Molecular demon

These macromolecules gather information in order to recognize their substrate or ligand within a myriad of other molecules floating in the intracellular or extracellular plasm.

When the demon is reset i.e. when the ligand is released, the information is erased, energy is dissipated and entropy increases obeying the second law of thermodynamics.

When ATP is consumed or hydrolyzed, the ligand is released and the demon acquires again information reverting to its basic state.

When this happens and the gliding back of the macromolecule from the movement it had made or the conformational change it underwent to its original state can be prevented, as is the case with molecular demons, the molecule works as a ratchet;[4][5] it is possible to observe for example the creation of a gradient of ions or other molecules across the cell membrane, the movement of motor proteins along filament proteins or also the accumulation of products deriving from an enzymatic reaction.

Even some artificial molecular machines and experiments are capable of forming a ratchet apparently defying the second law of thermodynamics.

[6][7] All these molecular demons have to be reset to their original state consuming external energy that is subsequently dissipated as heat.

[6] Serreli et al. constructed a nanomachine, a rotaxane, that consists of a ring-shaped molecule, that moves along a tiny molecular axle between two different equal compartments, A and B.

[9] Landauer established equivalence of one bit of information with entropy which is represented by kT ln 2, where k is the Boltzmann constant and T is room temperature.

Since the downward jumps following the gradient are more frequent than the upward ones, the particle falls down the stairs, on average.

[1] Naming the biological molecular machines in this way makes it easier to understand the similarities between these molecules and Maxwell's demon.

Because of this real discriminative if not 'cognitive' property, Jacques Monod attributed a teleonomic function to these biological complexes.

Fig.1 Schematic figure of Maxwell's demon thought experiment. The demon distinguishes fast-moving molecules from slow-moving ones and opens the small hatch selectively to let the fast-moving molecules pass from a A to B and the slow-moving molecules from B to A. Compartment B heats up whereas A cools down with respect to the average temperature although no work is done. It seems there is a contradiction with the second law of thermodynamics. But the ability to distinguish requires the gain of information, which is a form of energy; therefore. the system obeys the second law of thermodynamics inasmuch as information is first gained but then erased.
Fig.2 The protein demon (blue) and the substrate or ligand (orange) go through a cycle in which the electromagnetic interaction (1' --> 2) between the two, following the induced fit, causes a conformational change upon which the substrate is released (2'). Hydrolysis of ATP brings the protein back to its original state