A reversible spectroscopic change in chlorophyll (Chl) d has been demonstrated for the first time, using reduction with sodium dithionite and oxidation with potassium ferricyanide in an aqueous solution of detergent micelles. Chl d treated with a low concentration of NaBH4 in methanol showed the same results. The chemical structure of the reduced Chl d was determined by mass spectrometry to be 3-desvinyl-3-hydroxymethyl Chl a. These findings lead to a new hypothesis for the biosynthetic pathway of Chl d, being that Chl d is synthesized by oxidation of 3-desvinyl-3-hydroxymethyl Chl a, which is first formed enzymatically from Chl a. (c) 2006 Elsevier B.V. All rights reserved.