FREE AMINO GROUP CONTENT OF SERUM-ALBUMIN MICROCAPSULES .3. A STUDY AT LOW PH VALUES

Microcapsules were prepared by interfacial cross-linking of human seru m albumin (HSA) with terephthaloyl chloride (TC). Determination of fre e amino groups was performed on lyophilized microcapsules with trinitr obenzenesulfonic acid (TNBS). In two series of assays conducted with p hosphate buffers, pH 7.4 and 8, respectively, microcapsules exhibited a high -NH2 content (> 330 mu mol/g dry weight), with only a slight de crease when raising TC concentration from 2.5 to 5% TC (w/v) and/or re action time from 30 to 60 min. When the phosphate buffer, pH 7.4 (ioni c 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 con tent was reduced from 346 to 134 and 168 mu mol/g, respectively. Contr ol batches prepared with the acetate buffer without adding CaCl2 also had a reduced -NH2 content (182 mu mol/g), while a higher value (302 m u 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 microcapsu les prepared from phosphate buffers at the same pH values (438 and 388 mu mol/g, respectively).