Effect of mechanical denaturation on surface free energy of protein powders

Globular proteins are important both as therapeutic agents and excipients. However, their fragile native
conformations can be denatured during pharmaceutical processing, which leads to modification of
the surface energy of their powders and hence their performance. Lyophilized powders of hen eggwhite
lysozyme and �-galactosidase from Aspergillus oryzae were used as models to study the effects
of mechanical denaturation on the surface energies of basic and acidic protein powders, respectively.
Their mechanical denaturation upon milling was confirmed by the absence of their thermal unfolding
transition phases and by the changes in their secondary and tertiary structures. Inverse gas chromatography
detected differences between both unprocessed protein powders and the changes induced by their
mechanical denaturation. The surfaces of the acidic and basic protein powders were relatively basic,
however the surface acidity of �-galactosidase was higher than that of lysozyme. Also, the surface of
�-galactosidase powder had a higher dispersive energy compared to lysozyme. The mechanical denaturation
decreased the dispersive energy and the basicity of the surfaces of both protein powders. The amino
acid composition and molecular conformation of the proteins explained the surface energy data measured
by inverse gas chromatography. The biological activity of mechanically denatured protein powders can
either be reversible (lysozyme) or irreversible (�-galactosidase) upon hydration. Our surface data can be
exploited to understand and predict the performance of protein powders within pharmaceutical dosage
forms.