Free volume pockets inside a cell membrane play a prominent role in a variety of dynamic processes such as the permeability of small molecules across membranes and the diffusion of, e.g., lipids, drugs, and electron carriers in the plane of the membrane. Nonetheless, by now the chances for characterizing free volume voids in a nonperturbative manner through experiments have been very limited. Here we use lipid membranes as an example to show how positron annihilation spectroscopy (PALS) together with atomistic simulations can be employed to gauge changes in free volume pockets in biological macromolecular complexes. The measurements show that PALS is a viable technique to probe free volume in biomolecular systems. As examples, we consider the gel-to-fluid transition and the role of increasing cholesterol concentration in a lipid membrane. Further applications proposed in this work for PALS are likely to provide a great deal of insight into the understanding of the role of free volume in the dynamics of biomolecular complexes.
The function of mammalian ocular lens is to provide a sharp image to the retina. Accordingly, the lens needs to be transparent and minimize light scattering. To do so the lens fiber cells first loose intracellular organelles, organize the cytoplasm and arrange the fiber cell membranes. Because the fiber cells are metabolically inactive, the plasma membrane becomes the only cellular organelle and consequently, the phase behavior of these membranes determines the physiological state of the lens. Previous studies have shown that lipids extracted from the nuclear and cortical region of human lens show a temperature-induced phase transition close to the body temperature. Yet, the physiological function of this phase transition is not known, and even the presence of the phase transition in intact lenses is unknown. Positron annihilation lifetime spectroscopy (PALS) was used to characterize the sub-nanometer-sized local structure of intact porcine lens and these studies were complemented with differential scanning calorimeter and mass spectrometric analysis in extracted porcine lens lipids. Using PALS, we present evidence for the presence of a temperature-dependent structural transition centered at 35.5 degrees C in-situ in clear extracted porcine lenses. Further studies employing extracted lens lipids and purified egg-yolk sphingomyelin and cholesterol mixtures suggest that the nano-scale transition emerges from the phase behavior of lens lipids. Based on our results, PALS seems to be a viable method for gaining additional information on biological tissues, especially since it enables non-destructive studies on intact tissues.
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