Calculating a simple pI

[Louis Hom]

It's simple enough to calculate the pI for a molecule with one acidic group
and one basic group -- you just take the arithmetic mean of the two pKa's
(as in Lehninger).
But what about systems with, say, two acidic groups and one basic group?
Do you still just take the arithmetic mean?
-- 
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*   Lou Hom			*	"All folks are family."            *
*   lhom at ocf.berkeley.edu       *    			-- John Saponara   * 
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