Confining the χ space of basic natural amino acids: cyclobutane-derived χ1,χ2-constrained analogues of arginine, lysine and ornithine

Four χ1,χ2-constrained cyclobutane-derived basic amino acids—conformationally restricted analogues of arginine, lysine and ornithine—were prepared as the derivatives properly protected for Fmoc-solid phase peptide synthesis. Compatibility of the synthesized arginine analogues with standard procedures of the Fmoc solid-phase peptide synthesis was demonstrated by incorporating these residues into the small cyclic antimicrobial peptide c-(RRRWFW) substituting the arginine residues. These replacements did not affect much the antimicrobial activity of the peptides.