Fourier-transform infrared spectroscopic studies of cross-linked human serum albumin microcapsules. 2. Influence of reaction time on spectra and correlation with microcapsule morphology and size.

Microcapsules were prepared from human serum albumin (HSA) through interfacial cross-linking with terephthaloyl chloride (TC). Reaction times were increased from 2 to 60 min, while pH (9.8) and TC concentration (2.5% w/v) were kept constant. Fourier-transform infrared (FT-IR) spectra of lyophilized microcapsules were compared. Correlations were established with microcapsule morphology and size, as had been done in a previous study exploring the effect of increasing pH values. Microcapsules obtained after 2 min had to be considered separately. Minor alterations were observed in the spectrum as compared with pure HSA. They consisted of a decrease of the ester-assigned 1724-cm-1 band and of the carboxylate-assigned 1394-cm-1 band, attributed to a removal of contaminating lipids of HSA, and an increase of the 1624-cm-1 band, attributed to interchain H bonding following acylation of the NH2 groups. Prolonging the reaction time resulted in a progressive increase of the bands at 1724 (esters), 1795 (anhydrides), and 1624 cm-1 (beta-sheet), in a further decrease of the 1394-cm-1 band (carboxylates), and in marked alterations of the 1340-1080-cm-1 region. These important changes, which appeared after 5 min, reflect the progressive acylation of the hydroxy and carboxylate groups of HSA. As in the previous series of pH-based assays, important spectral changes were shown to correspond to a decrease in microcapsule mean size (from 32 to < 15 microns) and in important modifications of the membrane surface, made rough.