Chapter 25, Problem 64PP

Interpretation Introduction

Interpretation:

Form of Asp-Lys-Phe that predominates at physiological pH need to be drawn.

Concept introduction:

Bond-line structure is the representation of organic structural formulas in a shorthand manner.  In this case only the bond between carbon and other atoms are shown except hydrogen.  It is understood that all the remaining valency to be filled by hydrogen.  Apart from carbon atoms the other atoms are shown along with hydrogen.  For drawing a peptide sequence in bond-line structure the steps need to be followed are,

• Draw the peptide with correct number of residues
• The side chain associated with the residue has to be identified
• Assign proper configuration for the residues

Amino acid has both carboxylic acid and amine functional group.  In presence of acid or base either one form will predominate which depends upon the pK_a value.  Usually the carboxylic acid group is deprotonated first and then the amino group is deprotonated.  At higher pH the carboxylic acid gets deprotonated and at lower pH the amino group gets protonated.

• The carboxylic acid form will predominate over carboxylate form if ${\text{pH<pK}}_{\text{a}}$ and vice-versa.
• The protonated form of $\alpha$ amino will predominate over the  $\alpha$ amino form if ${\text{pH<pK}}_{\text{a}}$ and vice-versa.
• If side chain is present the same rule as above applies.

Physiological pH means it is the pH of 7.4

To draw : To draw structure of Asp-Lys-Phe and the form that predominates at physiological pH