Determine net charge of amino acid with different pH valueAspetic has the following pKA values.

alpha-COOH: 1.89 alpha-NH2: 8.6 R-COOH: 3.9

So i need to find the net charges at pH of 7.1, 12 and 1.

Can someone be kind enough to explain to me how its done? Its going to be tested in my upcoming test so i really appreciate if i can get a responseyou use Hinderson equation: pH= Pka + log(-H/HA). For each amino acid, there is three parts: amine, carboxyl and an R. The net charge is determined whether each part protonates or deprotonates. You manipulate the equation so figure that out. How? If the pKA> pH then it will protonate giving you a +1, if the pKA < pH it will deprotonate giving you a -1. In the end you add up the charges to get the net charge.

SO! Say you have an amino acid whose charges are 2, 5, and 9 for carboxyl, side chain and amine respectively. What is the net charge when pH=7? For carboxyl: 27 so it will protonate giving you a +1. For the side chain: 5 0 so [HA] > [A-]. This makes sense as it tells us that a stronger acid will cause the formation of HA, the protonated form. If instead we make the solution more basic, ie raise the pH, so pH > pKa and log [HA] / [A-] has to be < 0 so [HA] < [A-]. This makes sense as it tells us that a stronger base will cause the formation of A- , the deprotonated form.

IMPLICATIONS : Typical simple carboxylic acids, RCO2H, have a pKa of about 5, and typical simple ammonium ions, RNH3+ have a pKa of about 9. Therefore, since the acid is the stronger acid (lower pKa) the amino acid will exist in the zwitterionic form where the acid has protonated the amine in neutral aqueous solution (or normal physiological pH).

These principles can be extended to poly acidic / basic systems (such as amino acids) by thinking of each pKa value in turn. This information will let you decide which structure of an acid or base will dominate at a particular pH. Let's look at an example. To the left are the processes for the amino acid HISTIDINE which has an extra basic group. It has three acidic groups of pKa's 1.82 (carboxylic acid), 6.04 (pyrrole NH) and 9.17 (ammonium NH). Histidine can exist in the four forms shown, depending on the solution pH, from acidic pH (top) to basic pH. (bottom). Starting from the top, we can imagine that as we add base, the most acidic proton is removed first (COOH), then the pyrrole NH then finally the amino NH. These takes us through each of the forms in turn. At pH < 1.82, A is the dominant form. In the range 1.82 < pH < 6.02 B is the dominant form. In the range 6.02 < pH < 9.17 C is the dominant form, and when pH > 9.17, D is the major form in solution.