Fourier Transform Infrared Spectroscopic Studies of Cross‐Linked Human Serum Albumin Microcapsules. 3. Influence of Terephthaloyl Chloride Concentration on Spectra and Correlation with Microcapsule Morphology and Size

Abstract

Microcapsules were prepared from human serum albumin (HSA) by interfacial cross‐linking with terephthaloyl chloride (TC). TC concentrations were increased from 0.5 to 5% w/v, while pH (9.8) and reaction time (30 min) were kept constant. Fourier transform infrared (FT‐IR) spectra of lyophilized microcapsules were compared. Correlations were established with microcapsule morphology and size. The results were compared with those of previous studies exploring pH or reaction time and with those of parallel determinations of microcapsule free amino groups. With 0.5% TC, decreases of the ester‐assigned 1724‐cm−1band area and of the carboxylate‐assigned 1394‐cm−1band area were observed compared with pure HSA. This phenomenon was attributed to a removal of contaminating lipids of HSA. Increasing TC concentration resulted in a progressive increase of the band areas at 1724 cm−1(esters) and 1795 cm−1(anhydrides), in a further decrease of the 1394‐cm−1band area (carboxylates), and in marked alterations of the 1340–1080‐cm−1region. These changes, which revealed the progressive acylation of hydroxy and carboxylate groups of HSA, were accompanied by an increase of the 1624‐cm−1band area (β‐sheet), reflecting interchain H‐bonding due to cross‐linking. As observed in the previous studies of pH and reaction time, important spectral changes corresponded to low values of ‐NH2content, to a decrease in microcapsule mean size (from >30 to <15 µm), and to modifications of the membrane surface (made rough).