successful system for converting photoenergy to proton-motive force. However, it is not clear how this proton-motive force can be used in an energy production system on a large scale. Some participants asked what might be done to understand this system better. For example, are there synthetic analogs of the transformation in retinal? Is there any way to simplify the system so that protons can be pumped on a large scale? Instead of limiting it to the cell membrane, can the system be designed on a large enough scale to harness this energy?

Rational Design of Proteins

As Les Dutton indicated, it is possible through rational design to think about creating synthetic electron transfer proteins and components of future synthetic biology systems. Dutton also mentioned that it is important to focus on the rational design of proton transfer systems—proton pumps or proton-transfer-linked transducers that can run in two directions. In one direction, these proton-transfer systems would pump protons against a proton electrochemical gradient, and in the other direction, they would do some work, chemical or mechanical, by transducing protons back across the system. Tom Moore said that, right now, no one really has an idea of how to rationally design a proton pump. He pointed out that it is important to remember that biology never operates without the combination of proton-motive force and electromotive force.

However, the progress on understanding the two systems is uneven. A lot is understood about the fundamentals of electron transfer systems, but not as much is known about proton transfer. Moore said protons offer an incredibly rich research area, because protons are, in a sense, between classical and quantum mechanical particles. Sometimes their motion is not limited by mass transport considerations. Proton wires exist and, under short distances, protons can tunnel. Thus, he said that it is hoped that the environment for research in proton-motive force, particularly synthetic and artificial systems to generate proton-motive force and then couple that proton-motive force to either chemical or mechanical work, will increase in the future.

There was some discussion among participants about defining synthetic biology. Westpheling said that she thought the group meant it as the synthesis of microbial functions. Tom Moore added that he thinks it is an unlimited definition, because right now the field is open and no one knows where synthetic biology can lead. He said that if someone asked Bardeen to define the transistor in 1948, he probably would have given a similarly broad answer regarding semiconductor physics. Looking ahead, they knew they had to go somewhere that vacuum tubes could not take them, but it is worth noting that they did not have a path. They did not



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