The porphynoid pigments of the Caribbean tunicate Trididemnum solidum

Abstract

Tunichlorin, a nickel chlorin isolated from the Caribbean tunicate Trididemnum solidum, has been identified as nickel (II) 2-devinyl-2-hydroxymethylpyropheophorbide a by chemical and spectroscopic methods. A partial synthesis of dimethyl tunichlorin from chlorophyll a (7 steps, 5% overall yield) has also confirmed the proposed structure. In the living tunicate, tunichlorin appears to exist primarily as a series of C-14 to C-18 ester homologues (principally tunichlorin-COO-(C$\sb3$H$\sb4$O)-(CH$\sb2$)$\sb{14}$CH$\sb3$). Preliminary studies suggest that tunichlorin esters are biosynthesized by the tunicate from alga-produced chlorophyll a. Although the biological function of tunichlorin esters remains unclear, analogy to related compounds suggests that they may be enzyme cofactors involved in reductive processes.

In addition to tunichlorin and tunichlorin esters, examination of T. solidum extracts also revealed the presence of several known porphynoids, including chlorophyll a, pheophytin a, 10-hydroxypheophytin a, pheophorbide a, 10-hydroxypheophorbide a, pyropheophytin a, and 10-hydroxychlorophyll a. In addition, isopropyl pheophorbide a was identified from extracts of isopropyl alcohol-preserved tunicate, suggestive of an artifact of work-up and highlighting the pitfalls of natural product isolations. A previously unreported chlorophyll a degradation product, purpurin-18 phytyl ester, was also found in tunicate extracts.