Synthesis and characterization of indolicidin, a tryptophan‐rich antimicrobial peptide from bovine neutrophils

Indolicidin, a novel tryptophan‐rich microbicidal tridecapeptide amide isolated originally from granules of bovine neutrophils, has been prepared by optimized manual and automated protocols of stepwise solid‐phase synthesis with N^α‐9‐fluorenylmethyloxycarbonyl (Fmoc) amino acid derivatives. Both standard polystyrene (PS) and polyethylene glycol‐polystyrene (PEG‐PS) graft supports were used in combination with handles that provide C‐terminal peptide amides: 5‐(4‐Fmoc‐aminomethyl‐3,5‐dimethoxyphenoxy)valeric acid (PAL) or 5‐(9‐Fmoc‐aminoxanthen‐2‐oxy)valeric acid (XAL). Final deprotection/cleavage was carried out with reagent K, trifluoroacetic acid–phenol–water–thioanisole–1,2‐ethanedithiol (82.5:5:5:5:2.5), or reagent B, trifluoroacetic acid–phenol–water–tri(isopropyl)silane (88:5:5:2), and related cocktails. Initial purities as high as 93% were obtained immediately following cleavage. In the largest‐scale synthesis carried out, 0.8 g of HPLC‐purified indolicidin (> 99% pure) was obtained, representing a 39% overall yield based on C‐terminal Arg(Pmc) anchored to PAL‐PS‐resin. The main synthetic product, and some by‐products, were characterized by analytical high‐performance liquid chromatography (HPLC), sequencing, and fast atom bombardment mass spectrometry (FABMS). The antimicrobial potencies of natural and synthetic indolicidin, as determined by in vitro antibacterial and antifungal assays, were identical. Further, the reactivities of natural and synthetic peptides with anti‐indolicidin antibody were indistinguishable. © Munksgaard 1995.