Study of the synthesis of poly(isobutylene-b-amide-11) by polycondensation α,ω-dianhydride oligoisobutylene with α,ω-diamino oligoamide-11. II. Synthesis of the block copolymers by reaction in solution

Several di-, tri- or multiblock poly(isobutylene-b-amide-11)s are prepared by reaction of α-mono or α,ω-dianhydride oligoisobutylenes with α-mono or α,ω-diamino oligoamides, in o-dichorobenzene at 140°C. They are characterized mainly by ¹H- and ¹³C-NMR spectroscopy and GPC, using the trifluoroacetylation technique. Their thermal differential chromatograms are reported and analyzed.     

Tentoxin as a scaffold for drug discovery. Total solid-phase synthesis of tentoxin and a library of analogues

[reaction: see text] A solid-phase method for the synthesis of tentoxin has been developed. Two key steps-dehydration and N-alkylation-are carried out while the peptide is anchored to the resin. The method, which has been validated by the preparation of a library of tentoxin analogues, should be applicable to the generation of further libraries that have the tentoxin scaffold structure, as well as other structures containing N-alkylated didehydroamino acids.     

Amperometric urea biosensor based on urease and electropolymerized toluidine blue dye as a pH-sensitive redox probe

The electropolymerized toluidine blue film deposited on the glassy carbon electrode show amperometrically detectable pH sensitivity. This feature of polytoluidine blue (PTOB) film was used for a construction of an amperometric urea biosensor. We have observed a linear shift of the formal redox potential with increasing pH value between 4 and 8 giving the slope of 81 mV(Delta) pH(-1). Polytoluidine blue film has had a significantly increased stability and higher electrochemical activity compared to the adsorbed monomeric dye. The polytoluidine blue urea biosensor has been operating at a working potential of -200 mV vs. SCE. The sensitivity of the biosensor was 980 nA mM(-1) cm(-2). The biosensor showed linearity in concentration range up to 0.8 mM with the detection limit of 0.02 mM (S/N=3).     

Esmodil: an acetylcholine mimetic resurfaces in a Southern Australian marine sponge Raspailia (Raspailia) sp

Bioassay directed fractionation of a Raspailia (Raspailia) sp. (Order Poecilosclerida; Family Raspailiidae) collected during scientific trawling operations off the Northern Rottnest Shelf yielded as nematocidal agents the known metabolites, phorboxazoles A (1) and B (2). Further examination revealed the new natural product but known synthetic compound, esmodil (3). The structure for 3 was confirmed by spectroscopic analysis and total synthesis.     

Zeta potential of microfluidic substrates: 1. Theory, experimental techniques, and effects on separations

This paper summarizes theory, experimental techniques, and the reported data pertaining to the zeta potential of silica and silicon with attention to use as microfluidic substrate materials, particularly for microchip chemical separations. Dependence on cation concentration, buffer and cation type, pH, cation valency, and temperature are discussed. The Debye-Hückel limit, which is often correctly treated as a good approximation for describing the ion concentration in the double layer, can lead to serious errors if it is extended to predict the dependence of zeta potential on the counterion concentration. For indifferent univalent electrolytes (e.g., sodium and potassium), two simple scalings for the dependence of zeta potential on counterion concentration can be derived in high- and low-zeta limits of the nonlinear Poisson-Boltzman equation solution in the double layer. It is shown that for most situations relevant to microchip separations, the high-zeta limit is most applicable, leading to the conclusion that the zeta potential on silica substrates is approximately proportional to the logarithm of the molar counterion concentration. The zeta vs. pH dependence measurements from several experiments are compared by normalizing the zeta based on concentration.     

N2 activation by iron-sulfur complexes

Inspired by the determination of the structure of the nitrogenase enzyme cofactor by Rees et al., the binding of an N₂ molecule to some model iron-sulfur compounds was investigated usingab initio calculations. Side-on and end-on coordination to one two and four iron centers were investigated. In most cases, the N₂ is loosely bound and retains its internal triple bond, but a few examples are found where the N₂ is “activated” and has a longer N-N bond length.     

Acid-Catalyzed Diels-Alder Reactions of Cycloalkenones

Whereas the Diels-Alder reaction has been in vogue as a facile method of six-membered ring construction for many years, its application to a one-step cis-octalone synthesis was precluded by discouraging, early reports on the conditions required for the interaction of dienes with 2-cyclohexenones and on the yields of the resultant adducts.^1,2 It is quite conceivable that these observations were responsible in part for the minimization of use of an otherwise attractive route toward angularly alkylated six-membered polycycles in the field of steroid total synthesis. The discovery of Lewis acid catalysis of the cycloaddition of α, β-unsaturated carbonyl compounds³ has rekindled interest in the above reactions.⁴ The following discussion illustrates octalone     

Aβ40 and Aβ42 amyloid fibrils exhibit distinct molecular recycling properties

A critical aspect to understanding the molecular basis of Alzheimer's disease (AD) is the characterization of the kinetics of interconversion between the different species present during amyloid-β protein (Aβ) aggregation. By monitoring hydrogen/deuterium exchange in Aβ fibrils using electrospray ionization mass spectrometry, we demonstrate that the Aβ molecules comprising the fibril continuously dissociate and reassociate, resulting in molecular recycling within the fibril population. Investigations on Aβ40 and Aβ42 amyloid fibrils reveal that molecules making up Aβ40 fibrils recycle to a much greater extent than those of Aβ42. By examining factors that could influence molecular recycling and by running simulations, we show that the rate constant for dissociation of molecules from the fibril (k(off)) is much greater for Aβ40 than that for Aβ42. Importantly, the k(off) values obtained for Aβ40 and Aβ42 reveal that recycling occurs on biologically relevant time scales. These results have implications for understanding the role of Aβ fibrils in neurotoxicity and for designing therapeutic strategies against AD.