C.S. Wilding BGYCSW at
Thu Apr 2 10:59:25 EST 1998

In article <01bd5e46$c5481880$34280b0a at>,
   "Adelino Canario" <acanario at> wrote:
>Does anyone know where I could find a table indicating the size of duplex
>DNA (PCR products) that precipitate according to PEG8000 concentration.
>Adelino Canario
>Universidade do Algarve
>8000 Faro
>Email: acanario at

Dear Adelino,
I sent a similar(ish) query to this list some time ago. Here is the 
extremely helpful reply (including refs) that I receieved in response:

At 19:22 12/11/97 +0000, C.S. Wilding wrote:
>Is there a lower cut-off point for PEG/NaCl 
>precipitation of PCR products prior to sequencing (as 
>referred to in Current Protocols). I usually use it for 
>600-1000bp products and it is OK but failed in an effort 
>on 120-180bp products. Is the method unsuitable in this 


Sorry for the delay in replying to your post...

What number of bp are precipitated depends upon the composition of the PEG
solution.  Two the initial references describing the use of PEG to
precipitate DNA was Lis (1980) Meth. Enzy. 65:437 and  Paithankar & Prasad
(1991) NAR 19:1346.  PEG shuts down virtually all enzymes except for 
ligase  (Teraoka (1987) J. Biochem. 101:225-231).  Fortunately, one of the
many enzymes that PEG inhibits is Taq (Barnes, (1992) Gene 112:29-35).  Up
until recently, most papers employed using anywhere from 10-13% PEG for
"general" precipitation of DNA and almost always used  a solution
containing 40% PEG (w/v) and1.5 M NaCl within.

Then Nicoletti,VG et al published a very slick mini prep protocol in 1993
BioTechniques 14:532-536, wherein they claimed that PEG with NaCl
inconsistently precipitated DNA below 500 bp and when below a critical
threshold of 1 ug/ml.  However, when they substituted 30mM MgCl2, then
precipitation was consistent down to about 120 bp using a final of 13% 
 [Personal note, this is one of the few homegrown mini preps that is
moderately quick, cheap, and will yield DNA that can be sequenced on an
automated DNA sequencer and whose DNA is "transfectable".]

With no year printed on the single sheet article, I came across a "Focus"
article wherein the author (a Gibco res. scientist)  kept the MgCl2
concentration the same, 30mM, but varied the PEG concentration.  At 10% 
all bands down to about 120 were precipitated.  With 8.3% PEG, not much
below 200bp was recovered, with 6.7% I'd guesstimate not much below 
was recovered and at 5%, ZILCH...  As we know, the amount of PEG to 
ligations is roughly 4-4.5% so failure to precipitate DNA using 5% was no

I've used PEG for quite a number of purposes, namely trough elution where
the recovered yield can be quite high (see Zhen and Swank (1993)
BioTechniques 14(6):894-898.   However, one must be very religious about
washing it out of the DNA preparation if it is to be used for anything
other than ligations.  Superstitiously, I wash the DNA pellet no less than
5 times with vortexing using 70% EtOH and even with that sometimes I'll 
a probe that won't label as well as one prepared through PEG-less methods
(trough, freeze squeeze, etc.)

I hope this helps,

 David L. Haviland, Ph.D.
 Asst. Prof. Immunology 
 University of Texas - Houston, H.S.C.
 Institute of Molecular Medicine  
 2121 W. Holcombe Blvd.  
 Houston, TX  77030 
 Internet:"dhavilan at" 
 Voice: 713.500.2413  FAX: 713.500.2424
"A conclusion is simply the place where you got tired of thinking."

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