We report the chemical synthesis of 5'-amino- and 5'-thiol-hexaethylene glycol guanosine nucleotides and their enzymatic incorporation into RNA. followed by chemical modifications at their nucleophilic ends. By using two similar routes. the conjugates of guanosine- 5'-monophosphate and hexaethylene glycol with attached reactive groups (-SH or -NH2) were synthesized using phosphoramidite chemistry. and characterized by MALDI TOF mass spectrometry. These Initiator molecules were efficiently incorporated into RNA at the 5'-end by run-off transcription using T7 RNA polymerase. The potential of these RNA Conjugates for a broad reaction range with electrophiles is shown here. thereby enabling their use for diverse biochemical applications. (c) 2006 Elsevier Inc. All rights reserved.