Molecular size refers to the geometry of the molecule and is reported as the hydrodynamic radius (Rh). Rh is the absolute size, in nanometers, of the molecule in solution. On the other hand, molecular weight is calculated as the sum of the atomic weights of all the atoms in a molecule and is reported in kilodaltons (kDa).
In general, larger sized molecules tend to have higher molecular weight than smaller molecules, but the relationship between molecular size and weight is not always straightforward. This is especially true when considering different conformational states of proteins, such as globular versus unfolded, as the complexity of measurements increases in this case.
Rh can be used for various purposes, as listed here.
It is also possible to convert molecular size to molecular weight using calibration curve. The hydrodynamic radius of a protein changes when, for example, the protein is unfolded or missing a subunit. Plotting the altered hydrodynamic radius on the calibration curve quickly visualizes and identifies the structural state of the protein. When the sample falls above or below the molecular weight-molecular size calibration curve, there has been a change in protein conformation. Using FIDA to examine the relationship between molecular size (hydrodynamic radius) and molecular weight offers unparalleled precision and auxiliary insights thanks to a first principle approach to measurements.