In one embodiment, modifications are those wherein {TAIL ~ another chemical group} is attached to the 5 position of {HEAD ~ a pyrimidine}, the 8 position of a purine, or the 2 position of a sugar 
In one embodiment , modifications are those wherein { TAIL ~ another chemical group } is attached to the 5 position of { HEAD ~ a pyrimidine } , the 8 position of a purine , or the 2 position of a sugar
OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH