Covalently linked zinc chlorophyll dimers as a model of a chlorophyllous pair in photosynthetic reaction centers

Abstract

A heterodimer, where zinc pyropheophorbide-a was linked with zinc pyropheophorbide-d through ethylene glycol diester, was prepared, as well as the corresponding homodimers. The synthetic dimers were complexed with methanol in benzene to give folded dimers by mutual Zn⋯O(Me)–H⋯OC13¹ bonding. Such complexes had furthest red (Q_y) absorption bands at longer wavelengths than the monomeric species. These red-shifts were ascribable to excitonic coupling of the Q_y transition states in the chlorin π–π stacking conformer. In the heterodimeric system, a minor band was observed at the shorter wavelength side of the main Q_y band. This observation can be explained by an additional contribution of Q_y vibronic state to the exciton-coupled states. Based on the experimental results, a pair of chlorophyll(Chl)-d with Chl-a as well as a Chl-d homopair were proposed as dimers in reaction centers of Chl-d dominating cyanobacteria.