pH-Dependence Described for Key Membrane Bilayer Properties

“In nature, living things function at a delicate balance: acidity, temperature, all its surroundings must be within specific limits, or they die,” said Prof. Monica Olvera de la Cruz of Northwestern’s McCormick School of Engineering; “When living things can adapt, however, they are more functional. We wanted to find the specific set of conditions under which bilayers, which control so much of the cell, can morph in nature.”

In bilayer membranes, the two layers of amphiphile molecules form a crystalline shell around its contents. The density and arrangement of the molecules determine the membrane’s porosity, strength, and other properties. Taking advantage of the ionizable charge in the head groups, the team coassembled dilysine (+2) and carboxylate (-1) amphiphile molecules of varying hydrophobic-tail lengths into bilayer membranes at various (physiologically relevant) pH levels, which changed the effective charge of the heads. Then, using X-ray scattering technology at the DuPont-Northwestern-Dow Collaborative Access Team (DND-CAT) at Argonne National Laboratory’s Advanced Photon Source, the researchers analyzed the resulting crystallization formed by the bilayer molecules. Freezing has generally been used to produce electron microscope images of membrane structures, however this process is labor-intensive and changes the structural fidelity, making it less relevant for understanding membrane assembly and behavior under physiological conditions.

From the results, the researchers found that most molecules did not notably respond to the change in acidity, but for those that possessed a critical tail length (which correlates to the level of hydrophylia) the charge of the heads changed to the extent that their two-dimensional crystallization morphed from a periodic rectangular-patterned lattice in more basic pH solutions to a hexagonal lattice in more acidic pH solutions. Shells with a higher symmetry (e.g., hexagonal) are stronger and less brittle than those with lesser symmetry. The change in pH also altered bilayer thickness and compactness. Changing the crystallinity, density, and spacing of molecules within membranes could help researchers control diffusion rates and the encapsulation and release efficiency of molecules in vesicles, which would further shed light on cell function and could enable advances in drug delivery and other bio-inspired technology.

The study was published ahead of print online September 24, 2013, in the journal Proceedings of the National Academy of Sciences of the United States of America (PNAS).

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McCormick School of Engineering and Applied Science at Northwestern University