Although caffeine and guanine have very similar structures, the type, number, and location of the individual atoms makes them very different. In fact, many molecules with this rough chemical structure, known as purines, are found in tea and coffee, and have subtle differences, and as a result have different effects on the body. The reason why DNA incorporates molecules like guanine rather than those like caffeine or even theobromine, theophylline, or uric acid, is the ability for guanine to form a specific hydrogen bonding sequence of donor and acceptor inactions in DNA. Most other purines cannot make the same interactions with cytosine, specifically a donor-donor-acceptor interaction. For this system, the correct arrangement of hydrogen bond donors and acceptors make a stable structure, and in chemistry, stability is king.