Question

Predict the charge on the major species of the peptide Ile–Lys–Asp–Ala–Arg–Thr–Asn at pH 7.0. Please explain why!!!

Answer 1

The pI will be the only pH where the amino acid has a net neutral charge.

Isoleucine does not have any side chains.

For isoleucin, at pH 7, the net charge will be almost neutral, because 7 is well above the carboxylic acid pK of 2.32 and significantly below the amino pK of 9.76. There will be a small fractional negative charge because 7 is closer to the pK of 9.76 than to the pK of 2.32.

Lysine- The side chain of lysine is charged at all pH lower than 10.53 (according to the table you linked), and so it would be charged at pH 7. NH2 is more or less stable, but once you start to have any decent amount of hydrogen in your solution it will donate its lone pair to become NH3+. It has +1 charge at pH 7.

At pH 7 lysine has a net charge of very close to +1.

The carboxylic acid group is fully deprotonated (-1 charge).

The alpha amino group is about 99% protonated (+1 charge).

The side chain amino group is fully protonated (+1 charge).

Aspartic acid

Aspartic acid has two hydrogen donors so it will have -1 charge at pH 7 ([-1]+[-1]+[+1])

Alanine - in pH 7 the amino acid is in a neutral solution and will have both positive and negative charges. So its zero.

Arginine

Arginine has +1 charge at pH 7 (+1+1-1=+1)

Two NH2 and one OH

Threonine

Threonine has -1 at pH 7

-1-1+1 = -1

Asparagine

Asparagine has +1 charge at pH 7 (+1+1-1=+1)

Two NH2 and one OH