Gramicidin cleavage

Fig. 6.9 Dinuclear Zn(ll) complex of cyclic decapeptide as reported by Ka-wai et al. [53] modeled after the iono-phoric cyclic peptide gramicidin S and active in the cleavage of HPNP...

When the last condensation reaction has occurred, the linear precursor needs to be released from the enzyme. For this important last step, several mechanisms are known simple hydrolysis of the thioester (balhimycin, vancomycin), intramolecular cyclization leading to a lactam (tyrocidine, bacitracin) or a lactone (surfactin), or even reductive thioester cleavage (linear gramicidin). In some cases, the linear precursor is dimerized (gramicidin S) or even trimerized (bacillibactin, enterobactin) before cyclization (Fig. 2). Even though these reactions are critical for the compound s bioactivity, the catalytic domains responsible for the release are not found in all NRPS systems and will therefore be called modifying domains. 

Of more practical interest is the NBS cleavage of gramicidin A which... 

Gramicidin A, however, is attacked by JV -bromoacetamide (NBA) and N-bromosuccinimide (NBS) (Gross and Witkop, unpublished observation). In 50 % aqueous ethyl alcohol at room temperature 5 % of the peptide bonds (20% of the tryptophyl peptide bonds) are cleaved with NBS. Methyl alcohol must be avoided because it opens the spirodioxindole lactone from oxidized tryptophan to the ester even at room temperature. The cleavage mixture separates on electrophoresis (pH = 2.5, sodium-formate buffer) into four ninhydrin-positive components of which the fastest migrating one was identified as ethanolamine. Dinitrophenylation showed leucine and alanine to be additional NHs-terminals of the released fragments. 

Partial" Purification of Commercial Gramicidin hy Countercurrent Distribution and Cleavage by N-Bromoacetamide... 

An outline of the solid phase synthesis of LH-RH is presented in Fig. 8. As in the case of valine-gramicidin A, after the -butyloxy-carbonyl-tryptophan residue was coupled to the peptide-resin, cleavage of the N-protecting group was achieved by trifluoroacetic acid in the presence... 

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