Question

1. What is the net charge of the tripeptide Val-Ala-Glu at pH 6?

Answer 1

Step 1:

If pH < pKa, H+ is on. It means that the group is protonated

If it happens to be for -NH2 group, it will have +1 charge.

For -COOH group in this case, there will be no charge

If pH > pKa, H+ is off. It means that the group is de protonated

If it happens to be for -COOH group, it will have -1 charge.

For -NH2 group in this case, there will be no charge

Step 2: find individual amino acids

Val-Ala-Glu is composed of Val, Ala, Glu amino acids

Step 3: find pka's of all group in each amino acids from internet or textbook

For Valine:

pKa of acidic part, pKa1 = 2.32

pKa of basic part, pKa2 = 9.62

It has neutral side chain

For Alanine:

pKa of acidic part, pKa1 = 2.34

pKa of basic part, pKa2 = 9.69

It has neutral side chain

For Glutamic acid:

pKa of acidic part, pKa1 = 2.19

pKa of basic part, pKa2 = 9.67

pKa of side chain, pKa3 = 4.25

It has acidic side chain

Step 4: find charge on individual amino acid

Lets calculate charge on Val

acidic part is involved in forming bond with next amine group

pH is less than pKa2. So, basic part will have H+ on it

So, positive charge is present because of it

Hence, net charge on the Val is +1

Lets calculate charge on Ala

acidic part is involved in forming bond with next amine group

basic part is involved in forming bond with previous amine group

Hence, net charge on the Ala is 0

Lets calculate charge on Glu

pH is greater than pKa1. So, acidic part will not have its H+ on

So, negative charge is present because of it

basic part is involved in forming bond with previous amine group

pH is greater than pKa3 and since it is acidic, it will not have its H+ on it

So, negative charge is present because of it

Hence, net charge on the Glu is -2

Step 5: find charge on molecule

Hence, net charge on the molecule is -1