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Conformational Changes in SP-B as a Function of Surface Pressure

Wilfred K. Fullagar ^*, Karen A. Aberdeen ^*, David G. Bucknall , Paulus A. Kroon ^* and Ian R. Gentle ^*

^* School of Molecular and Microbial Sciences, The University of Queensland, Brisbane, Queensland 4072, Australia; and Department of Materials, University of Oxford, Oxford OX1 3PH, United Kingdom

Correspondence: Address reprint requests to Dr. Ian Gentle, School of Molecular and Microbial Sciences, The University of Queensland, Brisbane, Queensland 4072, Australia. Tel.: +61-7-3365-4800; Fax: +61-7-3365-4299; E-mail: i.gentle{at}uq.edu.au.

X-ray reflectivity of bovine and sheep surfactant-associated protein B (SP-B) monolayers is used in conjunction with pressure-area isotherms and protein models to suggest that the protein undergoes changes in its tertiary structure at the air/water interface under the influence of surface pressure, indicating the likely importance of such changes to the phenomena of protein squeeze out as well as lipid exchange between the air-water interface and subphase structures. We describe an algorithm based on the well-established box- or layer-models that greatly assists the fitting of such unknown scattering-length density profiles, and which takes the available instrumental resolution into account. Scattering-length density profiles from neutron reflectivity of bovine SP-B monolayers on aqueous subphases are shown to be consistent with the exchange of a large number of labile protons as well as the inclusion of a significant amount of water, which is partly squeezed out of the protein monolayer at elevated surface pressures.