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The diffusion of tracer proteins at low concentration was measured in solutions containing "background" proteins at concentrations of up to 200 g/liter. The fractional reduction of the diffusion coefficient of tracer in the presence of a given weight/volume concentration of background species generally increases with increasing size of tracer species and with decreasing size of background species. The dependence of the diffusion constants of three out of four tracer species upon the concentrations of four background species is accounted for semiquantitatively by a simple hard particle model. Extrapolation of model calculations to higher background concentrations suggests that in solutions containing proteins at concentrations comparable to those found in biological fluid media, the diffusive transport of larger proteins and aggregates may be slower than in dilute solution by several orders of magnitude.It has become increasingly appreciated that the kinetics and equilibria of important biological reactions, particularly those involving reversible self-or heteroassociation of macromolecules, may be greatly influenced by the high total concentration of macromolecules in many physiological fluid media (1). The rate and/or extent of a particular association reaction as observed in a particular physiological fluid medium may differ qualitatively from those of the same reaction in a solution that is identical in chemical composition except for the absence of significant amounts of so-called "inert" or unreactive macromolecules. These differences can be attributed in large part to exclusion of reactants from the volume of solution occupied by inert species (and vice versa). Hence media containing high total macromolecular content have been termed "volume-occupied" or "crowded" (2).Absolute reaction rate theory predicts that rate constants for association of globular proteins and other compact macromolecules should increase monotonically with increasing volume occupancy until the association reaction becomes diffusion limited (1, 2). Since volume occupancy hinders diffusion (3), further increases would be expected to bring about a decrease in association rate (1). Because quantitative information about the diffusion of proteins in highly volumeoccupied solutions is sparse, the consequences and possible biological significance of diffusion-limited kinetics in crowded media have not, heretofore, been considered.We have developed an automated method for the determination of diffusion coefficients through measurement of boundary spreading, which is well-suited to the measurement of the diffusion of a dilute "tracer" species in a solution containing an arbitrary concentration of a second "background" species. By using this method, we have measured the diffusion coefficients of each of four globular tracer proteins, ranging in size from Mr 17,000 to Mr 150,000, as functions of the concentrations of four globular background proteins ranging in size from Mr 13,000 to Mr 150,000.The method used to measure tracer diffusi...

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In an attempt to prepare monodisperse poly(D,L-lactide) and copoly(lactide-glycolide) microspheres, a novel emulsification technique (membrane emulsification) was employed and the preparation conditions which might affect the monodispersity were evaluated. With this technique nearly monodisperse poly(D,L-lactide) and copoly(lactide-glycolide) microspheres were successfully prepared and their sizes were controllable only by making use of microporous glass membranes of different pore sizes. However, in the present system of emulsion (methylene chloride/water) the surfactant used was limited to ionic ones and the amount of polymers available for the formation of microspheres was inevitably too small in concentration to entrap sufficient amounts of drug. As for the drug release, the effect of particle size was not appreciable but the method of solvent removal gave a great influence; the solvent extraction method showed a more drug-sustaining effect than did the solvent evaporation method. The present results suggest the possibility of making drug-loaded and biodegradable monodisperse microspheres.