Table I. Summary of Nitroxide Labeling Schemes that Use Solid-Phase Chemical Synthesis.

Nitroxide attachment site	Examples of chemical structure	References
1 Phosphate		[22]
2 Sugar: 2′-position		[23, 24]
3 Base: 4-position of uridine		[31, 46]
4 Base: acetylene linkage		[27, 30, 32, 34]
5 Base: 5-position of pyrimidines with variable tethers (2-11 atom)		[11]
6 Fused base		[28, 33]