Question

On paper draw a dipeptide, clearly showing the peptide bond joining the two amino acids together. If the two amino acids are valine and threonine, predict the overall charge of the dipeptide at pH 7. Do not forget to consider the amino (N-terminal) and carboxy (C-terminal) of the dipeptide, as well as the R groups.

Select one:

a. +2

b. -2

c. 0

d. -1

e. +1

Answer 1

The concept of peptide bond and the diagram is represented in the image below

Swo amino acid molecules can be covalently joined uch a substituted amide linkage termed a peptide hand such a linkage is formed by removal of the elements of water (dehydration) from the a carboxyl group of one amino acid and the x-amino group of another. H-CH3 - OH OH Is-OH не H₂N-CH-C-loH + t HA N-CH-coo THREONINE VALINE H₂O DHS Diese Seite ŞehgH-CH2-OH HĆ N-H₂N-CHI CH - QH- + d=0 R GROUPS MARKED IN RED

To know the charge of the dipeptide, we have to calculate the pI (Isoelectric point) of individual amino acids. pI is the characteristic pH at which the net electric charge is zero.

PI = (pk + pk 2) 1 pki (coot) pkz (- Nhat) Valine Ł (2:32 +9.62) -5.97 : Threoniene -> + (2.1179.62) - 5.86

pK values (of individual amino acids) are universal and constant. Now the concept is, any amino acid or peptide has a net negative charge above its pI and will move toward the positive electrode. In the question, the charge has to be found out at pH7. However, the pI of valine and threonine are less than ph7, which is why, they will possess (-1 for valine, -1 for threonine = -2) charge at pH7.

Therefore, the correct option is b. -2