Lysine basicity

[Meredith92]

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How is lysine basic at phsiologic pH? If its pKa is 10.5 then at normal pH the amino group will be protonated and would have much decreased nucleophicity and therefore be a poor base. Is it just defined as a basic amino acid because it has the capability at high pHs to be a base?
Thanks

 

[Trayshawn]

consider the pure substance. pure lysine exists as a zwitterion with an uncharged amino group. Put that in pH 7 water and it deprotonates the water to form the protonated amino group, i.e. it acts as a base.
As you mentioned, it will continue to do this at any pH water until you get to about 11.

 

[Meredith92]

Great thank you!