Fluorescence Study of Conformational Properties of Melanotropins Labeled with Aminobenzoic Acid

ABSTRACT The native hormone a-melanocyte-stimulating hormone (a-MSH) and its more potent analog [Nle4,D-Phe7]a-MSH (NDP-aMSH), labeled at the amino terminal with the fluorescent aminobenzoic acid (Abz) isomers, were examined by fluorescence methods. We observed energy transfer between the tryptophan9 residue acting as donor and Abz as acceptor, the transfer being more pronounced to the ortho-form of the acceptor. Within the hypothesis that different peptide conformations coexist in equilibrium during the fluorescence decay, we supposed that the intensity decay was modulated by an acceptor-donor distance distribution function f(r). From the time-resolved fluorescence experimental data, we recovered the distance distribution between Abz and Trp9, using the CONTIN program, within the framework of the Förster resonance energy transfer model. The methodology proved to be useful to provide quantitative information about conformational dynamics of melanotropins and its dependency on the solvent. In aqueous medium, a-MSH has a broad Abz-Trp9 distance distribution, reflecting the structural flexibility of the peptide. Three different distance populations could be identified in the labeled analog NDP-aMSH in water, indicating distinct conformational states for the synthetic peptide, compared with th