1. A compound having the structure according to Formula I: in which R 1 , R 2 , and R 3 are independently selected from substituted or unsubstituted alkyl and substituted or unsubstituted heteroalkyl; and R 4 is substituted or unsubstituted aryl or substituted or unsubstituted heteroaryl, wherein R 4 is other than adenine joined to the remainder of the compound through an endocyclic nitrogen atom of said adenine.