT) calculations with the (hybrid) B3LYP method were in good agreement with the experimental data. Computational analysis with the natural bond orbital (NBO) paradigm shows that the pucker preference of the substituted prolyl ring is due to the gauche effect. The backbone torsional angles, phi and psi, were shown to correlate with ring pucker, which in turn correlates with the known phi and psi angles in collagen-like peptides. The difference in K(trans/cis) between AcProOMe and Ac-4R-FlpOMe is due to an n-->pi interaction associated with the Bürg-Dunitz trajectory. The decrease in K(trans/cis) for Ac-4S-FlpOMe can be explained by destabilization of the trans isomer because of unfavorable electronic and steric interactions. Analysis of the results herein along with the structures of collagen-like peptides has led to a theory that links collagen stability to the interplay between the pyrrolidine ring pucker, phi and psi torsional angles, and peptide bond trans/cis ratio of substituted proline residues.     

Contrasting behavior in azide pyrolyses: an investigation of the thermal decompositions of methyl azidoformate, ethyl azidoformate and 2-azido-N, N-dimethylacetamide by ultraviolet photoelectron spectroscopy and matrix isolation infrared spectroscopy

The thermal decompositions of methyl azidoformate (N3COOMe), ethyl azidoformate (N3COOEt) and 2-azido-N,N-dimethylacetamide (N3CH2CONMe2) have been studied by matrix isolation infrared spectroscopy and real-time ultraviolet photoelectron spectroscopy. N2 appears as an initial pyrolysis product in all systems, and the principal interest lies in the fate of the accompanying organic fragment. For methyl azidoformate, four accompanying products were observed: HNCO, H2CO, CH2NH and CO2, and these are believed to arise as a result of two competing decomposition routes of a four-membered cyclic intermediate. Ethyl azidoformate pyrolysis yields four corresponding products: HNCO, MeCHO, MeCHNH and CO2, together with the five-membered-ring compound 2-oxazolidone. In contrast, the initial pyrolysis of 2-azido-N,N-dimethyl acetamide, yields the novel imine intermediate Me2NCOCH=NH, which subsequently decomposes into dimethyl formamide (HCONMe2), CO, Me2NH and HCN. This intermediate was detected by matrix isolation IR spectroscopy, and its identity confirmed both by a molecular orbital calculation of its IR spectrum, and by the temperature dependence and distribution of products in the PES and IR studies. Mechanisms are proposed for the formation and decomposition of all the products observed in these three systems, based on the experimental evidence and the results of supporting molecular orbital calculations.     

Reactions of methylated cyclopropanes and olefins with chloroplatinic acid. I. Diacylation and pyrylium ion formation

In contrast to cyclopropane itself, simple methyl-substituted cyclopropanes do not produce platinum insertion products upon reaction with chloroplatinic acid in acetic anhydride. Instead, the major products are aromatic heterocyclic cations, the pyrylium ions, which are formed as the result of diacylation of the cyclopropane ring and subsequent dehydration and ring closure to the aromatic heterocycle. Similar products are formed from the diacylation of certain olefins in the presence of chloroplatinic acid in anhydride solvents. The combination of solvent and substrate variations has led to a proposed mechanism for these reactions which demonstrates the essential and unique mole of platinum. In particular, the specific pyrylium ions produced are formed as a consequence of initial activation of the least-substituted cyclopropane ring bond by insertion of platinum, followed by acylation of platinum-bonded carbon, proton loss to a β,γ-unsaturated carbonyl, a second acylation, and, finally, ring closure.     

NMR chemical shifts in the low–pH form of a-chymotrypsin. A QM/MM and ONIOM–NMR study

 The relationship between hydrogen bonding and NMR chemical shifts in the catalytic triad of low-pH α-chymotrypsin is investigated by combined use of the effective fragment potential [(2001) J Phys Chem A 105:293] and ONIOM–NMR [(2000) Chem Phys Lett 317:589] methods. Our study shows that while the His57 N^δ1−H bond is stretched by a relatively modest amount (to about 1.060 ?) this lengthening, combined with the polarization due to the molecular environment, is sufficient to explain the experimentally observed chemical shifts of 18.2 ppm. Furthermore, the unusual down-field shift of H^ɛ1 (9.2 ppm) observed experimentally is reproduced and shown to be induced by interactions with the C=O group of Ser214 as previously postulated. The free-energy cost of moving H^δ1 from His57 to Asp102 is predicted to be 5.5 kcal/mol. Received: 26 September 2001 / Accepted: 6 September 2002 / Published online: 21 January 2003 Contribution to the Proceedings of the Symposium on Combined QM/MM Methods at the 222nd National Meeting of the American Chemical Society, 2001 Correspondence to: J. H. Jensen e-mail: jan-jensen@uiowa.edu Acknowledgements. This work was supported by a Research Innovation Award from the Research Corporation and a type G starter grant from the Petroleum Research Fund. The calculations were performed on IBM RS/6000 workstations obtained through a CRIF grant from the NSF (CHE-9974502) and on supercomputers at the National Center for Supercomputer Applications at Urbana-Champaign. The authors are indebted to Visvaldas Kairys for help with the CHARMM program, and to Daniel Quinn for many helpful discussions.     

