INTERACTION OF ANIONIC COMPOUNDS WITH GELATIN .1. BINDING-STUDIES

Even though gelatin is the most widely used polymeric excipient in pha rmaceutical products, scant attention has been paid to its interaction with small organic molecules. The present work deals with the interac tion of gelatin and four monosulfonated or monocarboxylated azo dyes h aving hydrocarbon moieties of different sizes. These dyes were used as models for anionic drugs, which make up a significant percentage of a ll new drugs. Most binding studies used ultrafiltration to separate fr ee from bound dye, followed by spectrophotometric dye assays. One bind ing study was based on the shift in pH when a dye was added to gelatin solutions. All binding isotherms consisted of two linear segments. Th e initial segments, which start at the origin, represent the partition ing of the dyes between the dissolved gelatin and the aqueous buffer s olution. They changed abruptly to horizontal plateaus, which represent the binding limit. Increases in pH from 5.00 to 7.00 reduced the bind ing of the sulfonated dyes but increased the binding of the carboxylat ed dye. At pH greater than or equal to 7.00, where even the carboxylic acid groups are fully ionized, the carboxylated dye and its sulfonate d analog were bound to gelatin to the same extent. The binding of all dyes decreased with increasing temperature (i.e., the standard enthalp y of binding was negative), with a change of the solvent medium from w ater to 0.15 M ammonium acetate, and with decreasing size of the hydro carbon moieties of the dyes. The binding of the dyes to gelatin was al ways reversible and the standard entropy change associated with it was negative. At the experimental conditions chosen, particularly pH valu es at least 1.9 units below the isoionic point of the gelatin of 8.9, electrostatic attraction between the dye anions and the basic sites of gelatin was the major binding force. Hydrophobic effects played a sec ondary but perceptible role, causing the dyes with the largest hydroca rbon moieties to be bound the most strongly and extensively to the gel atin.