Question

In complete sentences on the back of this page or on a separate piece of paper answer the following questions. (5 pts each) 1. Choose any two known amino acids with similar R and based on the size and polarity of their side chains, give an explanation for why their retention factors are similar. 2. Choose any two known amino acids with very different Rr values and based on the size and polarity of their side chains, give an explanation for why their retention factors are dissimilar. 3. Explain why lysine's Rris lower than that of leucine, even though they both have the same number of carbon atoms in their side chains. 4. When you supplied the beaker with solvent, you were cautioned that the solvent level should be below the samples spotted on the chromatography paper. Why? 34

Answer 1

Retention factor ( Rf ) is defined as the distance travelled by the compound divided by distance travelled by solvent.

1. Leucine (R- = (CH₃)₂ -CH-CH₂- ) and isoleucine (R- = C₂H₅-CH(CH₃)- ) amino acids will have almost similar Rf values , as both are nonpolar amino acids ( with small hydrocarbon side-chain) .These amino acids have same molecular formula thus similar in size. Since , Rf depends upon polarity and size , thus similar for these two.

2. Histidine (with imidazole ring in side chain ) and Leucine (R- = (CH₃)₂ -CH-CH₂- ) have very dissimilar Rf values. Since, histidine is with polar and basic side chain , whereas Leucine is with non-polar side chain. Size of histidine is bigger than leucine. Thus due to drastic difference in size and polarity these have very didsimilar Rf values.

3. Lysine and Leucine both have 4 carbon atoms in side-chain , but Rf of lysine is lower than leucine, because Leucine  is with non-polar side chain, where as lysine is with polar and basic side chain ( it has an extra -NH₂ group ). As with increasing polarity organic compound moves more slowly, thus lesser the Rf value for lysine than that of leucine.

4. Solvent level should be below samples spotted on the chromatography paper, otherwise the spots will washed off into solvent before it travel .