Probing interactions in OmpF Porin solutions via light scattering

Knowledge of protein structure and function is critical for understanding basic living cell function and interaction with its native environment. Protein structures are typically determined using X-ray crystallography, which is the only method that can locate the position of each amino acid residue and its constituent atoms in three-dimensional space for structures greater than 30 kDa. X-ray crystallography does, however, have an absolute prerequisite of large, high quality protein crystals.; To date, crystallization of macromolecular biological molecules has been most successful with soluble proteins. Membrane proteins have lagged significantly behind soluble proteins in terms of the number of structures determined and in terms of understanding the crystallization process. This is due to the amphiphilic nature of membrane proteins, which require detergents for solubilization and purification. These detergent molecules combine with protein molecules to form protein-detergent complexes.; This research attempts to develop a systematic method of crystallizing membrane proteins. The research supports prior suggestions that suggest that attractive interactions among the detergent portions of protein detergent complexes play a significant to dominant role in crystallization. This dissertation focuses on determining the role that detergent molecules play in crystallization of the membrane protein OmpF Porin. Attractive interactions among the detergent portions of protein detergent complexes that are optimal for crystallization were identified utilizing static light scattering measurements of the second osmotic virial coefficient (B₂₂). The B₂₂ behavior of protein-free micelles proved very similar to that of protein-detergent complexes suggesting that the detergent's contribution dominates the behavior of protein detergent complexes under crystallizing conditions. B₂₂ values for protein detergent complexes, as well as protein free micelles, were found to lie in Wilson's “Crystallization slot” under solution conditions conducive to crystallization. Attractive interactions between micelles increase as solution conditions approach the cloud point. The micelle size and molar mass were found to be constant as solution conditions were varied approaching the cloud point. Such information allows for the identification of which subset(s) of all possible crystallization conditions generate interaction potentials favorable for crystallization.