1. Synthesis of the marine sponge cycloheptapeptide phakellistatin 5(1)
G R Pettit, B E Toki, J P Xu, D C Brune J Nat Prod. 2000 Jan;63(1):22-8. doi: 10.1021/np990253+.
Phakellistatin 5 (1), a constituent of The Federated States of Micronesia (Chuuk) marine sponge Phakellia costada, was synthesized by solution-phase and solid-phase techniques. Because the linear peptide bearing (R)-Asn resisted cyclization, the synthesis of this peptide was repeated using the PAL resin attachment proceeding from N-Fmoc-D-Asp-alpha-OCH(2)CH=CH(2). After addition of the final unit (Ala), the allyl ester was removed under neutral conditions with Pd(o) [P(C(6)H(5))(3)](4). Removal of the final Fmoc-protecting group and cyclization with PyAOP provided (R)-Asn-phakellistatin 5 (2) in 28% overall yield. The same synthetic route from (S)-Asp led to natural phakellistatin 5 (1) in 15% overall recovery. The solution-phase and solid-phase synthetic products derived from (S)-Asp were found to be chemically but not biologically identical with natural phakellistatin 5 (1). This important fact suggested that a trace, albeit highly cancer-cell growth inhibitory, constituent accompanied the natural product or that there is a subtle conformational difference between the synthetic and natural cyclic peptides.
2. A 'conovenomic' analysis of the milked venom from the mollusk-hunting cone snail Conus textile--the pharmacological importance of post-translational modifications
Zachary L Bergeron, et al. Peptides. 2013 Nov;49:145-58. doi: 10.1016/j.peptides.2013.09.004. Epub 2013 Sep 18.
Cone snail venoms provide a largely untapped source of novel peptide drug leads. To enhance the discovery phase, a detailed comparative proteomic analysis was undertaken on milked venom from the mollusk-hunting cone snail, Conus textile, from three different geographic locations (Hawai'i, American Samoa and Australia's Great Barrier Reef). A novel milked venom conopeptide rich in post-translational modifications was discovered, characterized and named α-conotoxin TxIC. We assign this conopeptide to the 4/7 α-conotoxin family based on the peptide's sequence homology and cDNA pre-propeptide alignment. Pharmacologically, α-conotoxin TxIC demonstrates minimal activity on human acetylcholine receptor models (100 μM, <5% inhibition), compared to its high paralytic potency in invertebrates, PD50 = 34.2 nMol kg(-1). The non-post-translationally modified form, [Pro](2,8)[Glu](16)α-conotoxin TxIC, demonstrates differential selectivity for the α3β2 isoform of the nicotinic acetylcholine receptor with maximal inhibition of 96% and an observed IC50 of 5.4 ± 0.5 μM. Interestingly its comparative PD50 (3.6 μMol kg(-1)) in invertebrates was ~100 fold more than that of the native peptide. Differentiating α-conotoxin TxIC from other α-conotoxins is the high degree of post-translational modification (44% of residues). This includes the incorporation of γ-carboxyglutamic acid, two moieties of 4-trans hydroxyproline, two disulfide bond linkages, and C-terminal amidation. These findings expand upon the known chemical diversity of α-conotoxins and illustrate a potential driver of toxin phyla-selectivity within Conus.
3. Chiral N-Fmoc-beta-amino alkyl isonitriles derived from amino acids: first synthesis and application in 1-substituted tetrazole synthesis
Vommina V Sureshbabu, N Narendra, G Nagendra J Org Chem. 2009 Jan 2;74(1):153-7. doi: 10.1021/jo801527d.
A novel class of optically active N-Fmoc-protected amino isonitriles has been described for the first time. Conversion of the carboxyl group of Fmoc-beta-amino acids into an isocyano group has resulted in a new class of N-urethane-protected amino isonitriles. All the isonitriles have been isolated as stable solids, purified, and completely characterized. A synthetic application of the obtained isonitriles has also been demonstrated through the synthesis of 1-substituted tetrazole analogues of amino acids via a 2 + 3 cycloaddition with trimethylsilyl azide.
