Modification of the thiourea linkage of a fluorescein-oligonucleotide conjugate to a guanidinium motif during ammonia deprotection.

During the preparation of a fluorescent conjugate based on a manganese cationic porphyrin carboxylate derivative linked to a 5'-amino-3'-fluorescein-labeled oligonucleotide, we observed the chemical transformation of the thiourea linkage to a guanidinium function during the deprotection in ammonia of the 3'-fluorescein-oligonucleotide from CPG support. We also noted a secondary reactivity of the activated carboxylate group of the metalloporphyrin precursor with the fluorescein entity of the conjugated oligonucleotide.