Brownian Ratchets and Their Application to Biological Transport Processes and Macromolecular Separation

Abstract

Non-equilibrium fluctuations, whether generated externally or by a chemical reaction far-from-equilibrium, can drive directed motion along an anisotropic structure without thermal gradients or net macroscopic forces, simply by biasing Brownian motion. Systems operating on this principle are often referred to as “Brownian ratchets”, and the transport in such systems is called “fluctuation driven transport”. Brownian ratchets have been invoked as a possible explanation of the operation of molecular motors and pumps, but may also find application in other fields, such as particle separation or the design of nano-scale motors. Here we present a specific ratchet model for the operation of two microtubule based molecular motors, kinesin and ncd. This model can reproduce all the available mechanical data on the motion of these motors and, in addition, accounts for their directionality. We will also summarize some of the recently proposed macromolecular separation techniques based on geometrical ratchets. The main advantages of these new techniques are that the geometry of the devices can be chosen at will, the devices can be reused, molecules can be sorted continuously, and the separation can be easily automated.