Acid-Base Properties of Amino Acids

Investigate the pH-dependent protonation of amino acids.

Tasks

1. Create and optimize a glycine molecule (\(\ce{H2N-CH2-COOH}\)). Open Build Menu and select Add Hydrogens for pH... Choose different values of pH : 1,2,3,…,13 and note the effect of pH shift on electrical charge of glycine.
2. In a new View, create a (Glutamic acid - Lysine) dipeptide using Build Menu > Insert > Peptides... Set pH to 3 and calculate the correspondent electrical charge of the dipeptide. Do the same for pH=7 and 13.

When performing a paper electrophoresis at these pHs, in which direction will the dipeptide move (anode or cathode) ?

Solution

1. Glycine electrical charge according to pH shift:
   • pH<5: \(\ce{NH3+-CH2-COO-}\) \(\ce{H+}\); net charge =+1
   • 5≤pH<10: \(\ce{NH3+-CH2-COO-}\); net charge =0
   • 10≤pH≤13: \(\ce{NH2-CH2-COO-}\); net charge =-1
2. Glu-Lys dipeptide:

pH
Net charge	+1	-1	-2
Electrophoresis	→ cathode (−)	→ anode (+)	→→ anode (+)

Further readings

• chemguide.co.uk: The acid-base behaviour of amino acids
• Wikipedia:Isoelectric point