Optimal bond loads in designing molecular machines

One issue in designing molecular machine systems to do nanoscale work, such as molecular manufacturing, is how to transfer energy to implement guided molecular motions, to move components or to make or break chemical bonds. In Chemistry World Philip Ball comments on recent research that provides fresh insights into how this process is optimized in biology, and which could prove useful in designing artificial molecular machine systems. From “Make or break: the laws of motion“:

… the question biology has to face is: what is the optimal bond strength for a given mechanical function? This issue is tackled by Henry Hess of Columbia University, US, in a paper that is stimulating fresh thinking about molecular machines [abstract]. Consider a kinesin motor protein ‘walking’ along a tubulin track. The objective is to transfer impulse from the protein’s motor stroke – a conformational change driven by hydrolysis of adenosine triphospate – to the protein–tubule interface, propelling the molecule forward. Hess compares it to a car (kinesin) stuck in mud (tubulin). Anyone who has ever faced this situation knows how delicately the coupling must be managed, by engaging the clutch to just the right degree. Too much and the wheel just spins: the bond snaps. Too little, and the wheel’s coupling to the engine is insufficient to generate movement. The optimal point is found where the wheel–mud adhesion is just about to cease.

… Hess shows that as the load on a bond is increased, the transfer of impulse across the bond has a peak. The position of this peak depends on the distance to the transition state for bond rupture along the reaction coordinate. In other words, here is a design criterion for the ideal molecular machine that transfers energy during reversible binding: the bond should be just strong enough to be likely to survive during the power stroke. …

Proposals of how to advance from current nanotechnology to atomically precise manufacturing (see, for example, the Technology Roadmap for Productive Nanosystems) embody a range of proposals for different stages of development, from biological molecular machines based on networks of weak noncovalent bonds, to nanoscale versions of macroscopic machines constructed from hard materials like diamond comprising dense networks of strong covalent bonds. An important question to be clarified is how (or if) the design rules for molecular machine systems change at various points along this continuum.
—James Lewis, PhD

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