Free amino group content of serum albumin microcapsules. III. A study at low pH values

Abstract

Microcapsules were prepared by interfacial cross-linking of human serum albumin (HSA) with terephthaloyl chloride (TC). Determination of free amino groups was performed on lyophilized microcapsules with trinitrobenzene-sulfonic acid (TNBS). In two series of assays conducted with phosphate buffers, pH 7.4 and 8, respectively, microcapsules exhibited a high −NH 2 content (> 330 μmol/g dry weight), with only a slight decrease when raising TC concentration from 2.5 to 5% TC (w/v) and/or reaction time from 30 to 60 min. When the phosphate buffer, pH 7.4 (ionic strength (μ): 0.40), was replaced by an acetate buffer, pH 7.4 (μ: 0.66), supplemented with 0.125 or 0.25 M CaCl 2, microcapsule −NH 2 content was reduced from 346 to 134 and 168 μmol/g, respectively. Control batches prepared with the acetate buffer without adding CaCI 2 also had a reduced −NH 2 content (182, μmol/g), while a higher value (302 μmol/g) was found for microcapsules prepared using a phosphate buffer with a 0.66 ionic strength. Finally, a sharp decrease in −NH 2, content was also found for microcapsules prepared from acetate buffers at pH 5.9 (174 μmol/g) and 6.8 (158 μmol/g), as compared with microcapsules prepared from phosphate buffers at the same pH values (438 and 388 μmol/g, respectively).