The specific guanine binding site of the ribonuclease T1 family enzymes and of G-proteins is modeled in the cocrystal formed by 7-methylguanosine-5'-phosphate and phenylalanine

In the cocrystal formed by 7-methylguanosine-5'-phosphate.phenylalanine.6H2O, the interactions between guanine and phenylalanine are similar to those observed in the complex of ribonuclease T1 with 2'-guanylic acids, and those of the two G-proteins, Elongation Factor-Tu and ras oncogene p21, with GDP. They are similar in the following three points: (a) guanine N(1)H and N(2)H donate cyclic N-H...O hydrogen bonds to the carboxylate group of phenylalanine in the former cocrystal and to the side chain carboxylate group of Asp or Glu in the latter proteins, (b) O(6) of guanine accepts hydrogen bond(s) from main-chain NH group(s), and (c) the purine moiety is sandwiched between aromatic (or hydrophobic) amino acid side chains.