ammonium acetate in plasmid isolation

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dilini
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ammonium acetate in plasmid isolation

hi,
 i tried a modified alkaline lysis method, at there after the lysis of plasmids with NaOH and SDS, 7.5M CH3COONH4 was added folloed by immediate addition of chloroform. then it was kept on ice for 10 min and cetrifuged then the queous layer was separated and added precipitation solution( 30% PEG + 1.5M NaCl)child on ice for 15 min, centrifuged and pellet was separated. the authers of the protocol says that this can specifically isolate large plasmids. i suprized how this happen and how it is differe or useful than alkaline lysis.can any one please explain this.  in some litreture says ammonium acetate is preferbly precipitate HMW DNA but in this proccess  the DNA precipitation step i think PEG step. so wahat is the function of ammonium acetate here and how it is specify HMw DNA than potassium acetate in alkaline lysis?
 thank you