Comparative EPR and fluorescence conformational studies of fully active spin-labeled melanotropic peptides.

Similar to melanocyte stimulating hormone (alpha-MSH), its potent and long-acting analogue, [Nle(4), D-Phe(7)]alpha-MSH, when labeled with the paramagnetic amino acid probe 2,2,6,6-tetramethylpiperidine-N-oxyl-4-amino-4-carboxylic acid (Toac), maintains its full biological potency, thus validating any comparative structural investigations between the two labeled peptides. Correlation times, calculated from the electron paramagnetic resonance signal of Toac bound to the peptides, and Toac-Trp distances, estimated from the Toac fluorescence quenching of the Trp residue present in the peptides, indicate a more rigid and folded structure for the potent analogue as compared to the hormone, in aqueous medium.