Purity of DNA (Ratio of OD = 1.8-2.0) ?

Darren Natale camdna at ubvmsb.cc.buffalo.edu
Tue Jan 12 10:04:00 EST 1993

```In article <1993Jan12.061103.2973 at kudpc.kyoto-u.ac.jp>, a52041 at sakura.kudpc.kyoto-u.ac.jp (Michael CHENG) writes...

>  Why is a preparation of DNA considered to be pure if OD260/OD280 => 1.8?
>  Actually, I've never thought of it myself so I went around asking my labmates.
>Well, nobody really knows the answer. Then I heard that there's possibly a
>theoretical formula that sets the above value range.
>  If this is true, I would appreciate it if someone out there can point me to
>the reference.
>
Both proteins and nucleic acids absorb at 280 and 260. However, proteins
absorb more strongly at 280 than at 260, whereas the opposite is true for
nucleic acids. The formula you refer to is probably derived from the data
of Warburg and Christian in a paper (in German) from 1945 (?) or so. In that
paper, the authors were more interested in protein purity, and presented a
table of 280/260 ratios versus percent nucleic acid contamination. The
contamination ranged from 0% to 20%, and a single value of 100%. I have
derived a graph of their data, and found a linear correspondence between
the ratio and the %nucleic acid. 100% nucleic acid has a ratio (280/260)
of 0.49, which is a 2.04 260/280 ratio. Here is my formula for percent
nucleic acid using _280/260_:

%NA = 2.06-1.18r
----------
0.4962r-0.2274    where r = A280/A260

DN

```