ABSTRACT
The chemical modification of nucleic acids is not as complex as that of proteins since there are fewer monomer units and, for all practical purposes, only nitrogen as a nucleo-philic reactive group; the nitrogen is reactive as a primary and secondary amine. Reaction at the primary amine groups of, for example, adenine, is referred to an exocyclic modification whereas reaction at the imine nitrogens of pyrimidines and purine rings is referred to as an endocyclic modification. There are also ring-opening reactions and cross-linking reactions.
