Question

#1) For the following pentapeptide (Where pH =1):

# 1) his gonu ca < m ça & sau ca & marça &.com CH,SH CH₂ CH₂ (Cysteine) CH₂ CH2 CH-CH2 CH-CH₂ CH3 C=0 CH₃ NH2 (Leucine) (Leucine) (Glutamine) ΝΗ. C=NH₂ NH2 (Arginine)

a) What would the net charge be for the following pH?

pH 0 =

pH 3 =

pH 5 =

pH 7 =

pH 10 =

pH 13 =

b) Calculate the pl of the pentapeptide. (Cysteine: pK1 = 1.96, pK2 = 10.128) ( Glutamine: pK1 = 2.17 , pK2 = 9.13) (Arginine: pK1 = 2.17. pK2 = 9.04) (Leucine: pK1 = 2.36 , pK2 = 9.60)

c) If you were to place your pentapeptide into a buffer with a pH of 4, in what direction would it move? Choose one of the three options below:

i) Towards the positive electrode.

ii) It will not move.

iii) Towards the negative electrode.

d) If you were to separate 2 other peptides from your pentapeptide, at what pH would you be able to do so and why?

Peptide 1 is made of the amino acids : Alanine - Arginine - Lysine - Serine

Peptide 2 is made of the amino acids : Alanine - Aspartate - Glutamic Acid - Serine

Answer 1

To calculate the charge at different pH we need to find the pKa values of diiferent ionisable groups. In above eg

pKa for cysteine is 8.3, pKa for argenine is 12.48,  pKa for NH3 is 8, pKa for COO- is 3.1

When the pH is lesser then pKa it means that there are more H+ ions present in the media which will associate with NH2 and will get a a positive charge becoming NH3+

Similarly when the pH is more then pKa it means that there are more OH- ions present in the media which will deprotonate COOH and there would be a negative charge COO-.

1) At pH=0 charges would be as follows +1+0+1+1=3

2) At pH=3 charges would be as follows +1+0+1-1=1

3) At pH=5 charges would be as follows +1+0+1-1=1

4)At pH=7 charges would be as follows +1+0+1-1=1

5)At pH=10charges would be as follows -1-1+1-1=-2

6)At pH=13 charges would be as follows -1-1-1-1=-4

c)At pH=4 the net charge on the peptide would be +1+0+1-1=+1 so the peptide would move toward negative electrode. The positive charge would attract toward the negative charge.