Conformational analysis of galanin using end to end distance distribution observed by Förster resonance energy transfer

The structural dynamics of the flexible neuropeptide galanin in solution were studied by Förster resonance energy transfer measurements at different temperatures by time-resolved fluorescence spectroscopy to determine its conformational heterogeneity. Endogenous tryptophan at position 2 acted as the fluorescent donor and the non fluorescent acceptor dinitrophenyl or the fluorescent acceptor dansyl were selectively attached to lysine 25 in porcine galanin. The coexistence of different structures of the neuropeptide galanin in trifluoroethanol solution was revealed by the model independent analysis of the distribution of relaxation times from the time-resolved resonance energy transfer data. Multiple conformational states are reflected by distinct end-to-end distance populations. The conformations differ in mean donor-acceptor distance by about 15, and are consistent with the extended and folded backbone conformations of two alpha-helical regions separated by a flexible hinge. The effect that the labelling of galanin has on binding to the receptor was also evaluated. DNP-galanin showed the same high affinity to galanin receptors as unlabelled galanin, whereas DNS-galanin had significantly reduced affinity.