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