(Figure ). The rate of the reduction of 4a, rred, was determined by monitoring the reduction of 4a to 3a in a NaPi (100 mM, pH 7.0) / DMSO (10% v:v) mixture at 313 K using UV/vis spectroscopy (see Supplementary Information Section 7. The final proof that 3a undergoes continuous directional rotation under the conditions of the cyclic redox reaction network requires evidence that the oxidation of 3a proceeds stereoselectively, i.e., that rox(Sa)/rox(Ra) ≠ 1. This information cannot be provided by direct observation of enantiomeric enrichment in either starting material or product, because 3a is achiral, and (Ra)-4a and (Sa)-4a racemise too fast for analysis of the enantiomeric ratio.