Question

Draw the resonance forms of a protonated guanidino group and explain why arginine has such a strongly basic isoelectric point.

Short answer

Resonance forms of protonated guanidino group

Strongly basic isoelectric point of arginine is due to the high basicity of the guanidino group which is due to the resonance stabilisation in its protonated form.

Step-by-step solution

Step-1. Resonance forms of protonated guanidino group:

Protonation of guanidino group gives a resonance-stabilized cation which has all octets filled and positive charge delocalised over three nitrogen atoms. Delocalisation of charge and filled octets brings stability to the system. Though, positive charge on electronegative atom is not stable as in this case, positive charge is at nitrogen atom but nitrogen has filled octet in all resonating structures, so it is stable.

Resonance forms of protonated guanidino group

Step-2. Reason for arginine’s strongly basic isoelectric point:

The isoelectric point of arginine is 10.76 which is highest among amino acids. This is primarily due to higher basicity of the guanidino group which is attached to the side chain in arginine. Protonated form of guanidino group is resonance stabilised which increases the basicity of guanidino group and hence, isoelectric point of arginine.