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Tracking Unfolding and Refolding of Single GFPmut2 Molecules

Fabio Cannone ^* ¶, Sara Bologna  ^¶, Barbara Campanini  ^¶, Alberto Diaspro  ^¶, Stefano Bettati  ^¶, Andrea Mozzarelli  ^¶ and Giuseppe Chirico ^* ¶

^* Department of Physics, University of Milan Bicocca, 20126 Milan, Italy; Department of Biochemistry and Molecular Biology, and Department of Public Health, University of Parma, 43100 Parma, Italy; Department of Physics, MicroScoBio-LAMBS-IFOM Research Center, University of Genoa, 16146 Genoa, Italy; and ^¶ Italian National Institute for the Physics of Matter, Genoa, Italy

Correspondence: Address reprint requests to Giuseppe Chirico, E-mail: giuseppe.chirico{at}mib.infn.it.

The unfolding and refolding kinetics of >600 single GFPmut2 molecules, entrapped in wet nanoporous silica gels, were followed by monitoring simultaneously the fluorescence emission of the anionic and neutral state of the chromophore, primed by two-photon excitation. The rate of unfolding, induced by guanidinium chloride, was determined by counting the number of single molecules that disappear in fluorescence images, under conditions that do not cause bleaching or photoinduced conversion between chromophore protonation states. The unfolding rate is of the order of 0.01 min^–1, and its dependence on denaturant concentration is very similar to that previously reported for high protein load gels. Upon rinsing the gels with denaturant-free buffer, the GFPmut2 molecules refold with rates >10 min^–1, with an apparently random distribution between neutral and anionic states, that can be very different from the preunfolding equilibrium. A subsequent very slow (lifetime of 70 min) relaxation leads to the equilibrium distribution of the protonation states. This mechanism, involving one or more native-like refolding intermediates, is likely rate limited by conformational rearrangements that are undetectable in circular dichroism experiments. Several unfolding/refolding cycles can be followed on the same molecules, indicating full reversibility of the process and, noticeably, a bias of denaturated molecules toward refolding in the original protonation state.

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