Interfacial properties of B phycoerythrin extracted from the red microalga Rhodosorus marinus at hexadecane-water and air-water interfaces

In this work we report the effect of pH on the fluorescence spectra and on the interfacial behaviour at the air-water and hexadecane-water interfaces of the protein B phycoerythrin, extracted and purified from the red microalga Rhodosorus marinus. We performed isotherms and compressionexpansion cycles and demonstrate that the stability of B phycoreythrin monolayers at the air-water interface depends on its aggregation state conditioned by pH in the subphase. Stable monolayers were obtained for pH 4.5, 7 and 2.5 except for pH 10 were B phycoerythrin molecules showed to be dissociated. At the hexadecane-water interface and for pH 4.5 and 7, protein molecules were adsorbed producing a higher surface pressure in comparison with the surface pressure obtained with pH 2.5 and 10, probably due to the enhancement of hydrophobic interactions favoured by a globular conformation, also found at the air-water interface. Atomic force microscopy images of Langmuir-Blodgett monolayers showed higher monolayer homogeneity for pH 4.5 and 7. Fluorescence emission intensity was higher for pH 4.5 and 7 where B PE molecules remain in a more compact conformation.