Cloning and expression of full-lengthTrichoderma reesi cellobiohydrolase I cDNAs inEscherichia coli

The process of converting lignocellulosic biomass to ethanol via fermentation depends on developing economic sources of cellulases.Trichoderma reesei cellobiohydrolase (CBH) I is a key enzyme in the fungal cellulase system; however, specific process application requirements make modification of the enzyme by site-directed mutagenesis (SDM) an attractive goal. To undertake SDM investigations, an efficient, cellulase-free host is required. To test the potential ofEscherichia coli as a host, T.reesei CBH I cDNA was expressed inE. coli strain GI 724 as a C-terminal fusion to thermostable thioredoxin protein. Full-length expression of CBH I was subsequently verified by molecular weight, Western blot analysis, and activity on soluble substrates.

     

Nanorods of Various Oxides and Hierarchically Structured Mesoporous Silica by Sol-Gel Electrophoresis

In this paper, we report the template-based growth of nanorods of oxides and hierarchically structured mesoporous silica, formed by means of a combination of sol-gel processing and electrophoretic deposition. Both single metal oxides (TiO₂) and complex oxides (Pb(Zr_0.52Ti_0.48)O₃) have been grown by this method. This method has also been applied to the growth of nanorods of mesoporous silica having an ordered pore structure, where the pores are aligned parallel to the long axis of the nanorod. Uniformly sized nanorods of about 125–200 nm in diameter and 10 m in length were grown over large areas with near unidirectional alignment. Appropriate sol preparation yielded the desired stoichiometric chemical composition and crystal structure of the oxide nanorods, with a heat treatment (500–700°C for 15–30 min) for crystallization, densification and any necessary pyrolysis.     

Cinnamolid‐3β‐ol hemihydrate and 3β‐hydroxycinnamolide acetate,two drimanolide‐class sesquiterpene lactones from Warburgia ugandensis

3β‐Hydr­oxy‐7‐drimen‐12,11‐olide hemihydrate, C₁₅H₂₂O₃·0.5H₂O, (I), has two sesquiterpene mol­ecules and one water mol­ecule in the asymmetric unit. The OH groups of both mol­ecules and both H atoms of the water mol­ecule are involved in near‐linear inter­molecular hydrogen bonds, having O⋯O distances in the range 2.632 (3)–2.791 (2) Å. 3β‐Acet­oxy‐7‐drimen‐12,11‐olide, C₁₇H₂₄O₄, (II), has its ring system in very nearly the same conformation as the two mol­ecules of (I).     

A highly stable quadruply hydrogen-bonded heterocomplex useful for supramolecular polymer blends

The butyl urea of guanosine (UG) presents an ADDA hydrogen-bonding array that is complementary to the DAAD array of 2,7-diamido-1,8-naphthyridine (DAN). The stability of the DAN.UG complex was measured by fluorimetry using the fluorescence resonance energy transfer (FRET) from the naphthyridine ring to a coumarin 343 moiety linked covalently to the UG unit. The quadruply hydrogen-bonded complex is extremely stable with a measured association constant, Kassoc, of 3 x 108 M-1. Unlike related hydrogen-bonding modules, the guanosine urea, UG, contains a relatively fixed tautomeric form and only weakly self-associates (Kdimer = ca. 200 M-1). The DAN unit was linked to a styrene-based monomer and copolymerized with styrene to form a polymer (PS-DAN) containing a controlled number of the DAAD recognition units. Likewise, a methacrylate monomer containing the UG unit was copolymerized with butyl methacrylate to form a polymer (PBMA-UG). Blends formed from PS-DAN and PBMA-UG were characterized by DSC, SEC, and viscometry. The importance of selective heterocomplexation and weak self-association in forming the blended networks was demonstrated by using a ureidopyrimidinone (UPy) unit, which also forms strong heterocomplexes with DAN but is able to strongly self-associate.     

Highly selective entry to the azadirachtin skeleton via a Claisen rearrangement/radical cyclization sequence

[formula: see text] A highly diastereoselective, microwave-induced Claisen rearrangement of an appropriately substituted propargylic enol ether allows the formation of the sterically congested C8-C14 bond of azadirachtin. When combined with a radical-mediated cyclization of the corresponding allene, this sequence offers rapid entry to the framework of azadirachtin.