Depending on the IsoValue ( typical units are electrons per cubic bohr ), or the percentage of total electrons enclosed, the electron density surface can be used to locate atoms, emphasize electron densities associated with chemical bonds, or to indicate overall molecular size and shape.
42.
However, as there are various measures of the size of a macromolecule ( for instance, the radius of gyration and the hydrodynamic radius ), a fundamental problem in the theory of SEC has been the choice of a proper molecular size parameter by which molecules of different kinds are separated.
43.
Instead, we assay the relative population of folded molecules using various structural probes, e . g ., absorbance at 287 nm ( which reports on the solvent exposure of tryptophan and tyrosine ), far-ultraviolet circular dichroism ( 180-250 nm, which reports on the secondary structure of the protein backbone ), dual polarisation interferometry ( which reports the molecular size and fold density ) and near-ultraviolet fluorescence ( which reports on changes in the environment of tryptophan and tyrosine ).
