Century Old Overton Rule Shown False

An old rule devised by Ernst Overton, to estimate the amount of time a given chemical takes to pass through cell walls, has been overturned by scientists at the University of Warwick. The researchers directly observed and measured various substances rate of passage through cell membranes, and showed that Overton’s rule is simply backwards.

Overton’s rule says that the easier it is for a chemical to dissolve in a lipid (fat) the easier and faster it will be transported into a cell. The Rule was first outlined in the 1890s by Ernst Overton of the University of Zürich. He declared that substances that dissolve in lipids pass more easily into a cell than those that dissolve in water. He then set forth an equation that predicted how fast that diffusion would happen. One of the key parameters in that equation is K which defines the lipophilicity (oil-liking nature) of the chemical. The higher the value of K, the faster the predicted cell permeation rate. For over a century, medicinal chemists have used this relationship to shape their studies and clinical trials.
A team of electrochemists from the University of Warwick used a combination of a confocal microscope and an ultramicroelectrode to study what really happens when a chemical crosses a cell membrane. Advances in technology enabled them to position an ultramicroelectrode incrediblely close to the membrane boundary (roughly 20 microns away; ca. 1/3rd the thickness of a human hair) where it was used to generate a range of acids that should be able to diffuse relatively easily into a cell. These techniques allowed every step of the diffusion process to be directly examined. Previous studies had not been able to observe every step of the process and often required artificial stirring of the solutions.
The results stunned the researchers. While the acids did diffuse across a lipid membrane, they did so at rates that were diametrically opposite to the predictions of the Rule, i.e. the most lipophilic molecules were actually transported slowest. The researchers studied four acids (acetic, butanoic, valeric, and hexanoic) that had increasingly larger "acyl" (or carbon) chains. The longer the carbon chain, the easier the chemical dissolves in lipids and, therefore, according to Overton, the faster they should diffuse across a lipid membrane. In fact, the University of Warwick researchers observed that for these four acids the exact opposite is true: the easier it is for an acid to dissolve in a lipid, the slower it is transported across the membrane.

Press release: Century old rule of Chemistry overturned – major implications for drug delivery
Image: Model of a section of the lipid bilayer that makes up the cell membrane. A number of different types of proteins are embedded into the bilayer – some span the bilayer whilst others are only exposed to one side of the membrane. Some proteins carry carbohydrate side chains that are needed for them to function properly. These side chains are added after the protin is produced. Collection: Wellcome Images