The application of resonance fluorescence energy transfer measurements for the analysis of conformational states of neuropeptide galanin revealed the coexistence of different structures in solution. This method is complementary to other spectroscopic structural methods like 2D NMR. The energy transfer between a fluorescence donor and acceptor gives direct information about the long distance separation of defined residues of a molecule in analogy to the nuclear Overhauser effect (NOE) which is limited to short distances. A special analysis of experimental results has been performed to get the distribution function of these distances instead of one averaged value for the investigated system. This approach enables the investigation of the coexistence of different sub-populations and the dynamic equilibrium between them. Due to its high time resolution of sub-nanoseconds, energy transfer measurements show the 'frozen' distribution of conformations which are not averaged during the time range characteristic for this process, in contrast to NMR experiments (milliseconds). Large mobility of the energy donor and acceptor leads to high precision of the distance determination. The distribution of conformations corresponds to the diversity of structures found for galanin with NMR spectroscopy.
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