Under the experimental conditions reported by Phan and co-workers, the formation of the duplex/G4 hybrid is observed by two positive CD signals at 269 and 285 nm. Based on previous investigations, it is likely that the duplex stem contributes the signal at ~270 nm, whilst the G4 structure contributes the signal at ~290 nm. A strong negative band at 240 nm is also observed, another common spectral feature of G4 DNA secondary structures. Upon titrating the DNA structure with the inactive Z ligand, virtually no changes in the CD signal are observed, either in the DNA spectral region (λ < 320 nm) or in the ligand-only region (λ > 320 nm) where certain binding modes may be evidenced by the appearance of an induced CD in the achiral ligand upon binding to the chiral DNA structure (Figure ). n the case of the active E ligand however, a strikingly different behaviour is observed (Figure ). As the ligand concentration is increased, a hyperchromic shift in the positive CD signal of the DNA structure is clearly observed, and a strong CD signal is induced into the ligand (specifically, a negative band at λ = 330 nm and a positive band at λ = 480 nm). This provides compelling evidence for the association of the ligand with the achiral DNA and, together, the CD results observed for the Z and E isomers support the results of the UV/Visible titration studies that indicate the large difference in affinity of the two photoisomers for the DNA structure. Importantly, we found that the ligand binding and dissociation from the LTR-III G4 could be controlled reversibly by alternate irradiation with blue light (λ = 405 nm) and green light (λ = 520 nm) with no evidence of photofatigue after seven switches, and no disruption of the overall DNA structure was observed (Figure ). Moreover, in agreement with the preliminary experiments that demonstrated the back-isomerisation of E to Z, we observed the dissociation of the ligand from the DNA structure under dark conditions triggered by the reversion of the active E isomer to the inactive Z form (Figure ). These results are significant, as they demonstrate the presence of the DNA structure does not adversely affect the photochemical properties of the ligand and that its activity can be controlled in situ. ligand. However, more importantly, the E ligand, active in binding to the G4 structure, show no apparent binding to the duplex structure. Only a very modest hypochromicity in the DNA bands is observed, possibly due to non-specific binding. Moreover, in agreement with the preliminary experiments that demonstrated the thermal back-isomerisation of the ligand, we observed the dissociation of the ligand from the DNA structure under thermal conditions, demonstrating the presence of the DNA does not affect the electrostatic interactions, and no induced CD signal in the ligand region is seen, suggesting the lack of affinity of the ligand for the double-stranded structure. It should be noted that the appearance of induced CD signals is strongly dependent on the ligand binding mode to the chiral molecule and, as such, the absence of such an effect does not unequivocally rule-out the formation of a complex. Nonetheless, NMR experiments (vide infra) provided further evidence to support the ability of the E isomer to target the G4 region of the DNA structure.