Elucidation of the mechanism of supramolecular chirality inversion in bis(zinc porphyrin) by dynamic approach using CD and H-1 NMR spectroscopy

Dynamic CD and H-1 NMR spectroscopies have been applied for studying the ph enomenon of chirality inversion in the supramolecular system consisting of bis(Zn porphyrin) and (R)-amine upon stepwise titration with (S)-amine. The initially induced CD signals corresponding to the coupled B transitions of bis(Zn porphyrin) are gradually decreased during this process and transfor med into its mirror images that are the Cotton effects with the same extrem a positions, but with opposite signs. In the H-1 NMR spectra of this system upon titration there is inversion of the proton signals which are in close proximity to the neighboring porphyrin plane, and are thus the most affect ed by its ring current effect. The observed spectral changes are caused by shifting of the equilibrium from the left- to right-handed screw in bis(Zn porphyrin) upon titration with the amine of the opposite absolute configura tion. The CD and H-1 NMR experiments used to determine the reaction kinetic s explicitly corroborate each others results. This chirality inversion proc ess has no total energy change (DeltaG degrees = 0), as shown by the titrat ion curves passing through their zero values at the racemic point of an ena ntiomeric mixture.