Butyric acid diffuses easily across lipid bilayers

In a recent article published in the Biophysical Journal (Volume 101, August 2011, pages 700–708), Li et al. from the  University of Southern California studied the transport of low-molecular-weight carboxylic acids across cell membranes,. They have been using a new method called confocal microscopy to image the transport of the different carboxylic  acids. They wanted to confirm the rule of thumb called Overton’s rule, which holds that more lipophilic molecules transport across lipid membranes more quickly. The data shows that, among the short chain fatty acids (formic, acetic, propionic and butyric), formic acid had the lowest permeability and butyric acid has the highest value. Indeed, the quantity of butyric acid going across the lipid bilayers was almost 4 times more than that of propionic acid, 12 times greater than acetic acid and 20 times more than for formic acid. Previous studies have also shown that only uncharged (non dissociated) molecular species can penetrate through lipid membranes. It is well established that dissociation of the different acids will depend on their pKa value: when the pKa value increases, the acid is more stable (non dissociated) for a given pH value. This means that acids with a higher pKa value (such as butyric or propionic acids) will be more stable and will more easily diffuse through the lipid bilayers. As a conclusion, the results of this study confirm that indeed more lipophilic acids cross the lipid bilayer more quickly, which is important to know when selecting an organic acid for the control of gram negative bacteria