 Dear students, the amino acids have unique properties which contribute towards building the overall property of a protein after folding. So we continue to study these properties so that we can better understand the properties of the proteins. You know that some of the amino acids can be positively charged and some of them are negatively charged. So these charged states, they vary with the pH because at low pH a lot more protons would be available as opposed to a high pH. So the charged state of an amino acid is essentially the property of the R group. The property of the R group to accept protons or donate protons. So that is how the overall charge of an amino acid can be determined. For that we need to understand the concept of PK value. So the PK of charged amino acids is simply the pH at which half of the molecules of the amino acid are charged while the other half are not charged. So PK can be different for different amino acids. So any amino acid given a 100 molecules of that amino acid, if 50 of them are charged and 50 are not charged, then that pH will be the PK value for that amino acid. So the negatively charged amino acids, that is aspartic acid and glutamic acid, they have PK values of 3.9 and 4.2 while for the positively charged amino acid lysine, arginine and histidine PK values for the side chain are 10.5, 12.5 and 6. So this is a very important concept because it helps you to determine the charge on an amino acid at any given pH. If the pH is less than the PK value for an amino acid, the amine side group chain gains a proton and becomes positively charged, hence basic. As shown here in the figure that if a proton is added onto the amine group, then the overall charge is positive. So if the pH is less than PK, only then enough protons will be available to be added and give rise to a positively charged amine group. Dear students, what if the case is reversed? If pH is greater than PK for an amino acid, the carboxyl side chain loses a proton. As opposed to the previous case. So upon losing a proton, the carboxyl group obviously it becomes negatively charged and hence acidic. You can see here the overall charge of the carboxyl group is negative and this is only for a pH that is greater than the PK value. So these perturbations in the PK value, they will tell you how many charges an amino acid has and the state. So towards that an overview or a complete chart of these five amino acids is shown here. The pH is provided on the vertical axis here and it varies from 0 till 13 and here if you see in the red, we have the pH values which were the PK values for these amino acids. So once the pH is increased, then the charge state of each amino acid changes. For the positively charged amino acids, if the pH is reduced, the charge state will differ. So this is a very nice elaboration on the behavior of these charged states as the pH varies. So to conclude, depending upon the pH value, you can calculate the charge state of your amino acid. This is very helpful if you want to extrapolate it to the protein as well. So depending upon the amino acid in question, this charge may be positive or negative.