Euler-Lagrange Equations for Nonlinearly Elastic Rods with Self-Contact

 We derive the Euler-Lagrange equations for nonlinearly elastic rods with self-contact. The excluded-volume constraint is formulated in terms of an upper bound on the global curvature of the centre line. This condition is shown to guarantee the global injectivity of the deformation of the elastic rod. Topological constraints such as a prescribed knot and link class to model knotting and supercoiling phenomena as observed, e.g., in DNA-molecules, are included by using the notion of isotopy and Gaussian linking number. The bound on the global curvature as a nonsmooth side condition requires the use of Clarke's generalized gradients to obtain the explicit structure of the contact forces, which appear naturally as Lagrange multipliers in the Euler-Lagrange equations. Transversality conditions are discussed and higher regularity for the strains, moments, the centre line and the directors is shown. (Accepted December 20, 2002) Published online April 8, 2003 Communicated by S. S. Antman     

Thermally assisted and heavy-atom assisted intersystem crossing in meso-substituted anthracenes

A single linear correlation between the activation energy of S₁—T intersystem crossing and the logarithm of the bimolecular fluorescence quenching rate constant by haloalkanes for meso-substituted anthracenes in fluid solutions is established. It is concluded that both the monomolecular and the heavy-atom-assisted deactivation of S₁ depend in the same way on the change of S₁ energy by substitution or solvent relative to the energy of the accepting triplet T_n. The anomalous increase of fluorescence of Br-substituted anthracenes in heavy-atom containing solvents is consistent with a widening of the S₁—T_n energy gap due to the changing solvent.     

Comparison of strong anion-exchangers for the purification of a PEGylated protein

We have studied the effect of protein PEGylation on ion-exchange adsorption using bovine serum albumin as a model system. The free sulfhydryl group of BSA, located on cysteine 34, was PEGylated using the maleimido-PEG chemistry. Several different BSA preparations were screened for extent of reaction using a 30 kDa PEG reagent. The highest yielding BSA preparation was PEGylated using linear 12 kDa and 30 kDa PEG reagents at the 1 liter scale. The PEGylated reaction mixture was purified by anion-exchange gradient elution chromatography to remove native protein and aggregates. Purity following anion-exchange chromatography was >90% as determined by analytical size exclusion chromatography. The elution salt concentration decreased with increasing PEG chain length. Breakthrough studies on six commercially available anion-exchange stationary phases with purified PEG-BSA conjugates confirm a very large decrease in dynamic binding capacity compared to the native protein. The decrease in dynamic binding capacity is likely due to modulation of electrostatic interactions caused by the neutral PEG chain and increased mass transfer resistance associated with the large size of the molecule. Of the stationary phases evaluated, the open porous structure of the agarose based ion-exchangers resulted in the highest dynamic binding capacities for the PEG-BSA conjugates. Frontal analysis experiments demonstrate use of this technique for purification of PEGylated proteins. A stationary phase that tended to exclude the large PEG-BSA conjugate was very efficient in removing native protein from a crude reaction mixture by frontal analysis.     

On glancing scattering of light ions from tungsten single crystal surfaces

A computer calculation of light ion (200–800 keV protons and helium) scattering from a tungsten single crystal model with a (110)-type surface is presented. An axis (100) is considered. Thermal vibrations in 3 dimensions have been incorporated. The number, i.e. fraction, and spatial distribution of reflected ions for some angular scans are presented in an appendix. The importance of trajectory histories is demonstrated. 3 regimes of scattering behaviour as a function of the incidence angle ψ in to the string in the surface are found:

(i) for very small ψ_in surface-semi-proper-channelling of all ions;

(ii) for intermediate ψ_in mainly planar channelling and dechannelling resulting in a decreasing reflective index for increasing ψ _in;

(iii) axial channelling and transverse plane focussing are mainly of importance leading to a small reflective index, and surface semi-channelling like for (ii) occurs for very few ions.

The importance of the scattering potential is demonstrated and an expression is discussed.     

Binding and elution behavior of proteins on strong cation exchangers

This work provides a broad survey of binding and elution behavior of proteins on strong cation exchangers. Four proteins comprising two monoclonal antibodies, lysozyme, and cytochrome c were used as models in the investigation. Seven chromatography resins with different base matrices were compared. Dynamic binding capacity as a function of salt concentration was examined for a monoclonal antibody and lysozyme. Elution behavior as a function of gradient slope was modeled to determine the characteristic charge, essentially a measure of the number of sites involved in binding, for each protein on each resin. Trends with respect to dynamic binding capacity and elution behavior are analyzed and discussed.     

Magnetically alignable phase of phospholipid "bicelle" mixtures is a chiral nematic made up of wormlike micelles

We have studied the phase behavior of binary mixtures of long- and short-chain lipids, namely, dimyristoyl phosphatidylcholine (DMPC) and dihexanoyl phosphatidylcholine (DHPC), using optical microscopy and small-angle neutron scattering. Samples with a total lipid content of 25 wt %, corresponding to ratios Q ([DMPC]/[DHPC]) of 5, 3.2, and 2, are found to exhibit an isotropic (I) --> chiral nematic (N) --> lamellar phase sequence on increasing temperature. The I-N transition coincides with the chain melting transition of DMPC at Q = 5 and 3.2, but the N phase forms at a higher temperature for Q = 2. All three samples form multilamellar vesicles in the lamellar phase. Our results show that disklike "bicellar" aggregates occur only in the lower temperature isotropic phase and not in the higher temperature magnetically alignable N phase, where they were previously believed to exist. The N phase is found to consist of long, flexible wormlike micelles, their entanglement resulting in the very high viscosity of this phase.     

Photochemical and Photobiotogical Studies with Acridine and Phenanthridine Hydroperoxides in Cell-free DNA

Abstract— The acridine and phenanthridine hydroperoxides 3 and 7 were synthesized as photochemical hydroxyl radical sources for oxidative DNA damage studies. The generation of hydroxyl radicals upon UVA irradiation (Λ. = 350 nm) was verified by trapping experiments with 5,5-di-methyl-1-pyrroline N -oxide and benzene. The enzymatic assays of the damage in cell-free DNA from bacteriophage PM2 caused by the acridine and phenanthridine hydroperoxides 3 and 7 under near-UVA irradiation revealed a wide range of DNA modifications. Particularly, extensive single-strand break formation and DNA base modifications sensitive to formamidopyrimidine DNA glycosylase (Fpg protein) were observed. In the photooxida-tion of calf thymus DNA, up to 0.69±0.03% 8-oxo-7,8-dihydroguanine was formed by the hydroperoxides 3 and 7 on irradiation, whose yield was reduced up to 40% in the presence of the hydroxyl radical scavengers mannitol and fert-butanol. The acridine and phenanthridine hydroperoxides 3 and 7 also induce DNA damage through the type I photooxidation process, for which photoinduced electron transfer from 2'-deoxyguanosine to the singlet states of 3 and 7 was estimated by the Rehm-Weller equation to possess a negative Gibb's free energy of cα -5 kcal/ mol. Control experiments with the sensitizers acridine 1 and the acridine alcohol 4 in calf thymus and PM2 DNA confirmed the photosensitizing propensity of the UVA-ab-sorbing chromophores. The present study emphasizes that for the development of selective and efficient photochemical hydroxyl radical sources, chromophores with low photosensitizing ability must be chosen to avoid type I and type II photooxidation processes.