2011 Annual Science Report
University of Wisconsin Reporting | SEP 2010 – AUG 2011
Project 2B: Proto-Cell Membrane Evolution May Have Been Directed by Mineral Surface Properties
Metal oxides have been studied widely in the biogeochemical literature for understanding the adsorption and other surface interactions of dissolved organic and inorganic molecules with mineral surfaces. The goal of our study is to understand whether the earliest lipid membranes or “protocells” would have been stable in contact with different mineral surfaces on early Earth, and whether the surface properties of the minerals control their relative affinity to cell membranes. In previous years of this study, we used bulk adsorption isotherms and classical DLVO theory modeling approaches to examine the stability lipid bilayers in contact with micron-sized quartz (α-SiO2), rutile (α-TiO2) and corundum (α-Al2O3) particles. By understanding the role of natural geochemical parameters such as mineral surface chemistry, solution chemistry and temperature cycling on protocell membrane stability, we attempted to model potential aqueous environments where life may have originated such as lacustrine, tidal pool, and sub-aerial or submarine hydrothermal vents. In the present project year, we used neutron reflectivity to determine if the geometry of the mineral surface (sub-spherical particles versus planar single crystal surfaces) affects membrane stability. The results of our various approaches were consistent showing that lipid membrane stability depends on (1) lipid head-group charge and (2) surface charge of the mineral, which in turn depend on pH, ionic strength, presence or absence of Ca2+, (3) van der Waals interactions, and (4) relative hydrophobicity of the surface, as well as purely physical parameters such as relative size of the model membrane relative to the mineral surface. Our project addresses NASA Astrobiology Institute’s (NAI) Roadmap goals of understanding the origins of cellularity and the evolution of mechanisms for survival at environmental limits, and NASA’s Strategic Goal of advancing scientific knowledge of the origin and evolution of the Earth’s biosphere and the potential for life elsewhere.
Keywords: lipid, protocell, vesicle, self-assembly, pre-biotic, mineral surface, hydrophilic, hydrophobic, bilayer
Metal oxides have been studied widely in the biogeochemical literature for understanding the adsorption and other surface interactions of dissolved organic and inorganic molecules with mineral surfaces. In the present project year, we report on neutron reflectivity results to study the role of surface chemistry of sapphire (α-Al2O3) (1120) in controlling the structure of supported bilayers (SPBs) of dipalmitoylphosphatidylcholine (DPPC). Specifically, we studied the effects of surface charge, charge screening, and binding of Ca 2+ by adjusting solution pH, ionic strength, and the absence or presence of divalent Ca2+. Our study is unique in focusing on the surface chemistry effects of the supporting oxide compared to previous neutron reflectivity studies of SPBs that have focused on elucidating bilayer structure responses to external stimuli such as an applied electric field or UV illumination. Our study is also the first neutron reflectivity study on bilayers deposited directly on sapphire.
We find that the amount of DPPC adsorbed increases as the surface charge of the sapphire substrate becomes increasingly positive. These results agree with our previous work on SPB formation and structure at (i) micron-size α-Al2O3 particle surfaces using adsorption isotherms, and (ii) using single crystal planar surfaces by atomic force microscopy, confirming the consistency of surface chemistry effects across substrate morphologies. We also found that the effects of increasing ionic strength (set with NaCl) and adding Ca2+, are competitive rather than additive, with Ca2+ providing the dominant control, which has not been described previously. Finally, our results suggest that the iso-electric point for (1120) the sapphire surface is closer to that of corundum particles (~ 9), contrary to lower values (~ 5 – 6) suggested by previous studies of planar sapphire surfaces.