Here is a table with the predicted and experimental pI values for the amino acids.
2 The pH-charge plot for the histidine molecule showing a more complex plot with a pI 8.02. See a more complex pH-charge plot for the histidine molecule as an example.įig. 1 The pH-charge plot of the amino acid valine with a pI value of 6.16.įor molecules that have more ionizable sites the prediction uses the same principles however the pH-charge plot becomes more complex because of the number of microspecies. As the two dissociation constants (the pK a value of the COOH group and the pK b value of the NH 2 group) are far away from each other, we get a very simple pH-charge plot.įig. So there are only 2 microspecies (1 with +1 and 1 with -1 charge ) with exactly opposite charges. The following examples were calculated with the default settings of the Isoelectric Point Plugin.įor amino acids that have just 2 ionizable sites it is easy to predict the pI as there are only 3 microspecies, and 1 of them has 0 net charge. The accuracy of the prediction is heavily dependent on the accuracy of the pK a prediction and the pH step size. Predicting the isoelectric point for molecules with many ionizable sites is not easy in general as there are many microspecies. The pH step-size can be set for the algorithm. Our pI prediction algorithm approaches the (unique) zero point of the total charge function (which is a function of the pH). The isoelectric point is the pH where this sum is 0, that is The total charge can be expressed by the weighted sum of the net charges of the microspecies (let the charge of the i th microspecies be C i ), using the distributions as weights (let the distribution of the i th microspecies be d i ): (A molecule with N ionizable sites have 2 N -1 microspecies we do not count the natural form here as a microspecies) At a given pH the molecule is present as a set of microspecies with a given distribution, where each microcpecies has a net charge. Molecules with ionizable sites have many microspecies in aqueous solution. To determine the total charge of a protein at a given pH, the fractional number of positive and negative charges for each of the amino acids in the proteins. the solubility of a molecule at a given pH predicting the pI for proteins can help in their separation and purification processes. Knowing the pI can be important for predicting e.g. The isoelectric point (pI) of a molecule is the pH at which the molecule carries no net charge. This background material discusses the theory behind our isoelectric point calculation:
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