Tunichlorins

Bible KC, M Buytendorp, PD Zierath, KL Rinehart (1988) Tunichlorin a nickel chlorin isolated from the Caribbean tunicate Trididemnum solidum. Proc Natl Acad Sci USA 85 4582-4586. 

Rinehart, K.L., Jr. Kishore, V. Bible, K.C. Sakai, R. Sullins, D. Li, K.-M. (1988) Didemnins and tunichlorin novel natural products from the marine tunicate Trididemnum solidum. J. Nat. Prod., 57,1-21. 

Figure 16-27 (A) Structure of the nickel-containing prosthetic group F430 as isolated in the esterified (methylated) form. From Pfaltz et al.i59 The "front" face, which reacts with methyl-coenzyme M, is toward the reader.458 (B) Structure of a representative member of a family of tunichlorins isolated from marine tunicates.460 For tunichlorin R = R = H. Related compounds have R = CH3 and/or R = an alkyl group with 13-21 carbon atoms and up to six double bonds.

In the tunicates (sea squirts and their relatives) it occurs in a fixed ratio with cobalt, suggesting a metabolic role.460 A new class of nickel chelates called tunichlorins have been isolated. An example is shown in Fig. 16-27B. The function of tunichlorins is unknown but their existence suggests the possibility of unidentified biochemical roles for nickel. 

A highly unusual porphynoid (117), named tunichlorin, was isolated from the Caribbean tunicate Trididemnum solidum from which didemnins (Section VIII), anticancer cyclopeptides, had previously been obtained... 

Because of their long excited-state lifetimes, chlorophyll derivatives are rare outside phototrophic organisms. However, some chlorophyll-like pigments perform other functions in nature Certain deep-sea hsh use Chls as visual pigments in the marine worm, Bonella viridis, a chlorophyll-derivative acts as a sex determinant and in certain tunicates, the tunichlorins may be involved in nonphotosynthetic electron transport (5). 

The structural type of C. (2,3-dihydroporphyrins) is formally derived from that of porphyrin by reduction of a peripheral double bond in a pyrrole ring of the porphyrin skeleton. The name is based on that of chlorophyll which possesses the C. skeleton with a central magnesium(ll) ion. Other substances with the C. skeleton are, in addition to the chlorophylls and their degradation products, some bacterioch)orophylls, bo-nellin, chlorophyllone, cyclopheophorbide, factor I, heme d, and tunichlorin. 

The nickel containing tunichlorin (57) and its derivatives (58-60) were discovered in the tunicate Trididemnum solidum which occurs in the Carribean Sea (35). Structural elucidation was carried out by UV/Vis, CD, MS, and NMR spectroscopy on dimethyltunichlorin (58), which was prepared by etherification of the 3-hydroxymethyl group and esterification of the 17-propionic acid side chain. 

Tunichlorin (57) 13, 35a,b) and cyclopheophorbide (61) (13, 37) as well as derivatives of both structures were obtained by partial synthesis starting from methylpheophorbide as a common intermediate (59). 

Tunichlorin is a greenish-blue pigment that was isolated in 1988 from the ascidian Trididemnum solidum (Bible et al. 

It is the first known example of a porphyrin containing a nickel atom, and the function of this element for the ascidian is still unknown. Later on, tunichlorin was identified in the mollusk Dolabella auricularia, suggesting an exogenic origin of this compoimd (Pettit et al, 1993). Some other derivatives similar to tunichlorin have been characterized in the same ascidian Trididemnum soUdum (Rinehart et al, 1988 Sings, Bible, and Rinehart, 1996 and Chapter 28). 

Pettit, G.R., Kantoci, D., Doubek, D.L., Tucker, B.E., Pettit, W.E., and Schroll, R.M. (1993) Isolation of the nickel-chlorm chelate tunichlorin from the South Pacific Ocean sea hare Dolabdla auricularia. J. Nat. Prod., 56,1981-1984. 

Sings, H.L., Bible, KG., and Rinehart, K.L. (1996) Acyl tunichlorins a new class of nickel chlorins isolated from the Caribbean tunicate Trididemnum solidum. Proc. Natl Acad. Sci. USA, 93,10560-10565. 

The tunichlorin content is very low 1.25 mg was obtained from 82.6 kg of ascidian (roughly 10 %). The amount of acyltunichlorins present is about 1000-fold higher (10 %). 

---