1. Novel N omega-xanthenyl-protecting groups for asparagine and glutamine, and applications to N alpha-9-fluorenylmethyloxycarbonyl (Fmoc) solid-phase peptide synthesis
Y Han, N A Solé, J Tejbrant, G Barany Pept Res. 1996 Jul-Aug;9(4):166-73.
The N alpha-9-fluorenylmethyloxycarbonyl (Fmoc), N omega-9H-xanthen-9-yl (Xan), N omega-2-methoxy-9H-xanthen-9-yl (2-Moxan) or N omega-3-methoxy-9H-xanthen-9-yl (3-Moxan) derivatives of asparagine and glutamine were prepared conveniently by acid-catalyzed reactions of appropriate xanthydrols with Fmoc-Asn-OH and Fmoc-Gln-OH. The Xan and 2-Moxan protected derivatives have been used in Fmoc solid-phase syntheses of several challenging peptides: a modified Riniker's peptide to probe tryptophanalkylation side reactions, Briand's peptide to assess deblocking, at the N-terminus and Marshall's ACP (65-74) to test difficult couplings. Removal of the Asn and Gln side-chain protection occurred concomitantly with release of peptide from the support, under the conditions for acidolytic cleavage of the tris(alkoxy)benzylamide (PAL) anchoring linkage by use of trifluoroacetic acid/scavenger mixtures. For each of the model peptides, the products obtained by the new protection schemes were purer than those obtained with N omega-2,4,6-trimethoxybenzyl (Tmob) or N omega-triphenylmethyl (Trt) protection for Asn and Gln.
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. Multiple synthesis by the multipin method as a methodological tool
A M Bray, R M Valerio, A J DiPasquale, J Greig, N J Maeji J Pept Sci. 1995 Jan-Feb;1(1):80-7. doi: 10.1002/psc.310010110.
The multipin method of peptide synthesis is demonstrated as a potent methodological tool, where large numbers of comparative studies can be performed concurrently. Two studies are presented. In each study, the test peptides were simultaneously synthesized, and the products examined by high throughput ion spray mass spectrometry and reverse-phase HPLC. In the first study, comprising 24 experiments, peptides 1 (AELFSTHYLAFKEDYSQ-NH2) and 2 (LKDFRVYFREGRDQLWKGPG-NH2) were prepared using Fmoc-Axx/BOP/HOBt/NMM [100 : 100 : 100 : 150 mM) and Fmoc-AXX/HATU/HOAt/NMM (100 : 100 : 100 : 150 nM) with 60, 90 and 120 min coupling times. The two reagent combinations were found to give comparable results. The second study compared the N-terminal coupling of Fmoc-Asn-OH, Fmoc-Asn(Mbh)-OH, Fmoc-Asn(Mtt)-OH, Fmoc-Asn(Tmob)-OH and Fmoc-Asn(Trt)-OH in the synthesis of seven test peptides: 3, NVQAAIDYIG-cyclo(KP): 4. NTVQAAIDYIG-cyclo(KP): 5. NRVYVHPFNL: 6. NRVYVHPFHL: 7. NEAYVHDAPVRSLN: 8. NQLVVPSEGLYLIYSQVLFK; 9, NPNANPNANPNA. A total of 33 experiments were performed. Peptides 3 and 4 were selected to highlight the effect of steric bulk of each Asn derivative on coupling efficiency. Reagent efficiency, as measured by target peptide purity, was as follows: Fmoc-Asn(Tmob)-OH > Fmoc-Asn-OH > Fmoc-Asn(Mtt)-OH = Fmoc-Asn(Trt)-OH > Fmoc-Asn(Mbh)-OH.
