Fig. 2: Structure and photochemistry of Pap as well as complex formation between its trans- or cis-state and α-CD or β-CD.

A Photo-induced cis/trans-isomerization of Pap. B, C While the structure of the azobenzene side chain in trans-Pap is mostly planar and elongated (B), the one of cis-Pap is twisted and more bulky due to the steric repulsion between the two aromatic rings (C). D UV-Vis spectra of the trans- and cis-states of Pap. A 50 µM solution of Pap in 100 mM Tris/HCl pH 8.0 either equilibrated under daylight (orange) or after irradiation with 355 nm UV light for 30 min (violet), respectively. Wavelengths of absorption maxima are indicated. E Time-dependent increase in absorption at 326 nm during irradiation of a 50 µM trans-Pap solution at 355 nm with a UV LED from the top. F Thermal re-isomerization of cis-Pap (50 µM) at 25 ± 1 °C in the dark as spectrophotometrically monitored at long measurement intervals (12 h). The data in (E) and (F) were subjected to exponential curve fit (applying asymptotic values for (F) from (E)). G, H Structural model of the complex between trans-Pap and α-CD (side view and front view, respectively; energy-minimized with the MM2 method using ChemDraw3D). I Titration of 50 µM trans-Pap in 2 mL 100 mM Tris/HCl pH 8.0 with a 50 mM solution of α-CD in the same buffer. J Titration of cis-Pap (100 µM) with α-CD (solid circles) in comparison with buffer alone (hollow circles). K Titration of trans-Pap (50 µM) with β-CD. L Titration of cis-Pap (100 µM) with β-CD. For all titrations – except for (J), where a straight line fit was applied – the small change (negative or positive, respectively) in absorbance at the diagnostic wavelengths of 426 nm for cis-Pap and 326 nm for trans-Pap during complex formation with the CD was monitored and the curves were fitted using Eq. 1.