Dolastatin 15, structure determination

In fact, some of the early examples of structure determination were particularly favourable in terms of practical MS (even though they appear to be more structurally complex examples than would be expected for pioneering studies) because they were sufficiently volatile without derivatisation. They could be described as naturally derivatised peptides (many natural peptides are TV-acylated and ester-ified). Dolastatin 15 (Figure 4.7) has no zwitterionic characteristics, since it is TV-methylated and the C-terminus is cyclised as part of a lactone. It is already suitable for MS since it has adequate volatility without derivatisation. 

The extracts from a large collection of Dokhdla auricuhria sea hares from the Indian Ocean were evaluated at the National Cancer Institute and showed an anticancer effect in the P388 lymphocytic leukemia mouse model. After 15 years of intense effort, an exceptionally potent toxin known as dolastatin 10 was isolated and its structure determined using extensive degradation by hydrolysis in concert with 2D NMR and various MS methods. The peptide was... 

The antineoplastic, cyclic peptide dolastatin 3 (197) was isolated from the sea hare Dolabella auricularia in small quantities [187]. It bears much structural resemblance to the cyclic peptides of tunicates. Synthetic attempts indicated that the original published structure was incorrect [188]. Three reports of research directed towards synthesis of possible components of dolastatin 3 (197) failed to help with the correct structure [189-191]. Reisolation of 197 allowed the determination of the correct sequence of amino acids in this cyclic pentapeptide and the new structure was confirmed by synthesis [192]. Synthesis of dolastatin 3 (197) and the corresponding 12R diastereoisomer permitted study of the solution... 

D. auricularia from Japan contained dolabellin (199), a cytotoxic bis-thiazole. The structure was elucidated by spectral data examination and chemical degradation [200], D. auricularia also contained the cyclic hexapeptide dolastatin E (200) [201], the stereochemistry of which was determined by chemical degradation and total synthesis [202], A further cytotoxic hexapeptide, dolastatin I (201) was isolated from D. auricularia from Japan [203] and the stereostructure was confirmed by enantioselective synthesis [204], Dolastatin 18 (202) was isolated as a very minor cytotoxic component of D. auricularia from Papua New Guinea [205]. 

New thiazole-containing cyclicpeptides were reported very recently. Homodolastatin 3 (479) and kororamide (480) were isolated from a Palau collection of the macroscopic cyanophyte Lingbya majuscula. The structures of 479 and 480 were determined by interpretation of their spectroscopic data and chemical degradation. This work constitutes an additional support of the cyanobacterial origin of the dolastatin peptides [364]. 

Note added in proof The structure of tubulin in complex with two other vinca domain ligands, namely with soblidotin, a dolastatin 10 analog, and with phomopsin A, has now been determined. These new results show that the site of these molecules overlaps only in part with the one of vinblastine. They put a structural explanation for the different properties of these compounds (e.g. an higher efficiency to inhibit the nucleotide exchange on tubulin) as compared to vinblastine [72]. 

The sea hare Dolabella auricularia (Aplysiidae) has been extensively investigated as a rich source of bioactive natural products, and a number of bioactive peptides and depsipeptides such as dolastatin 10 were isolated from both Western Indian Ocean specimens and Japanese specimens of this animal [for example, 134, 135]. This mollusk was also found to contain macrolide-type natural products. From D. auricularia, collected by hand at a depth of 0-1 m off the coast of the Shima Peninsula, Mie Prefecture, Japan, a 22-membered macrolide, dolabe-lide A (65), was isolated as a cytotoxin with an IC50 value of 6.3 pg/ml against HeLa-S3 cells. The gross structure of 65 was determined by spectroscopic analysis, and its absolute stereochemistry was elucidated by a combination of chemical means and the NMR spectroscopic method [136]. 

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