What is the role of glucose in plasmid isolation?
This is a common misconception: in fact, in the alkaline lysis method for plasmid isolation, the glucose does not act as an osmotic stabiliser.


Glucose is present in the resuspension buffer at 50 mM. This concentration is not high enough for osmotic protection of spheroplasts formed by lysozyme treatment: from memory, 0.5 M glucose is typically used for stabilisation. So why is the glucose included?

The original method for plasmid preparation by alkaline lysis was published by Birnboim and Doly in 1979. They explain that the method relies upon achieving a pH of between 12.0 and 12.5 after addition of NaOH so that linear dsDNA (sheared chromosomal DNA) is denatured but closed-circular DNA (plasmid) is not. They state in the paper that:


  further pH control is obtained by including glucose as a pH buffer.


According to this site glucose has a pKₐ of 12.3, which aligns nicely with the claim that it helps to keep the pH within the desired range.