Free amino group content of serum albumin microcapsules .3. A study at low pH values

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 trinitrobenzenesulfonic acid (TNBS). In two series of assays conducted with phosphate buffers, pH 7.4 and 8, respectively, microcapsules exhibited a high -NH2 content (> 330 mu mol/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 (mu): 0.40), was replaced by an acetate buffer, pH 7.4 (mu: 0.66), supplemented with 0.125 or 0.25 M CaCl2, microcapsule -NH2 content was reduced from 346 to 134 and 168 mu mol/g, respectively. Control batches prepared with the acetate buffer without adding CaCl2 also had a reduced -NH2 content (182 mu mol/g), while a higher value (302 mu mol/g) was found for microcapsules prepared using a phosphate buffer with a 0.66 ionic strength. Finally, a sharp decrease in -NH2 content was also found for microcapsules prepared from acetate buffers at pH 5.9 (174 mu mol/g) and 6.8 (158 mu mol/g), as compared with microcapsules prepared from phosphate buffers at the same pH values (438 and 388 mu mol/g, respectively).