Fluorescence correlation spectroscopy conformational changes

Fluorescence correlation spectroscopy (FCS) measures rates of diffusion, chemical reaction, and other dynamic processes of fluorescent molecules. These rates are deduced from measurements of fluorescence fluctuations that arise as molecules with specific fluorescence properties enter or leave an open sample volume by diffusion, by undergoing a chemical reaction, or by other transport or reaction processes. Studies of unfolded proteins benefit from the fact that FCS can provide information about rates of protein conformational change both by a direct readout from conformation-dependent fluorescence changes and by changes in diffusion coefficient. 

IV. Advantages and Disadvantages of Using Fluorescence Correlation Spectroscopy to Study Protein Conformational Changes... 

Picosecond photon correlation experiments have some similarities to fluorescence correlation spectroscopy (PCS). PCS investigates the fluctuations of the fluorescence intensity of a small number of molecules confined in a small sample volume (see Sect. 5.10, page 176). The intensity fluctuations are correlated on a time scale from microseconds to milliseconds. Therefore, PCS differs from picosecond correlation in the way the photons are correlated. Moreover, PCS effects are driven by diffusion, conformational changes, or other sample-internal effects, while antibunching is driven by the absorption of the photons of the excitation light. 

Fluorescence correlation spectroscopy (FCS) is based on exciting a small number of molecules in a femtoliter volume and correlating the fluctuations of the fluorescence intensity. The fluctuations are caused by diffusion, rotation, intersystem crossing, conformational changes, or other random effects. The technique dates back to a work of Magde, Elson and Webb published in 1972 [335]. Theory and applications of FCS are described in [51, 429, 430, 431, 456, 457, 497, 537, 556]. 

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