Question

1. Electrophoresis is a general term that describes the migration and separation of charged particles (ions) under the influence of an electric field. An electrophoretic system consists of two electrodes of opposite charge (anode, cathode), connected by a conducting medium called an electrolyte. The separation effect on the ionic particles results from differences in their velocity (v), which is the product of the particle's mobility (m) and the field strength (E):

v = mE

The mobility (m) of an ionic particle is determined by particle size, shape, and charge, and the temperature during the separation, and is constant under defined electrophoretic conditions.  At a pH of 9.5, tyrosine is strongly attracted to the positive end of an electrophoresis gel, whereas lysine moves a very small distance from the origin.

1. Draw both Tyrosine and Lysine at pH 9.5
2. Based on their structure what is the most likely reason for this observation
3. Would it be possible to alter the pH such that Lysine is strongly attracted to the positive end of an electrophoresis gel, and not Tyrosine? Why or why not?

Amino Acid	Acidic Group	pKa
Tyrosine	α-COOH	2.2
	α-NH3+	9.1
	R = CH2C6H4OH	10.1
Lysine	α-COOH	2.2
	α-NH3+	9.0
	R = (CH2)4NH3+	10.5

Answer 1

Structure of tyr and lys at pH 9.5 shows net neative charge and thats why attracted towards positive end of electrophoresis.

Ans B. The change in charge and structure is due to pKa values of COOH and NH2 of both amino acids because pH is 9.5 which is more than the pKa of COOH 2.2 and that leads to release of H++ ion and charge is COO- where as in NH2 pka is 9.1 and 9.0 for tyr and lys respectively which is also lower than the pH 9.5 and leads to release of H+ ion and NH2 is formed.

Ans C If we keep the pH of 9.1 than the neutral charge on the Tyr will be zero due to equal pka and pH of NH2 and thats why it will not move in gel electrophoesis where as 9.1 pH will cause the lysine to relelase H+ ion from NH2 and thats why overall charge will be negative on lysine due to COO-. So it is possible to get such results by changing pH from 9.5 to 9.1.