For example, if a highly hydrophobic moiety A is chosen, with particularly hydrophobic substituents on the purine moiety, then a more hydrophilic moiety B can be used. [0031] In one alternative, B is a moiety containing at least one carboxyl, carboxamide, carboxyl ester, or carbonyl function. [0032] In another alternative, B is a cyclic or acyclic moiety containing at least one hydroxyl, primary a