IsCT is a non-cell-selective antimicrobial peptide isolated from the scorpion Opisthacanthus madagascariensis that has potent cytolytic activity against both mammalian and bacterial cells. To investigate the structure-activity relationships of IsCT and to design novel peptide antibiotics with bacterial cell selectivity, we synthesized several analogs of IsCT and determined their three-dimensional structures in solution by 2D-NMR spectroscopy. IsCT has a linear alpha-helical structure from Gly(3) to Phe(13), and [K-7]-IsCT has a linear alpha-helical structure from Leu(2) to Phe(13). [K-7, P-8, K-11]-IsCT, which has a bend in its middle region, exhibited the highest antibacterial activity without hemolytic activity, suggesting that its proline-induced bend is an important determinant of this selectivity. Tryptophan fluorescence showed that the high selectivity of [K-7 P-8, K-11]-IsCT toward bacterial cells is closely correlated with its highly selective interaction with negatively charged phospholipids. Its potent activity against antibiotic-resistant bacteria suggests that [K-7, P-8, K-11]-IsCT may serve as a promising lead candidate in the development of new peptide antibiotics. (C) 2004 Elsevier Inc. All rights reserved.