Acid-base titration of melanocortin peptides: Evidence of Trp rotational conformers interconversion

• Published in: Biopolymers Vol. 80; no. 5; pp. 643 - 650
• Main Authors: Fernandez, Roberto M., Vieira, Renata F. F., Nakaie, Clóvis R., Lamy, M. Teresa, Ito, Amando S.
• Format: Journal Article
• Language: English
• Published: Hoboken Wiley Subscription Services, Inc., A Wiley Company 2005
• Subjects: alpha-MSH - analogs & derivatives; alpha-MSH - chemistry; Cyclic N-Oxides - chemistry; electron spin resonance; Electron Spin Resonance Spectroscopy; Hydrogen-Ion Concentration; melanocortins; Models, Chemical; Molecular Conformation; NDP-MSH; pH titration; Spectrometry, Fluorescence; time-resolved fluorescence; Toac; Trp; Tryptophan - chemistry; α-melanocyte stimulating hormone
• ISSN: 0006-3525; 1097-0282
• DOI: 10.1002/bip.20210

Tryptophantime‐resolved fluorescence was used to monitor acid–base titration properties of α‐melanocyte stimulating hormone (α‐MSH) and the biologically more potent analog [Nle4, D‐Phe7]α ‐MSH (NDP‐MSH), labeled or not with the paramagnetic amino acid probe 2,2,6,6‐tetramthylpiperidine‐N‐oxyl‐4‐amino‐4‐carboxylic acid (Toac). Global analysis of fluorescence decay profiles measured in the pH range between 2.0 and 11.0 showed that, for each peptide, the data could be well fitted to three lifetimes whose values remained constant. The less populated short lifetime component changed little with pH and was ascribed to Trp g+ χ1 rotamer, in which electron transfer deactivation predominates over fluorescence. The long and intermediate lifetime preexponential factors interconverted along that pH interval and the result was interpreted as due to interconversion between Trp g‐ and trans χ1 rotamers, driven by conformational changes promoted by modifications in the ionization state of side‐chain residues. The differences in the extent of interconversion in α‐MSH and NDP‐MSH are indicative of structural differences between the peptides, while titration curves suggest structural similarities between each peptide and its Toac‐labeled species, in aqueous solution. Though less sensitive than fluorescence, the Toac electron spin resonance (ESR) isotropic hyperfine splitting parameter can also monitor the titration of side‐chain residues located relatively far from the probe. © 2005 Wiley Periodicals, Inc. Biopolymers (Pept Sci), 2005