Do nonplanar distortions of porphyrins bring about strongly red-shifted electronic spectra? Controversy, consensus, new developments, and relevance to chelatases.

TDDFT calculations confirm a controversial proposal by DiMagno and co-workers that ruffling distortions, by themselves, do not bring about sizable red shifts in the electronic absorption spectra of "simple" nontransition-metal porphyrins. We now report that the same also holds for saddling distortions. The situation is more complex for transition metal porphyrins. For example, ruffling does bring about strong red shifts in the electronic spectra of nickel porphyrins because of a specific metal(d)-porphyrin(pi) orbital interaction.