 we 
 present a chemical method to selectively tag and enrich thymine 
 modifications , 5-formyluracil ( 5-fu ) and 5-hydroxymethyluracil ( 5-hmu ) , 
 found naturally in dna . 
 inherent reactivity differences have enabled 
 us to tag 5-fu chemoselectively over its c modification counterpart , 
 5-formylcytosine ( 5-fc ) . 
 we rationalized the enhanced reactivity of 
 5-fu compared to 5-fc via ab initio quantum mechanical calculations . 
 
 we exploited this chemical tagging reaction to provide proof of concept 
 for the enrichment of 5-fu containing dna from a pool that contains 
 5-fc or no modification . 
 we further demonstrate that 5-hmu can be 
 chemically oxidized to 5-fu , providing a strategy for the enrichment 
 of 5-hmu . 
 these methods will enable the mapping of 5-fu and 5-hmu 
 in genomic dna , to provide insights into their functional role and 
 dynamics in biology . 
Supplementary Material