Structure of a Chlorophyll-RC1

Abstract

The primary donor in the reaction center of photosystem I (p 700) has a red-shifted absorption maximum ascompared to the bulk of the antenna pigments. P7oo is generally believed to be one or a pair of chlorophyll a molecule(s). The recent isolation of a new chlorophyll (chlorophyll-RC1) which has been related quantiatively to the content of P700 in a variety of preparations (1) has raised the possibility that a structurally different molecule may be responsible for this function. Chlorophyll-RCI had originally been isolated from Scenedesmus, and subsequently also from a cyanebacterium and from spinach (2). Following a gift of a chlorophyll-RCl sample from Scenedesmus to us by Doernemann and Senger, we have also isolated a similar pigment from a different cyanobacterium, Spirulina geitleri and converted it to its methylphophorbide(s). Here, we wish to report its molecular structure as 132(R)-Hydrexy-20-chloro-17(s), 18(s)-Methyl-pheophorbide a (structure 1), which corresponds structure 2 for chlorophyll-RC1.