DYNAMICS OF THE PEPTIDE-HORMONE MOTILIN STUDIED BY TIME-RESOLVED FLUORESCENCE SPECTROSCOPY

Time resolved fluorescence was used to study the dynamics on the nanos econd and subnanosecond time scale of the peptide hormone motilin. The peptide is composed of 22 amino acid residues and has one tyrosine re sidue in position 7, which was used as an intrinsic fluorescence probe . The measurements show that two rotational correlation times, decreas ing with increasing temperature, are needed to account for the fluores cence polarization anisotropy decay data. Viscosity measurements combi ned with the fluorescence measurements show that the rotational correl ation times vary approximately as viscosity with temperature. The shor ter rotational correlation time (0.08 ns in an aqueous solution with 3 0% hexafluoropropanol, HFP at 20 degrees C) should be related to inter nal movement of the tyrosine side chain in the peptide while the longe r rotational correlation time (2.2 ns in 30% HFP at 20 degrees C) desc ribes the motion of the whole peptide. In addition, the interaction of motilin or the derivative motilin (Y7F) -23W (with tyrosine substitut ed by phenylalanine and with a tryptophan fluorophore added to the C-t erminal) with negatively charged phospholipid vesicles (DOPG) was stud ied. The results show the development of a long anisotropy decay time which reflects partial immobilization of the peptide by interaction wi th the vesicles.