Determining the occurrence of a 3(10)-helix and an alpha-helix in two different segments of a lipopeptaibol antibiotic using TOAC, a nitroxide spin-labeled C-alpha-tetrasubstituted alpha-amino acid

Trichogin GA IV is a 11-residue lipopeptaibol antibiotic exhibiting membran e modifying properties. We synthesized step-by-step by solution methods thr ee trichogin analogues, each with a double Aib (alpha-aminoisobutyric acid) -->TOAC (2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid) re placement. The strict similarity in the conformational propensities of Aib and TOAC allowed us to exploit these analogues in a detailed investigation of the conformation of this lipopeptaibol in different organic solvents and in a membrane-mimetic environment using in particular the double spin labe ling ESR technique. We conclude that the secondary structure in solution re mains essentially unchanged if compared to that previously found in the cry stal state for trichogin. More specifically, the N-terminal region of the p eptide folds in a 3(10)-helix, while the central and C-terminal regions are mainly alpha-helical. An additional, significant proof for the modest plas ticity of the trichogin structure was obtained by an X-ray diffraction anal ysis of the nOct-[TOAC(4,8), Leu-OMe11] analogue. For the three analogues p ermeability measurements revealed membrane-modifying properties comparable to those of natural trichogin. (C) 1999 Published by Elsevier Science Ltd. All rights reserved.