How is the isoelectric point of Lys Ala Glu Gly Glu calculated?

The isoelectric point is a state where the total charge is zero, then it should not be calculated as a single lysine, but 3 should be put together, that is, an amino group, a carboxyl group, plus 3 lysine side chains have a total charge of zero. Because Lys is basic AA, in the isoelectric point state, the amino group is not charged, and the carboxyl group has a negative charge, then each side chain is said to be dissociated by one-third. It is a one-third positive charge.

At the isoelectric point of this tripeptide, the carboxyl group has a negative charge, and the three side-chain amino groups have a total of one positive charge, which is balanced with the carboxyl group. Therefore, each side chain amino band 1 3 positive charges, apply the formula: pi=pka+lg (alkaline acid), and pi= is obtained.

Isoelectric point refers to the pH value of a solution when the number of positive and negative charges carried by a molecule is equal, which is usually represented by PI. Most biochemistry textbooks will mention the isoelectric point of amino acids, pi=(pka+pkb) 2 (for those without acid-base free radicals on the amino acid branch, such as ala, gly、...).or pi=(pka1+pka2) 2 (for those with acidic free radicals on the branched chains, such as glu, asp、....)) or pi=(pkb1+pkb2) 2 (for those with basic free radicals on the branched chains, such as lys, arg, 、....)And none of these books say why. In this paper, we will illustrate the calculation principle of amino acid pi by using a simple method of acid-base equilibrium and differential extreme value.

At the isoelectric point, the vast majority of amino acids in solution are electrically neutral, but there are very few amino acids that are positively or negatively charged. In a solution, these amino acids with different charge numbers are in equilibrium. Taking lysine as an example, when the solution changes from extreme acid to extreme base, we can list the main charge state and the change of acid-base free from lysine (hereinafter referred to as L) as follows

Browse Originals] Using equations (1), (2) and (3), we can denote L2+, L1+ and [L1- as L0, H+, Ka, KB1, and KB2. After moving items and sorting, you can get:

Browse the original] The total concentration of ionic acid in solution [l 0 is the sum of the concentrations of ionic acid with various charges, i.e.,

Browse Original] At the isoelectric point, the concentration of the electroneutral ionic acid [l0 has a maximum, that is, the following formula has a maximum: [Browse the original].

Substituting Eq. (4), (5), (6) and (7) into Eq. (8) yields:

Browse Original] at the isoelectric point [Browse Original].

Substituting Eq. (8a) into Eq. (9), and simplifying, we finally get:

Browse Original] At the isoelectric point, h "Therefore [Browse Original].

At this time, [Browse Originals].

Using the same principle, interested readers can derive the isoelectric points of the other two types of amino acids and oligosheng chains, and further can ** the isoelectric points of proteins, and then compare them with experimental values, and can preliminarily ** the rough distribution of charge on the surface and inside of the protein.

The isoelectric point of a small peptide is similar to the calculation of the isoelectric point of an amino acid! First of all, let's not look at the solution of such a problem!The separation between the free amino group and the free carboxyl group in the peptide bond is larger than in the average amino acid!

So the value of the pka of the carboxyl group is larger than that of the free amino acids, while the amino group is smaller! For details, please refer to the third edition of Biochemistry published by Higher Education Press!