The
pK_(a)
value for N 1 of adenine is 3.64 , whereas the
pK_(a)
value for N 1 of guanine is 9.50 . Explain this difference.
The energy of N1 in a purine ring is lowest when it is in its
◻
trivalent form. Thus, protonated and therefore tetravalent and
◻
charged N1 in adenine readily gives up its proton to water (has a fow pKa value) to yeld its
◻
trivalent form. In contrast, the trivalent N1 of neutral guanine resists donating its proton to water (has a high pKa value) to yield a divalent and therefore
◻
charged ion.