Understanding Casein's Reaction in Water: pH 4.6 Residue Formation

[vilhelm]

What happens when you put protein in water?

I know that an amino acid in water vill dissolve into:
$$^{+}H_3 N-CHR-COO^{-}$$
As protein is built up by over 50 amino acids, will the very same thing (as the formula above) happen to it?

(My homework is to explain why Casein produces a residue when put into a water solution with pH≈4,6)

 

[Borek]

Protein is in a way similar to a single amino acid. You should be able to see it if you draw a simple protein, say GlyGlyGly.

 

[vilhelm]

I think this is the answer to the question why Casein produces a residue when put into a water solution with pH≈4,6 (but I'm very uncertain)

At that pH, this is what happens:

$H_2N-CHR-COOH \rightarrow ^{+}H_3N-CHR-COO^{-}$ (Amino acid put into water solution)

$pH ≈ pI \Rightarrow [^{+}H_3N-CHR-COOH] = [H_2N-CHR-COO^{-}]$

This leads us to the fact that there are no repulsive forces between the amino acids, thus the R-group of the amino acids of Casein are able to attract each other via van der Waals-bonds.
When pH≠pI the protein can not come together, because of the repulsive forces.

Correct, or incorrect?