Toward intelligent molecular machines: directed motions of biological and artificial molecules and assemblies.

In the last two decades, considerable progress has been made in the field of molecular biology, which has enabled a molecular-level understanding of a number of interesting biological events. In particular, the discovery of a family of moving proteins and their assemblies has attracted particular attention not only of biologists but also of chemists and physicists.1-5 In response to certain biological stimuli, these proteins perform directed or programmed motions, similar to many tools and machines used in our daily life. Such biological molecular machines play essential roles in a wide variety of biological events, particularly those related to the activities of cells,1 and realize specific functions through their stimuliresponsive mechanical motions. Cytoplasmic proteins such as myosins, kinesins, and dyneins are called “molecular motors” and are the most extensively studied molecular machines.2-5 These protein-based supramolecular conjugates are known to switch back and forth along linear tracks of actin filaments or microtubules and transport substrates at the expense of adenosine triphosphate (ATP) as fuel. The flagellum is a huge protein conjugate that controls the swimming motion of bacteria.6 The bacterial flagellar motor consists of (1) flagellar filaments that are 15 μm long and 120-250 Å in diameter and a (2) motor domain that rotates the flagellar filaments alternately in clockwise and anticlockwise directions. This unique behavior of rotation allows bacteria to swim desirably. ATP synthase is a different type of a molecular machine, which synthesizes and hydrolyzes ATP through its rotary motion.7-9 ATP synthase is built up of two different machinery components, that is, F1 and F0, where the rotation of the former is driven by the free energy released from the hydrolysis of ATP to adenosine diphosphate (ADP); the latter rotates by a flux of ions passing through a membrane and synthesizes ATP. This huge protein complex has a shaft, which rotates just like real rotary motors. Since the rotary motion of ATP synthase occurs in a stepwise manner in response to the hydrolysis of ATP, it is regarded as a biological stepping motor. In addition to these molecular motors, some other biological machines are known, where the hydrolysis of ATP triggers different types of mechanical motions. Representative examples include the family of chap† The University of Tokyo. ‡ PRESTO. 1377 Chem. Rev. 2005, 105, 1377−1400