We studied the hydration characteristics of room-temperature ionic liquids (IL). We experimentally determined the excess chemical potentials, , the excess partial molar enthalpies, , and the excess partial molar entropies in IL−H2O systems at 25 °C. The ionic liquids studied were 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim]BF4) and the iodide ([bmim]I). From these data, the excess (integral) molar enthalpy and entropy, and , and the IL−IL enthalpic interaction, , were calculated. Using these thermodynamic data, we deduced the mixing schemes, or the “solution structures”, of IL−H2O systems. At infinite dilution IL dissociates in H2O, but the subsequent hydration is much weaker than for NaCl. As the concentration of IL increases, [bmim]+ ions and the counteranions begin to attract each other up to a threshold mole fraction, x IL = 0.015 for [bmim]BF4 and 0.013 for [bmim]I. At still higher mole fractions, IL ions start to organize themselves, directly or in an H2O-mediated manner. Eventually for x IL > 0.5−0.6, IL molecules form clusters of their own kind, as in their pure states. We show that , a third derivative of G, provided finer details than and , second derivatives, which in turn gave more detailed information than and , first derivative quantities.

It is well established that small sugars exert different types of stabilization of biomembranes both in vivo and in vitro. However, the essential question of whether sugars are bound to or expelled from membrane surfaces, i.e., the sign and size of the free energy of the interaction, remains unresolved, and this prevents a molecular understanding of the stabilizing mechanism. We have used smallangle neutron scattering and thermodynamic measurements to show that sugars may be either bound or expelled depending on the concentration of sugar. At low concentration, small sugars bind quite strongly to a lipid bilayer, and the accumulation of sugar at the interface makes the membrane thinner and laterally expanded. Above ∼0.2 M the sugars gradually become expelled from the membrane surface, and this repulsive mode of interaction counteracts membrane thinning. The dual nature of sugar-membrane interactions offers a reconciliation of conflicting views in earlier reports on sugar-induced modulations of membrane properties.membrane interface | membrane structure | preferential binding | preferential exclusion | interaction free energy S mall sugars such as the disaccharides sucrose and trehalose are among the so-called osmolytes (1) or compensatory solutes (2), which are accumulated in response to environmental stress in virtually all taxa. Their function is to act as inert regulators of the osmotic pressure, but they also optimize the physical properties of the cytosol (3) and stabilize biomolecular conformations against cold, drought, and heat (4-7). The same small carbohydrates have also proven useful in vitro as protectants or excipients for biopreservation (8). Many reports have shown that membranous structures are particularly stabilized by small sugars (4, 6, 9), but the definition of stabilization covers a wide range of biological and physical parameters. Thus, studies on intact cells have documented improved survival following exposure to heat, cold, drought, or chemical stressors (6,10,11). Other works have analyzed stabilization on the basis of phenomenological properties of model membranes, for example, the leakage or intermixing of probes in liposomes (12, 13). Finally, stability has been discussed with respect to rigorous physical parameters such as the structure or mechanical properties of lipid bilayers (14, 15). The current work addresses membrane dimensions and the thermodynamics of interaction with the purpose of elucidating fundamental aspects of membrane-sugar interrelationships. The different observations of sugar stabilization have sparked a large number of studies on sugars and model membranes (usually phospholipid bilayers) over the past 30 y. Investigations of fully hydrated membranes show an interesting tendency to fall into two groups with mutually conflicting conclusions. Thus, many investigations have suggested direct (favorable) interaction of sugars and the phospholipid interface (16-23), and it is obvious that such interactions could be the origin of sugar effects, for example, through int...

Background:The molecular understanding of factors that limit enzymatic hydrolysis of cellulose remains incomplete. Results: Pre-steady-state analysis of cellulolytic activity provides rate constants for basic steps of the overall reaction. Conclusion: Slow dissociation of inactive enzyme-cellulose complexes governs the hydrolytic rate at pseudo-steady state. Significance: Kinetic constants elucidate molecular mechanisms and structure-function relationships for cellulases.

The excess partial molar enthalpy of 1-propanol (1P), , was measured at 28 degrees C in the ternary mixture of 1P-1-butyl-3-methylimidazolium chloride ([bmim]Cl)-H(2)O in the H(2)O-rich composition range. From these data we evaluated what we call the 1P-1P enthalpic interaction function, . Its changes induced by addition of [bmim]Cl of the pattern of were used as a probe to elucidate the effect of [bmim]Cl on the molecular organization of H(2)O. It was found that the effect of Cl(-) was not conspicuous within this methodology, and the observed dependence is predominantly due to the hydration of [bmim](+). The changes in the x(1P)-dependence of were compared with those brought about by temperature increase, or by the addition of fructose or glycerol. It was found that the effect of [bmim](+) is similar to that of fructose or increased temperature. We speculate that in the H(2)O-rich composition region a number of H(2)O molecules are attracted to the delocalized positive charge of the imidazolium ring and the bulk of H(2)O is influenced in such a manner that the global hydrogen bond probability is reduced.