Toby M. Chapman

Aldehyde Dehydrogenase Catalytic Mechanism

Elsewhere in this volume we detail findings from an alignment of 145 ALDH sequences (Perozich et al., 1999), and previously at these meetings we reported that the crystal structure of a class 3 ALDH (E-NAD binary complex) revealed a non-traditional mode of NAD-binding within an open β/α domain otherwise familiar in the NAD-binding “ossmann folds” of other dehydrogenases (Liu et al., 1997a). The variability of residues in the substrate-binding site clearly indicates evolutionary tailoring of the substrate specificities of inddual ALDHs. However, farther to the interior of the active site-between the catalytic thiol and NAD molecule where hydride transfer from aldehyde to NAD occurs-strict conservations are compatible with a common chemical mechanism (Liu et al., 1997b)he position of NAD in an isomorphous class 3 ALDH derivative and the emergence of Asn4/169 as a strictly conserved residue prompted us to consider the catalytic mechanism we present here.

Polymyxin B. NMR evidence for a peptide antiobiotic with folded structure in water

Abstract Hydrochlorides of several members of the polymyxin family of peptide antibiotics have been studied by NMR at 250 MHz in H 2 O, D 2 O, and DMSO-d6. Deuterium exchange experiments carried out in D 2 O indicate the presence of at least five exchange-resistant amide protons in polymyxins B and E and in circulin. Polymyxin A shows no slowly exchanging amide protons. A cross-beta conformation is proposed for polymyxin B in which two intramolecular hydrogen bonds form between backbone peptide segments of the linear and cyclic parts of the molecule. This cross-beta conformation is suggested to be a general phenomenon in antibiotics and hormones containing a cyclopeptide linked to a linear peptide by a peptide bond. In particular, conformational similarities are seen between polymyxin B and bacitracin A. A space-filling model of the latter suggests a conformation which explains chemical properties of this peptide.

Easily grafted polyurethanes with reactive main chain functional groups. Synthesis, characterization, and antithrombogenicity of poly(ethylene glycol)‐grafted poly(urethanes)

A novel synthesis of poly(ethylene glycol) (PEG)-grafted poly(urethanes) (PURs) is described based on a precursor PUR containing free amino groups in the main chain. Three different poly(urethane) backbones were prepared: a homopoly(urethane) comprised of N-Bocdiethanolamine (BDA) and 4,4′-methylenebis(phenyl isocyanate) (MDI), a copoly(urethane) (COPUR) consisting of BDA, N-benzyldiethanolamine and MDI, and a poly(urethane urea) (PUU) that was prepared from BDA, MDI, and ethylenediamine as the chain extender. The Mn of these poly(urethanes) ranged from 32,000 to 72,000 g/mol. PEG (750, 1,900, and 5,000 g/mol) was grafted onto the boc-deprotected poly(urethanes) via the chloroformate. Films of the polymers were spin cast from dilute solutions, annealed, and the surfaces analyzed by goniometry. Water contact angle data indicates increasing PEG surface coverage of the poly(urethanes) with increasing PEG molecular weight. Reorientation of the polymer films is evidenced by contact angle hysteresis. Polymer thrombogenicity, which was studied using blood perfusion experiments, shows that COPUR-g-PEG5000 and PUU-g-PEG5000 exhibit very little platelet adhesion.

Mechanical degradation of drag reducing polymers in suspensions of blood cells and rigid particles

Natural and synthetic soluble drag reducing polymers (DRP) have been shown to produce beneficial effects on blood circulation in various animal models and may represent a novel bioengineering way to treat cardiovascular disorders. These polymers are known to degrade when subjected to high shear stresses which could be a part of the process of their elimination from the vascular system. However, the relative rate of their degradation was not known especially in the presence of blood cells or particles. The hydrodynamic tests in this study demonstrated that DRP mechanical degradation was significantly increased by the presence of red blood cells (RBC) and even more so by the presence of rigid particles of similar size. Degradation rates increased with an increase in RBC or particle concentration. The natural DRP (derived from aloe) was shown to be much more resistant to flow-induced degradation than polyethylene oxide in the presence or absence of RBC.

POLY(N-VINYLFORMAMIDE) AS A DRAG-REDUCING POLYMER FOR BIOMEDICAL APPLICATIONS

Water-soluble drag-reducing polymers (DRPs) were previously demonstrated to significantly increase blood flow, tissue perfusion, and tissue oxygenation when injected intravenously at nanomolar concentrations in various animal models. Turbulent flow drag-reducing ability was proven to be the most important factor defining the potential of polymers to favorably affect blood circulation. Several DRPs were applied in previous in vivo tests, but the search continues for suitable DRPs for biomedical applications. We demonstrated that poly(N-vinylformamide) (PNVF) with a molecular weight of 4.5 x 10(6) Da significantly reduced resistance to turbulent flow in a pipe and thus presents a DRP. We also found that the PNVF mechanical degradation is much slower than that of the most commonly used DRP, poly(ethylene oxide). PNVF is known to have low toxicity. Furthermore, our pilot in vivo study showed that PNVF had acceptable biocompatibility and hemodynamic effectiveness and thus could be considered as a DRP candidate for potential clinical use.