Peptide-lipid interaction monitored by spin labeled biologically active melanocortin peptides

The present work comparatively analyzes the interaction of alpha-MSH and its more potent and long-acting analog [Nle(4), D-Phe(7)]alpha-MSH (NDP-MSH) with lipid bilayers. The peptides were spin labeled with Toac (2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid) at the N-terminal, as those derivatives had been previously shown to keep their full biological activity. Due to the special rigidity of the Toac covalent binding to the peptide molecule, this spin label is highly sensitive to the peptide backbone conformation and dynamics. The peptides were investigated both by the electron spin resonance (ESR) of Toac(0) and the time resolved fluorescence of Trp(9) present in the peptides. The Toac(0) ESR of the membrane-bound peptides indicates that the two peptides are inserted into the bilayer, close to the bilayer surface, in rather similar environments. A residue titration around pK(a) 7.5, possibly that of His(6), can be clearly monitored by peptide-lipid partition. Trp(9) time resolved fluorescence indicates that the peptides, and their Toac-labeled derivatives, present rather similar conformations when membrane bound, though Trp(9) in NDP-MSH, and in its Toac-labeled derivative, goes somewhat further down into the bilayer. Yet, Toac(0) ESR signal shows that the Toac-labeled N-terminal of NDP-MSH is in a shallower position in the bilayer, as compared to the hormone. (c) 2005 Elsevier Inc. All rights reserved.