Functional Stabilization of Cellulase from Aspergillus niger by Conjugation with Dextran-aldehyde

The covalent conjugates of cellulase from Aspergillus niger were prepared with various molar ratios by using dextran. The conjugate (n_E/n_D: 1/5) showed higher activity than purified enzyme at all temperatures after 1 h of incubation and its activity could also be measured at higher temperature. Also, this conjugate lost only 60% activity in 2 h at 70°C in comparison to the purified enzyme, which lost all its activity. In addition, conjugation protected cellulase against denaturation in the presence of sodium dodecylsulfate (residual activity of about 80%) and inactivation by air bubbles (residual activity of about 50% for 4 h).     

Stabilization of horseradish peroxidase by covalent conjugation with dextran aldehyde against temperature and pH changes

Stabilization of Horseradish Peroxidase (HRP; EC 1.11.1.7) against temperature and pH via the formation of the conjugates obtained by multipoint covalent bonding of dextran aldehyde (DA) to the enzyme were studied. Hence, three different molar weighted dextrans (17.5 kD, 75 kD, 188 kD) were covalently bonded to purified enzyme with different molar ratios (n_HRP/n_DA 20/1, 10/1, 1/1, 1/5, 1/10, 1/15, 1/20). The thermal stabilities of the obtained conjugates were evaluated with the activities determined at different temperatures (25, 30, 35, 40, 50, 60, 70, 80°C) applying 60 minutes incubation time. Conjugates formed were characterized by gel-permeation chromatography (GPC) and fluorescence techniques. The conjugate synthesized using dextran 75 kDa with n_HRP/n_DA 1/10 molar ratio showed better thermal stability than other conjugates and purified enzyme at pH 7. This conjugate also has wider activity pH range than purified enzyme. In addition, mentioned conjugate at pH 7 had very long storage lifetime compared to purified enzyme at +4°C and room temperature; which is considered a favorable feature for usage in practice.      

Micromechanical modeling of stress-induced phase transformations. Part 2. Computational algorithms and examples

Based on the theory developed in Part 1 of this paper [Levitas, V.I., Ozsoy, I.B., 2008. Micromechanical modeling of stress-induced phase transformations. Part 1. Thermodynamics and kinetics of coupled interface propagation and reorientation. Int. J. Plasticity. doi:10.1016/j.ijplas.2008.02.004], various non-trivial examples of microstructure evolution under complex multiaxial loading are presented. For the case without interface rotation, the effect of the athermal thresholds for austenite (A)–martensite (M) and martensitic variant M_I–variant M_II interfaces and loading paths on stress–strain curves and phase transformations was studied. For coupled interface propagation and rotation, two types of numerical simulations were carried out. The tetragonal–orthorhombic transformation has been studied under general three-dimensional interface orientation and zero athermal threshold. The cubic–tetragonal transformation was treated with allowing for an athermal threshold and interface reorientation within a plane. The effect of the athermal threshold, the number of martensitic variants and an interface orientation in the embryo was studied in detail. It was found that an instability in the interface normal leads to a jump-like interface reorientation that has the following features of the energetics of a first-order transformation: there are multiple energy minima versus interface orientation that are separated by an energy barrier; positions of minima do not change during loading but their depth varies; when the barrier disappears (i.e. one of the minima transforms to the local saddle or maximum points), the system rapidly evolves toward another stable orientation. Depending on the loading and material parameters, we observed a large continuous change in interface orientation, a jump in interface reorientation, a jump in volume fractions and stresses, an expected stress relaxation during the phase transition and unexpected stress growth during the transition because of large change in elastic moduli.     

Selection of a pentameric host in the host-guest complexes [[[[P(mu-NtBu)]2(mu-NH)]5].I]-[Li(thf)4]+ and [[[P(mu-NtBu)]2(mu-NH)]5].HBr.THF

The structures of the host-guest complexes [[[[P(mu-NtBu)]2(mu-NH)]5]I]-.[Li(thf)4]+ [2.I[Li(thf)4]] and [[[P(mu-NtBu)]2(mu-NH)]5].HBr.THF (2.HBr.THF) show that increased distortion of the framework of the pentameric macrocycle [[[P(mu-NtBu)]2(mu-NH)]5] (2) occurs with the larger halide ions. Theoretical studies show that the thermodynamic stabilities of the model host-guest anions [2.X]- (X=Cl, Br, I) are in the order Cl- approximately Br->I-, that is, the reverse of the templating trend observed experimentally. These studies support the view that the selection of the pentamer 2 over the tetramer [[[P(mu-NtBu)]2(mu-NH)]4] (1) is kinetically controlled, a conclusion which is also consistent with the previous observation that the frameworks of 1 and 2 are not in dynamic equilibrium with each other.     

