How polypeptide chains acquire specific conformations to realize unique biological functions is a central problem of protein science. Single-molecule spectroscopy, combined with fluorescence resonance energy transfer, is utilized to study the conformational heterogeneity and the state-to-state transition dynamics of proteins on the submillisecond to second timescales. However, observation of the dynamics on the microsecond timescale is still very challenging. This timescale is important because the elementary processes of protein dynamics take place and direct comparison between experiment and simulation is possible. Here we report a new single-molecule technique to reveal the microsecond structural dynamics of proteins through correlation of the fluorescence lifetime. This method, two-dimensional fluorescence lifetime correlation spectroscopy, is applied to clarify the conformational dynamics of cytochrome c. Three conformational ensembles and the microsecond transitions in each ensemble are indicated from the correlation signal, demonstrating the importance of quantifying microsecond dynamics of proteins on the folding free energy landscape.

Elucidating the protein folding mechanism is crucial to understand how proteins acquire their unique structures to realize various biological functions. With this aim, the folding/unfolding of small globular proteins has been extensively studied. Interestingly, recent studies have revealed that even such small proteins represent considerably complex processes. In this study, we examined the folding/unfolding process of a small α-helical protein, the B domain of protein A (BdpA), at equilibrium using two-dimensional fluorescence lifetime correlation spectroscopy with 10 μs time resolution. The results showed that although the BdpA is a two-state folder, both the native and unfolded states are highly heterogeneous and the conformational conversion within each ensemble occurs within 10 μs. Furthermore, it was shown that the average structures of both ensembles gradually change and become more elongated as the denaturant concentration increases. The analysis on two mutants suggested that fraying of the N-terminal helix is the origin of the inhomogeneity of the native state. Because the direct observation of the ensemble nature of the native state at the single-molecule level has not been reported, the data obtained in this study give new insights into complex conformational properties of small proteins.

Sum frequency generation (SFG) spectroscopy is a unique and powerful tool for investigating surfaces and interfaces at the molecular level. Phase-sensitive SFG (PS-SFG) is an upgraded technique that can overcome...

Human serum albumin (HSA) plays important roles in transport of fatty acids and binding a variety of drugs and organic compounds in the circulatory system. This protein experiences several conformational transitions by the change of pH, and the resulting conformations were essential for completing the physiological roles in vivo. Steady-state and time-resolved fluorescence spectroscopy was applied to single tryptophan residue solely arranged in HSA to study subtle conformational change around single tryptophan residue in HSA at various pH. The results showed the characteristic feature of local conformation around tryptophan residue in domain II responding to the change in entire structure. The study of time-resolved area-normalized fluorescence emission spectra (TRANES) also showed the peculiar dielectric property of water molecule trapped nearby tryptophan residue depending on pH. These results suggested that microenvironment around tryptophan residue was tightly packed at acidic and basic pH although entire conformation was loosened.

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