Computing with molecules

It’s easy to forget that, looking at biology as a whole, computing and information processing is more often done by individual molecules than by brains and nervous systems. After all, most organisms don’t have a nervous system at all, yet they still manage to sense their environment and respond to what they discover. And a multi-cellular organism is itself a colony of many differentiated cells, all of which need to communicate and cooperate in order for the organism to function at all. In these processes, signals are communicated not by electrical pulses, but by the physical movement of molecules, and logic is performed, not by circuits of transistors, but by enzymes. Modern systems biology is just starting to unravel the operation of these complex and effective chemical computers, but we’re very far from being able to build anything like them with our currently available nanotechnology.

A news story on the New Scientist website (seen via Martyn Amos’s blog) reports an interesting step along the way, with an experimental demonstration of an enzyme-based system that chemically implements simple logic operations like a half-adder and a half-subtracter. The report, from Itamar Willner’s group at the Hebrew University of Jerusalem, is published in Angewandte Chemie International Edition (abstract here, subscription required for full paper). No-one is going to be doing complicated sums with these devices for a while; the inputs are provided by supplying certain chemical species (glucose and hydrogen peroxide, in this case), and the answers are provided by the appearance or non-appearance of reaction products. But where this system could come in useful is in providing a nanoscale system like a drug delivery device with some rudimentary mechanisms for sensing the environment and acting on the information, maybe by swimming towards the source of some chemicals or releasing its contents when it has detected some combination of chemicals around it.

This is still not quite a fully synthetic analogue of a cellular information processing system; it uses enzymes of biological origin, and it doesn’t use the ubiquitous chemical trick of allostery. In this, the binding of one molecule to an enzyme changes the way it processes another molecule, effectively allowing a single molecule to act as a logic gate. But it suggests many fascinating possibilities for the future.