the principle of double-sided size selection of Illumina DNA prep bead clean-up?

[szy0931]

I don't fully understand the principle of double-sided size selection of Illumina DNA prep bead clean-up. Can someone explain to me?
I know the first clean-up is using diluted SPB beads to remove right-side large fragments and the second clean-up is using undiluted SPB beads to remove left-side small fragments.
I also know in both clean-up steps, the larger fragments are bound to beads. In the first clean-up, we keep the supernatant which contains the smaller fragments and in the second clean-up we keep the beads which contains the larger fragments.
What I do not know is why the fragments larger than a certain size are bound to the diluted beads and the fragment larder than another size are bound to the undiluted beads,

[SethDennison]

From what I understand about the SPB beads, the behavior of the DNA is dependent on the concentration of PEG and salt in the reaction, the volumetric ratio of beads to DNA is critical. Part of the reason for this effect is that DNA fragment size affects the total charge per molecule with larger DNAs having larger charges; this promotes their electrostatic interaction with the beads and displaces smaller DNA fragments. When you decrease the ratio there only larger DNA fragments will be allowed to bond with the beads while if you you increase the ratio smaller DNA fragments have the chance to bond.

[Lindsey]

I know it's not really an answer to your question, but I think that this figure is very helpful to understand bead purification.