Stability of highly compressed spread monolayers of ¹²⁵ I-labelled and cold BSA

Abstract

Compression-expansion cycles were performed on BSA spread monolayers at the air/aqueous interface using both unlabelled and radioiodinated protein. Irreversible losses in area were observed when films were held at surface pressures above 18 mNm⁻¹ although the remaining films were qualitatively unchanged. Film was transferred quantitatively from the labelled monolyer to glass slides by a Blodgett-type technique and the measured specific activity used to evaluate the surface density of protein. Constancy of specific activity while films were maintained at high pressure confirmed that area losses were caused by desorption and that there was no selective desorption of tagged protein molecules. Specific activity measurements of aliquots of the substrate verified desorption and were in agreement with the amounts of protein calculated from monolyaer losses. Rates of desorption increased with increasing surface pressure and were higher for the radiolabelled protein above a critical surface pressure. A decrease in activity of BSA monolayers, much greater than that corresponding to normal decay of ¹²⁵ I, was observed when they were left for extended periods, apparently due to loss of iodine from the protein. The technique of transferring radiolabelled film to glass slides for analysis appears to have much potential for studying the stability of proteins at surfaces both alone and in association with other surface active substances.