Simulations of a lattice model of two-headed linear amphiphiles: influence of amphiphile asymmetry

Using a 2D lattice model, we conduct Monte Carlo simulations of micellar aggregation of linear-chain amphiphiles having two solvophilic head groups. In the context of this simple model, we quantify how the amphiphile architecture influences the critical micelle concentration (CMC), with a particular focus on the role of the asymmetry of the amphiphile structure. Accordingly, we study all possible arrangements of the head groups along amphiphile chains of fixed length N = 12 and 16 molecular units. This set of idealized amphiphile architectures approximates many cases of symmetric and asymmetric gemini surfactants, double-headed surfactants, and boloform surfactants. Consistent with earlier results, we find that the number of spacer units s separating the heads has a significant influence on the CMC, with the CMC increasing with s for s < N/2. In comparison, the influence of the asymmetry of the chain architecture on the CMC is much weaker, as is also found experimentally.     

Hammett correlations in the chemistry of 3-phenylpropyl radicals

The energetics and kinetics of the reaction of variously substituted benzyl radicals with a model alkene were calculated at the G3(MP2)-RAD//B3-LYP/6-31G(d) level of theory to determine whether such reactions are amenable to Hammett analysis. The reactions were studied both in the gas phase and in toluene solution in the temperature range 298-353 K; calculations include 1D-hindered rotor corrections for low frequency torsional modes, and the solvation energies were calculated using COSMO-RS at the BP/TZP level of theory. The addition reaction was found to be dominated by radical stabilization effects, but under circumstances where olefin substituent effects were decoupled from aryl substituent effects, a modest polar effect comes into play, which is enhanced by solvation. Reasonable correlations with empirical substituent parameters such as Hammett σ and σ(?) are observed for the enthalpy of activation, but additional entropic factors act to decrease the degree of correlation with respect to free energies and rate coefficients, confirming hypotheses from earlier experimental work. Substituent effects on the reverse β-fragmentation reaction, and potential cyclization of the 3-phenylpropyl radicals formed by addition are also discussed.     

Conformational studies of the robust 2-Cys peroxiredoxin Salmonella typhimurium AhpC by solution phase hydrogen/deuterium (H/D) exchange monitored by electrospray ionization mass spectrometry

This is the first comprehensive HX-MS study of a "robust" 2-Cys peroxiredoxin (Prx), namely Salmonella typhimurium AhpC (StAhpC). Prx proteins control intracellular peroxide levels and are abundant antioxidant proteins in eukaryotes, archaea and bacteria. Crystal structural analyses and structure/activity studies of several bacterial and mammalian 2-Cys Prxs have revealed that the activity of 2-Cys Prxs is regulated by redox-dependent oligmerization and a sensitivity of the active site cysteine residue to overoxidation. The propensity to overoxidation is linked to the conformational flexibility of the peroxidatic active site loop. The HX-MS results emphasize the modulation of the conformational motility of the active site loop by disulfide formation. To obtain information on the conformational impact of decamer formation on the active site loop motility, mutants with Thr77 substituted by Ile, a decamer-disrupting mutation or by Val, a decamer-stabilizing mutation, were studied. For the isoleucine mutant, enhanced mobility was observed for regions encompassing the α4 helix located in the dimer-dimer interface and regions surrounding the peroxidatic loop. In contrast, the T77V mutation resulted in an increase in conformational stability in most regions of the protein except for the active site loop and the region encompassing the resolving cysteine.     

A semi-empirical determination of the Alkali metal conduction band relaxation energies during photoelectron emission

Using a semi-empirical method the alkali metal conduction band relaxation energies are determined to ca. 0.1 eV. Comparison with theoretical values given by Ley et al. shows that the localized hole model is superior to the nonlocalized hole model.     

Improved method for fitting falloff data

An extension of the dimensionless treatment of Troe is developed for fitting falloff data from unimolecular and recombination reactions. This method using dimensionless parametrizations derived from accurate numerical solutions of the master equation for system with any type of activated complex and energy transfer probability distribution, is as easily implemented as Troe's method, and allows global fitting of an entire set of pressure- and temperature-dependent rate coefficients. © 1994 John Wiley & Sons, Inc.     

Spin labelling of human erythrocytes with nitroxide radicals

Summary Human erythrocytes were labelled with nitroxide, the spin label SYNVAR 101, under various experimental conditions. A study was made of the influence of antireductants on the labelling efficiency and the kinetics of the radical decay during the labelling process. The antireductant hydrogen peroxide was effective in suppressing the decay, whereas ferricyanide was not very effective in this role. The nitroxide radical concentration in a nondialyzed plasma suspension of spin-labelled erythrocytes continuously decayed exponentially with a time constant of 35 min. Nitroxide radicals in nondialyzed plasma exhibited a similar decay to a final residual value which was small relative to the initial concentration. Dialyzing the plasma raised the residual concentration to three quarters of the initial value without changing the decay time.

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Riassunto Eritrociti umani sono marcati con ossido di azoto, il marchio di spin SYNVAR 101, in varie condizioni sperimentali. Si è fatto uno studio dell’influenza degli agenti antiriducenti sulla capacità di marcare e sulla cinetica del decadimento dei radicali durante il processo di marcatura. Il perossido d’idrogeno antiriducente era efficacie nel sopprimere il decadimento, mentre il ferrocianuro non era molto efficacie in questo roulo. La concentrazione dei radicali dell’ossido di azoto in una sospensione di plasma non dializzato degli eritrociti marcati secondo lo spin decade continuamente in maniera esponenziale con una costante di tempo di 35 min. I radicali dell’ossido di azoto nel plasma non dializzato esibiscono un decadimento simile al valore residuo finale che era piccolo in relazione alla concentrazione iniziale. Dializzare il plasma aumenta la concentrazione residua a tre quarti del valore iniziale senza cambiare il tempo di decadimento.

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Резюме В различных эксрериментальных условиях метятся человеческие эритроциты с окисью азота. Исследуются влияния антивосстановителя на эффективность нанесения метки и кинетику распада радикалов в процессе нанесения метки, Перекись водорода, как антивосстановитель, является эффективной для подавления распада, тогда как феррицианид не очень эффективен в этой. Концетрация радикалов окиси азота в суспензии недиализированной плазмы эритроцитов со спиновой меткой уменьшается эксоненциально с временной постоянной, равной 35 мин. Радикалы окиси азота в недиализированной плазме обнаруживают аналогичный распад, причем конечная остаточная величина концентрации оказывается малой по сравнений с начальной концентрацией. Диализация рлазмы увеличивает остаточную концентрацию до трех четвертых начальной без изменения времени распада.

     

On-column injection in the stopped-flow mode with open-tubular columns

Summary A simple, inexpensive, on-column injector of the septum moving-needle kind is described for quantitative analysis using open-tubular columsn. when operated in the stopped-flow mode an improvement incolumn efficiency is observed combined wit a diminished tendency to produce split-top peaks, particularly for late eluting peaks. Using electronic integration a relative error of about 2.7% was observed for peaks spanning a wide range of sample concentrations and volatilities.Dedicated to Professor A. Zlatkis on the occastion of his 60th birthday.