Question

TABLE 4.1 Amino Acid Alanine Arginine Asparagine Aspartic acid Cysteine....« Glutamic acid Glutamine Glycine Histidine Isoleucine Leucine Lysine … Methionine Phenylalanine Proline Serine Threonine Tryptophan Tyrosine Valine pKa Values of Common Amino Acids α-COOH pKa R group pka 1949 4499909 00 0099 04990000 440990 44490909 41400 094044 004 04004 91400 044 1 9+499 9+99 19999* 19 * * * 송송

The one-letter sequence is: WATER

a) Draw the peptide (R-groups trans), indicating charges, in predominant form found at pH = 0.

b) What is the isoelectric point?

c) What is the average charge on the population of peptide macromolecules at pH = 2.2?

d) What is the average charge on the population of peptide macromolecules at pH = 12.5?

Answer 1

Answer ; One letter sequence is : WATER  

Where W = Tryptophan , A= alanine , T= Threonine , E= Glutamic acid , R = Arginine .

(a) structure of peptide (with R group trans )in predominant form at pH =0 , is shown in the image attached

Overall charge on peptide at pH = 0 is +2

(b) Isoelectric point is the pH at which there is no net charge on the peptide . Isoelectric point of given peptide is achieved when deprotonation of COOH group of arginine and COOH group of side chain of Glutamic acid become Deprotonated , because by their deprotonation overall charge on peptide become 0 , this is achieved at pH range 5~6 . So, isoelectric point is in between 5~6 . (for exact isoelectric point you will have to do titration) .

(C) charge on peptide at pH 2.2 is +1 ( due to deprotonation of COOH of arginine .

(D) charge at pH 12.5 is -2 (due to deprotonation of NH_{​​​​​3}^{​​​​+} of Tryptophan , COOH group of side chain of Glutamic acid and NH_{​​​​​2}^{​​​​+} of arginine .)

Stuuchuru st au s omins aurd C A) Grlutomiie. Lej ㄩ 1- at buso rang 1+1 접。 Tote 3 1-1