In a recent publication by a famous chemist, I came across this illustration in which they have calculated the pKa values of guanine and xanthine, among other bases.
Not only does the NH in the five membered ring in xanthine have a lower pKa than the NH next to the carbonyl in the six membered ring, but the pKa of the NH next to the carbonyl in the six membered ring does not change at all when you go from guanine to xanthine.
Maybe I have just had a long day, but this does not make sense to me at all. For the love of god, explain.
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