Understanding the mechanism for sucroseinduced protein stabilization is important in many diverse fields, ranging from biochemistry and environmental physiology to pharmaceutical science. The current study measures the preferential exclusion of 1 M sucrose from a protein drug, recombinant interleukin 1 receptor antagonist (rhIL-1ra). It is proposed that the degree of preferential exclusion and increase in chemical potential are directly proportional to the protein surface area and that, hence, the system will favor the protein state with the smallest surface area. This mechanism explains the observed sucroseinduced restriction of rhIL-1ra conformational f luctuations, which were studied by hydrogen-deuterium exchange and cysteine reactivity measurements. Furthermore, infrared spectroscopy of rhlL-1ra suggested that a more ordered native conformation is induced by sucrose. Electron paramagnetic resonance spectroscopy demonstrated that in the presence of sucrose, spin-labeled cysteine 116 becomes more buried in the protein's interior and that the hydrodynamic diameter of the protein is reduced. The preferential exclusion of sucrose from the protein and the resulting shift in the equilibrium between protein states toward the most compact conformation account for sucrose-induced effects on rhIL-1ra.

No abstract

No abstract

The behaviour of a Lennard-Jones fluid confined within a straight cylindrical pore has been studied using mean-field theory. The fluid potential parameters were chosen to model argon, and a range cf wall-fluid parameters, including values approximating carbon dioxide and graphite walls, was investigated. We calculated the density profile and grand potential of the fluid, and examined the effect of varying the pore radius, pressure, temperature, and strength of the wall-fluid fxces on these properties, and especially on the gas-liquid phase transitions that occur. We found that the gas-liquid transition occurs at pressures below the bulk fluid vapour pressure in all cases studied. For a fixed temperature, when the pore radius is decreased the gas-liquid coexistence curve ends in a critical point, as has been observed for fluids between parallel plates. The strength of the wall-fluid forces had a dramatic effect on the phase diagram, changing both the range of pore sizes in which phase transitions occur, and the effect of temperature on them.

No abstract

Simulated annealing is applied to the determination of distributions of silicon and aluminum atoms on the faujasite lattice from 29Si NMR data. The method is described and compared to others. The local silicon environments, Si(nAl), n = 0−4, are reproduced to high accuracy by the technique. More detailed features of local structure are then available, such as numbers of Al on different ring systems in the zeolite. Dempsey's rule is found to be strictly followed in the four rings of the zeolite X sample studied, while this is not the case for zeolite Yin agreement with previous work. The results are consistent with a possible small violation of Lowenstein's rule in the zeolite X sample.

Results from a molecular dynamics simulation of xenon in silicalite at 298 K and 4 atoms per unit cell (A&, = -26.9 kJ/mol, D = 1.86 X m2/s) are in good agreement with the experimental value of -24.5 kJ/mol and the diffusion coefficient derived from the NMR pulsed field-gradient method (4.00 X m2/s). The diffusivity is predicted to be negligible at temperatures around 77 K and then increases over the investigated range to D = 3.25 X m2/s at 450 K, yielding an activation energy of 5.5 kJ/mol. Increasing the concentration from 4 to 16 atoms per unit cell results in a decreased internal energy of adsorption (-28.1 kJ/mol) and a decreased diffusion coefficient (D = 0.37 X lo4 m2/s). The anisotropy of diffusion is also examined. IntroductionComputer simulations of zeolite/adsorbate systems are currently of increasing importance in the context of understanding the catalytic properties of zeolites. They are used in conjunction with experimental techniques but possess the significant advantage that a wide range of conditions may be simulated, conditions which may be difficult to obtain in an experimental setup. Early based upon molecular mechanics procedures, employed simple atom-atom potentials to calculate heats of adsorption and adsorption isotherms and to estimate diffusion coefficients. More recently, adsorption sites and potential energy maps were calculated for xenon in zeolite rho,3 pyridine in zeolite L,4 and benzene in silicalite and zeolite theta-1.5 Monte Carlo simulations have been employed as a useful means of probing the potential energy space of zeolite/adsorbate systems as a function of temperature and sorbate concentration. Yashonath et al. examined the temperature dependence of the behavior of methane in zeolite Y6 and Smit and den Ouden the adsorption of methane in mordenite with variable zeolite composition.' More recently the molecular dynamics (MD) technique has been applied to such systems with a view to obtaining time-dependent data such as diffusion coefficients for methane and benzene in zeolite NaY8s9 and water in ferrierite.I0 These studies have been based upon the approximation of a rigid zeolitic framework, but MD framework simulations of natrolite" and zeolite AIz have been reported recently by Demontis et al.The emergence of '29Xe N M R as a means of probing the internal structure of zeolitesI3 and of exploring xenon as an adsorbate14 prompted us to investigate the silicalite/xenon system using the MD technique. There is also the added value that the diameter of xenon is close to that of methane, a molecule of increasing importance in heterogeneous catalysis. We developed three separate computer programs which were first used to investigate the test system, xenon in silicalite, at 298 K and a concentration of 4 xenon atoms per unit cell, followed by the use of the separate programs for varying the system temperature from 77 to 450 K and the adsorbate concentration from 4 to 16 ad-

No abstract