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Store and read programs Genetic system By analogy with macroscopic devices, feasible molecular machines presumably include manipulators able to wield a variety of tools.
Thermal vibrations in typical structures are a modest fraction of interatomic distances; thus, such tools can be positioned with atomic precision. As present microtechnology 2 can lay down conductors on a molecular scale 10 nm and molecular devices can respond to electric potentials through conformation changes, etc.
Further, by analogy with biological sensors, molecular scale instruments can evidently produce macroscopic signals, indicating the feasibility of feedback control in molecular manipulations.
Together, these arguments indicate the feasibility of devices able to move molecular objects, position them with atomic precision, apply forces to them to effect a change, and inspect them to verify that the change has indeed been accomplished.
It would be foolish to minimize the A modest proposal the environment and effort that will be required to develop the needed components and assemble them into such complex and versatile systems. Still, given the components, the path seems clear. Ordinary chemical synthesis relies on thermal agitation to bring reactant molecules in solution together in the correct orientation and with sufficient energy to cause the desired reaction.
Enzyme-like molecular machines can hold reactants in the best relative positions as bonds are strained or polarized.
Like some enzymes, they can do work on reactant molecules to drive reactions not otherwise thermodynamically favored. These are clearly techniques of great power, yet the synthetic capabilities of systems based on polypeptide chains might seem limited by amino acid properties.
However, enzymes show that other molecular structures bound to the polypeptide such as metal ions and complex ring structures 11 can extend protein capabilities.
The range of such tools is large and greater than found in nature. Thus, the synthetic capabilities of enzymes set only a lower bound on the capabilities of engineered protein systems. Indeed, as tool-wielding protein systems can control the chemical environment of a reaction site completely, they should be able, at a minimum, to duplicate the full range of moderate-temperature synthetic steps achieved by organic chemists.
Further, where chemists must resort to complex strategies to make or break specific bonds in large molecules, molecular machines can select individual bonds on the basis of position alone. Conventional organic chemistry can synthesize not only one- two- and three-dimensional covalent structures but also exotic strained and fused rings.
With the addition of controlled site-specific synthetic reactions, a broad range of large complex structures can doubtless be built.
Still, the synthetic abilities of protein machines will be limited by their need for a moderate temperature aqueous environment although applied forces can sometimes replace or exceed thermal agitation as a source of activation energy and reaction sites and reactive groups can be protected from the surrounding water, as in some enzymatic active sites.
These limits may be sidestepped by using the broad synthetic capabilities outlined above to build a second generation of molecular machinery whose components would not be coiled hydrated polypeptide chains but compact structures having three-dimensional covalent bonding.
There is no reason why such machines cannot be designed to operate at reduced pressure or extreme temperatures; synthesis can then involve highly reactive or even free radical intermediates, as well as the use of mechanical arms wielding molecular tools to strain and polarize existing bonds while new molecular groups are positioned and forced into place.
This may be done at high or low temperature as desired.
The class of structures that can be synthesized by such methods is clearly very large, and one may speculate that it includes most structures that might be of technological interest.
Firmness of the argument The development path described above should lead to advanced molecular machinery capable of general synthesis operations.
As the results of this path can be shown to have consequences for the present, it is of interest to discuss the degree of confidence that should be placed in its feasibility. It might be argued that complex protein or nonprotein machines are impossible or useless, on the grounds that, if they were possible and useful, organisms would be using them.
A similar argument would, however, conclude that bone is a better structural material than graphite composite, that neurons can transmit signals faster than wires, and that technology based on the wheel is impossible or useless.
Nature has been constrained less by what is physically possible than by what could be evolved in small steps. Thus, the absence of a proposed kind of molecular machinery in organisms in no way suggests its infeasibility. To deny the feasibility of advanced molecular machinery, one must apparently maintain either i that design of proteins will remain infeasible indefinitely, or ii that complex machines cannot be made of proteins, or iii that protein machines cannot build second-generation machines.
In light of the expected improvements in computation, the simplified task of design engineers compared with scientiststhe possibilities offered by sheer trial-and-error modification of natural proteins, and the progress already made in protein design, the first seems difficult to maintain.
Further, even if protein design were to prove intractible because of difficulties in predicting conformationsthis would in no way preclude developing an alternative polymer system with predictable coiling and using it as a basis for further development. In light of the presence of the needed components for mechanical devices in the cell, the second seems difficult to maintain.
Indeed, the cytoskeleton provides a fair counterexample.
In light of the results of synthetic organic chemistry and the ability of molecular machines to make reactions site specific, it seems difficult to maintain that nonprotein machine components cannot be built and assembled. Each of the development steps outlined above seems closely analogous to past steps taken by nature or by technology.
Each of these steps can be accomplished in many ways. To argue their infeasibility would seem to require some general principle precluding success, and it is difficult to see what such a principle might be like. Thus, the claim that advanced molecular technology can be developed seems well founded.
Although the existence of molecular machinery in cells indicates the feasibility of some sort of artificial molecular machinery, errors in assembly might limit the synthesis of structures of great complexity.Sean Hecht is co-executive director of the UCLA Emmett Institute on Climate Change and the Environment.
His interests cover coastal management, adaptations to impacts of climate change, federal public lands, and the use of science in informing government decision making. A modest proposal for combatting climate change: satire. By Elizabeth. A Modest Proposal to Mitigate Sexual Harassment and Misconduct in the Workplace.
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