Fig. 1.

The relationship between the shape of the landscape and the generated torque is illustrated by the schematic landscapes for chemical and rotational coordinates (omitting the chemical or conformational barrier). Two extreme situations occur when the chemical free energy is (A) tightly coupled and (B) loosely coupled to the rotational coordinate. In A the chemical free energy decreases more when the rotational coordinate ϕ approaches 120° than when it approaches 0°. Hence the average trajectory ultimately moves from ϕ = 0° to ϕ = 120° and the system generates torque. In B due to loose coupling of the rotational and chemical coordinates, the system can move to different rotational angles over time so that we obtain zero torque in the ϕ direction.