Show which of the nitrogen atoms in purine are basic, and which one is not basic. For the nonbasic nitrogen, explain why its nonbonding electrons are not easily available to become protonated.
Basic nitrogen are those which can donate its lone pair of electrons to acid. In purine base the two nitrogen six member ring are basic and one nitrogen of five membered ring is non basic.
They nonbonding electrons of nonbasic nitrogen are used in the ring for the delocalization. Hence they are not available for the protonation.