Anomalous translational diffusion and subdiffusion, respectively, is a breakdown of the laws of mass action. As opposed to normal translational diffusion, in which the movement of molecules is not correlated with their previous position, anomalous translational diffusion molecules are spatially and temporally correlated.
This spatio-temporal correlation reflects a fundamentally different behavior compared with the one in dilute or very dilute solution, which, for example, affects the spread of molecules within live cells.
Until now, characterization of translational diffusion in live cells has relied almost exclusively on measurements of constant translational diffusion coefficients. There is some evidence that in live cells the apparent translational diffusion coefficients may not be constant, but instead can vary over time, even for inert molecules.
In order to take account of temporal randomness of molecular interaction, i.e. time (rate)-dependent sources of anomalous translational diffusion behavior, we decoupled spatial and temporal coordinates of anomalous diffusion. The exponent α (alpha) now quantifies the crowding conditions (spatial heterogeneity) and the exponent γ (gamma) the temporal heterogeneity in anomalous diffusion.
Knowing the crowding parameter α (alpha) for the cell type as well as the cellular compartment, the heterogeneous parameter γ (gamma) can be extracted from the measurements in the presence of the interacting reaction partner, e.g. ligand, for the same (fixed) α value.
Fluorescence correlation spectroscopy (FCS) and two-color fluorescence cross-correlation spectroscopy (FCCS) are the methods of choice for measuring the exponents of anomalous subdiffusive translational motion of molecules in live cells as demonstrated for the first time.