The Tunichromes. A Class of Reducing Blood Pigments from Sea Squirts: Isolation, Structures, and Vanadium Chemistry

Marine tunicates (“sea squirts”) display a remarkable propensity to sequester and reduce vanadium (or iron) in specialized blood cells termed vanadocytes (or ferrocytes). Characterization of the reducing blood pigments designated as tunichromes (TC's) suggested a plausible mechanism for accomplishing this. TC refers to a class of hydroxy-Dopa-containing peptides whose purification entailed several unusual chromatographic techniques, all performed anaerobically. The first TC characterized from Ascidia nigra (An-1) is one such modified tripeptide (1a).⁴ The structural elucidation of two other major TCs from Ascidia nigra (An-2 and An-3), as well as two additional TC's from an iron-sequestering tunicate, Molgula manhattensis (Mm-1 and Mm-2), is reported here. Aqueous An/V complexation reactions exhibited a preferred stoichiometry of 2-3:1. Moreover, A. nigra blood cells afforded a green fraction possessing the spectroscopic features of an An/V complex. These and other findings regarding tunichrome-vanadium complexation chemistry are presented.