Physiological ionization and pKa prediction

http://www.daylight.com/meetings/emug00/Sayle/pkapredict.html

Physiological ionization and pKa prediction
Roger Sayle
Bioinformatics Group,
Metaphorics LLC,
Santa Fe, New Mexico.

HA <=> H+ + A- or HB+ <=> H+ + B

This provides a measure of the ease of proton disassociation of a site in a compound, the disassociation (or ionization) constant pKa, defined by the equation:

pKa = pH + log(protonated/unprotonated)

Alternatively, the pKa of a site can be thought of the pH at which the protonated and deprotonated fractions are equal. If the pH is higher than the pKa, the site is mostly deprotonated, and if the pH is lower than the pKa, the site is mostly protonated.