Adsorption and desorption behavior of plasmid DNA on ion-exchange membranes. Effect of salt valence and compaction agents

In this study, we report the effect of salt type and compaction agents on adsorption and desorption behavior of plasmid DNA on strong anion-exchange membranes. Both divalent cations and compaction agents are known to reduce the effective charge density of plasmid DNA in solution, and compaction agents decrease the radius of gyration of plasmids. Differences in the batch uptake adsorption of a 6.1 kilo base pair plasmid in solution with sodium and magnesium salts were observed at low ionic strengths. Recoveries at high salt conditions, however, were independent of the cation, and measured only 63-76%. Similarly, no improvement in recoveries were observed when using sulfate rather than chloride anions as displacers. The compaction agents, spermine and spermidine, showed no strong effect on the uptake adsorption, capacity, or recovery of three different-sized plasmids on membrane sheets. It is recommended that further efforts to improve plasmid recoveries from anion-exchange membranes focus on properties of the adsorbent surface.     

Position of coordination of the lithium ion determines the regioselectivity of demethylations of 3,4-dimethoxymorphinans with L-selectride

[reaction: see text] L-Selectride is an efficient agent for the 3-O-demethylation of opioids and is known to cleave the least hindered methoxyl group in a molecule. The treatment of a 3,4-dimethoxymorphinan containing a 6-ketal with L-Selectride gave selective 4-O-demethylation, rather than cleavage of the less hindered 3-methoxyl. In contrast, a 3,4-dimethoxymorphinan lacking a 6-ketal gave selective 3-O-demethylation, suggesting that the regiochemistry of L-Selectride-mediated O-demethylation can be manipulated through altering the position of coordination of the lithium ion.     

Effects of spiroconjugation on the calculated singlet-triplet energy gap in 2,2-dialkoxycyclopentane-1,3-diyls and on the experimental electronic absorption spectra of singlet 1,3-diphenyl derivatives. Assignment of the lowest-energy electronic transition of singlet cyclopentane-1,3-diyls

The effect of a 2,2-ethylene-ketal functionality on the singlet-triplet energy gap (Delta E(ST)) and on the first electronic transition in singlet cyclopentane-1,3-diyls (1) has been investigated. UDFT calculations predict a significant increase in the preference for a singlet ground state in the diradical with the cyclic ketal at C2 (1g; Delta E(ST) = -6.6 kcal/mol in C(2) symmetry and -7.6 kcal/mol in C(2v) symmetry), compared to the 2,2-dihydroxy- and 2,2-dimethoxy-disubstituted diradicals (1d, Delta E(ST) = -3.6 kcal/mol in C(2) symmetry, and 1e, Delta E(ST) = -3.4 kcal/mol in C(2) symmetry). Spiroconjugation is shown to be responsible for the larger calculated value of absolute value Delta E(ST) in 1g, relative to 1d and 1e. A strong correlation between the calculated values of Delta E(ST) and the computed electronic excitation energies of the singlet diradicals is found for diradicals 1d, 1e, and 1g and for 2,2-difluorocyclopentane-1,3-diyl (1c). A similar correlation between Delta E(ST) and lambda(calcd) is predicted for the corresponding 1,3-diphenylcyclopentane-1,3-diyls 3, and the predicted blue shift in the spectrum of 3g, relative to 3e, has been confirmed by experimental comparisons of the electronic absorption spectra of the annelated derivatives 2c, 2e, and 2g in a glass at 77 K. The wavelength of the first absorption band in the singlet diradicals decreases in the order 2e (lambda(onset) = 650 nm) > 2g (lambda(onset) = 590 nm) > 2c (lambda(onset) = 580 nm). The combination of these computational and experimental results provides a sound basis for reassignment of the first electronic absorption band in singlet diradicals 2c, 2e, and 2g to the excitation of an electron from the HOMO to the LUMO of these 2,2-disubstituted derivatives of cyclopentane-1,3-diyl.     