(TlMes2)[BF4] – Ein Salz mit linearem (Mes‐Tl‐Mes)+‐Ion

(TlMes₂)[BF₄] – A Salt with the Linear Cation (Mes‐Tl‐Mes)⁺ TlMes₃ was reacted with [BF₃(OEt₂)] in Et₂O at 20 °C to give (TlMes₂)[BF₄] ( 1 ). 1 was characterized by NMR techniques, IR spectroscopy as well as by an X‐ray structure determination. According to this, 1 is built‐up by ifinite chains of cations and anions along [001]. The linear cations are rotated 90° to each other along the chains due to the coordination of the [BF₄]^? ion.     

An efficient and selective enzymatic oxidation system for the synthesis of enantiomerically pure D-tert-leucine

[reaction: see text] d-tert-Leucine was prepared with an enantiomeric excess of >99% by an enzyme-catalyzed oxidative resolution of the racemic mixture of dl-tert-leucine with use of leucine dehydrogenase. The l-amino acid was oxidized completely due to coupling of the primary reaction with a highly efficient irreversible NAD(+)-regenerating step by NADH oxidase.     

Proton Affinities of Cationic Carbone Adducts [AC(PPh3)2]+ (A=Halogen,Hydrogen, Methyl) and Unusual Electronic Structures of the Cations and Dications [AC(H)(PPh3)2]2+

This work reports the syntheses and the first crystal structures of the cationic carbone adducts [FC(PPh₃)₂]⁺ and [BrC(PPh₃)₂]⁺ and the protonated dication [FC(H)(PPh₃)₂]²⁺, which are derived from the carbone C(PPh₃)₂. Quantum chemical calculations and bonding analyses were carried out for the series of cations [AC(PPh₃)₂]⁺ and dications [AC(H)(PPh₃)₂]²⁺, where A=H, Me, F, Cl, Br, I. The bonding analysis suggests that the cations are best described as phosphane complexes L→(CA)⁺←L (L=PPh₃), which are related to the neutral borylene adducts L→(BA)←L (L=cyclic carbene; A=H, aryl) that were recently isolated. The carbone adducts [AC(PPh₃)₂]⁺ possess a π electron lone pair at carbon and they can easily be protonated to the dications [AC(H)(PPh₃)₂]²⁺. The calculations of the dications indicate that the molecules are best represented as complexes L→(CHA)²⁺←L (L=PPh₃) where a carbene dication is stabilized by the ligands. The central carbon atom in the cations and even in the dications carries a negative partial charge, which is larger than the negative charge at fluorine. There is also the peculiar situation in which the carbon–fluorine bonds in [FC(PPh₃)₂]⁺ and [FC(H)(PPh₃)₂]²⁺ exhibit the expected polarity with the negative end at fluorine, but the carbon atom has a larger negative charge than fluorine. Given the similarity of carbodiphosphorane C(PPh₃)₂ and carbodicarbene C(NHC)₂, we expect that analogous compounds [AC(NHC)₂]⁺ and [AC(H)(NHC)₂]²⁺ with similar features as [AC(PPh₃)₂]⁺ and [AC(H)(PPh₃)₂]²⁺ can be isolated.     

On the Solution Behaviour of Benzyllithium⋅(−)‐Sparteine Adducts and Related Lithium Organyls – A Case Study on Applying 7Li,15N{1H} HMQC and Further NMR Methods,Including Some Investigation into Asymmetric Synthesis

2D ⁷Li,¹⁵N heteronuclear shift correlation through scalar coupling has successfully been applied to several lithium organyls consisting of polydentate N ligands such as N,N,N′,N′‐tetramethylethylenediamine (tmeda), N,N,N′,N′,N′′‐pentamethyldiethylentriamine (pmdta) and (?)‐sparteine. Structural insights on the conformation of benzyllithium ? pmdta ( 5 ) in a toluene solution and the strength of ion pairing in combination with PGSE NMR measurements, ¹H,¹H‐NOESY and ¹H,⁷Li‐HOESY experiments are presented. By studying in detail the formation of 5 in solution, a transient species has been observed for the first time and assigned to a pre‐complex of nBuLi and pmdta. In addition, the solution behaviour of the complex formed between benzyllithium and (?)‐sparteine ( 8 ) has been studied by PGSE and multinuclear NMR spectroscopy. The straightforward synthesis and first applications in asymmetric lithiations are also reported, which show that the new system benzyllithium ? (?)‐sparteine ( 8 ) provide poorer enantioselective induction than the classical nBuLi ? (?)‐sparteine ( 6 ). The results were supported by deprotonation experiments confirming that the formation of 8 relies on two relevant factors, namely temperature and lithiating reagent. The existence of 8 may thus interfere with the asymmetric induction when the system nBuLi ? (?)‐sparteine is used in the enantioselective deprotonations of N‐Boc‐N‐(p‐methoxyphenyl)‐benzylamine conducted in toluene.