Bio-Rad Mini-PROTEAN® Tetra Cell - Manual (Page 22)

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Note: The pH attained in the resolving gel of the Ornstein-Davis
system approaches pH 9.5, which may be outside the range of
stability for some proteins, causing denaturation. Additionally,
the pI of the protein of interest may be too close to or above the
Ornstein-Davis buffer pH (9.5), which may result in a very low net
charge or a positive net charge that may significantly reduce or
even prohibit migration to the anode. Alternative discontinuous
systems can be found in an article by Chrambach and Jovin
(1983).
Note: It is very desirable to know the pI of the protein of interest
before selecting a buffer system.
2. Continuous Buffer Systems
A continuous buffer system will be required if discontinuous
systems cannot be used due to stacking-induced protein
aggregation. In a continuous system the same buffer is used
in the upper and lower electrode chambers as in the gel. Since
stacking does not occur, proteins migrate in bands at least as
wide as the applied sample. Consequently, sample volumes
should be minimized. The mobility of proteins in a continuous
system is dictated by pH rather than by sieving through the
polyacrylamide gel. For this reason, 6% polyacrylamide gels are
recommended for most applications. For very large proteins, 4%
or 5% gels may be used. McLellan describes various continuous
buffer systems from pH 3.8–10.2. Detailed protocols are
provided in section 4.3.