Fluorescence and Monte Carlo conformational studies of the (1-15) galanin amide fragment

Galanin (GAL) is a 29 amino acid C-terminally aminated linear neuropeptide showing diverse biological activities. The N-terminal (1-15)GAL-NH2 fragment was shown to have a very high affinity to the galanin receptor. In this work we describe the results of a combined fluorescence and Monte Carlo studies, the latter carried out using the ECEPP/3 force field with and without including hydration, on the (1-15)GAL-NH2 fragment. Using the time-domain technique we measured fluorescence decay times of the tyrosine residue in position 9. Based on the Forster energy transfer theory we calculated the distance and distance distribution between the Trp2 (acceptor) and Tyr9 (donor) aromatic side chains. The distance obtained was about 10.5 angstrom and half-width, hw, of the distance distribution was 5.6 angstrom. This results were found to be in good agreement with the chromophore distances calculated for the low-energy solution confirmations obtained in Monte Carlo simulations. All the low-energy conformations obtained in the absence of water were almost all-helical with the exception of a few C-terminal residues. In contrast, none of the low-energy solution conformations contained any significant amount of secondary structure. These findings are in agreement with the results of earlier CD and NMR conformational studies of galanin in water and non-aqueous solvents. On the other hand, the conformations obtained in the presence of water turned out to be largely compact in the N-terminal hydrophobic part. This explains the relatively short distance between chromophores and narrow distance distribution obtained in fluorescence measurements.