We first folded the minimal 1HP-junction and loaded it with an excess of pso-TFO. We then irradiated the complexes with the LED for 1, 3, and 9 seconds and separated the products using PAGE (Fig ). Analysis of the gel revealed that, after 1 second of irradiation, ~89% of the Cy5-labelled staple 1 was converted into products of higher molecular weight. The main product contained ~60% of signal and was consistent with the formation of a bis-adduct comprised of staple 0, staple 1 and the TFO. An additional product of intermediate molecular weight that contained 19% of the signal likely corresponded to the mono-adduct formed between the Cy5-labelled staple 1 and the TFO. To support this, it was observed that the proportion of the mono-adduct decreased by ~7% upon irradiation for 9 seconds, while the proportion of the bis-adduct increased by ~8%. In addition, both products migrated slower when pso-TFO-PEG5K was used in place of the pso-TFO. Overall, these results indicate that the 1HP-junction loaded with pso-TFO gets rapidly and specifically crosslinked upon irradiation, with formation of a bis-adduct containing pso-TFO and the two hairpin-forming staples.