Re: how DNA precipitates during isolation of DNA by CTAB method?

CTAB, aka cetyltrimethylammonium bromide or hexadecyltrimethylamonium 
bromide, is a cationic surfactant that forms insoluble complexes with 
nucleic acids at sodium chloride concentrations below 0.5 M. Most 
carbohydrates and proteins remain soluble under those conditions and can 
be separated from the precipitated DNA by centrifugation. CTAB/nucleic 
acid complexes are soluble at higher salt concentrations, and this 
property is used to rid the nucleic acid from the detergent. After 
increasing the salt concentration to solubilize the precipitated 
CTAB/nucleic acid complexes, the nucleic acid is precipitated by the 
addition of ethanol. The detergent, which is more soluble in ethanol than 
in water, remains in the supernatant after centrifugation.

The interaction between CTAB and nucleic acids is predominantly ionic in 
nature, via the negatively charged phosphate groups on the nucleic acid 
and the positively charged detergent (1). Hydrophobic interactions might 
also play a role. The shielding of charges renders the nucleic acid less 
soluble in water. An increase in ionic strength by addition of salt leads 
to displacement of the detergent and solubilization of the nucleic acid.

CTAB is often used to purify DNA from plant tissues, which are rich in 
contaminating carbohydrates (2). A variation of the CTAB DNA 
precipitation method is to extract carbohydrates and proteins with 
chloroform in the presence of CTAB and 0.7 M NaCl (3) prior to removal of 
CTAB by ethanol-precipitation of the DNA.

(1)  H. Morimoto, P.A. Ferchmin and E. L. Bennett. Anal. Biochem. 62: 436 
(1974)
(2)  http://iprotocol.mit.edu/protocol/120.htm
(3)  http://www.biosci.ohio-state.edu/~awolfe/ISSR/protocols.DNA.html