What accounts for the difference between singlet phenylphosphinidene and singlet phenylnitrene in reactivity toward ring expansion?

(8/8)CASSCF and (8/8)CASPT2 calculations have been performed in order to investigate the potential surface for the ring expansion of the (1)A(2) state of phenylphosphinidene (1c) to 1-phospha-1,2,4,6-cycloheptatetraene (3c). Unlike the comparable ring expansion of the (1)A(2) state of phenylnitrene (1b) to 1-aza-1,2,4,6-cycloheptatetraene (3b), ring expansion of 1c to 3c is computed to be quite endothermic. Nevertheless, cyclization of 1c, to form the bicyclic intermediate 2c in the ring expansion reaction, is computed to be only slightly more endothermic than the comparable cyclization reaction of 1b to 2b. The origins of these differences between the ring expansion reactions of 1b and 1c have been elucidated through the calculation of the energies of relevant isodesmic reactions.     

Reactions of sterically congested 1,5-hexadienes: Ab initio and DFT calculations on the competition between cope rearrangements and disrotatory cyclobutene ring-opening reactions of bridged syn-tricyclo[4.2.0.0(2,5)]octa-3,7-dienes

The thermolytic behavior of four syn-tricyclo[4.2.0.0(2,5)]octa-3,7-dienes, each spanned by four propano bridges (13, 14, 21, and 26), has been investigated by means of calculations at the UB3LYP/ 6-31G* and CASPT2/6-31G levels. These calculations predict that 13 should undergo a degenerate Cope rearrangement (E(A) = 19.6 kcal/mol), whereas the other three C(20)H(24) isomers should prefer a necessarily disrotatory cyclobutene ring-opening reaction. In the case of 14, the ring-opening reaction (E(A) = 27.2 kcal/mol) is concerted and leads directly to 15, a 4-fold bridged cyclooctatetraene. In the ring opening of 21, the 1,6-bridge in the 4-fold bridged bicyclo[4.2.0]octa-2,4,7-triene 31 prevents formation of the corresponding cyclooctatetraene. In the ring opening of 26, the 4-fold bridged bicyclo[4.2.0]octa-2,4,7-triene derivative 36 is predicted to form the corresponding bridged cyclooctatetraene 38, which should undergo bond shift. The results of these calculations are found to be in very good agreement with the results of experiments on these hydrocarbons.     

Performance and scale-up of adsorptive membrane chromatography

Separation efficiency and scalability of Pall Corporation's new Mustang stacked membrane chromatographic devices were investigated, using both the 10-ml and l(-1) models and comparing the responses of tracer pulses obtained for conventional and reverse-flow operation. Tracers included AMP, lysozyme, and thyroglobulin, which vary in relative molecular mass from less than 1000 up to 650000. Both devices showed marked insensitivity to tracer size and flow-rate and gave sharper peaks than would have been expected from conventional 15-microm bead packings. However, reverse-flow peaks were always significantly sharper than those for conventional operation, and the differences were ascribed primarily to non-uniform header residence times. Numerical simulations of the macroscopic flow confirmed that this was indeed the case. This problem was much less pronounced for the l(-1) device so scale-up is conservative.     

Locally mass-conserving Taylor–Hood elements for two- and three-dimensional flow

By supplementing the pressure space for Taylor–Hood elements, elements that satisfy continuity locally are produced. These elements are shown to satisfy the Babuska–Brezzi compatibility condition by using the patch argument. Two examples are presented, one illustrating the convergence rates and the other illustrating a difficulty with a Taylor–Hood element that is overcome by the element presented here.     

The free surface flow of Newtonian and non-Newtonian fluids trapped by surface tension

In this paper the position of the free surface of a swirling fluid held in by surface tension is calculated by the finite element method. A new locally mass-conserving quadratic velocity, linear pressure triangular element is used to overcome non-physical solutions produced by the well known Taylor-Hood element.