Fig. 4

Mechanisms underlying fluorescence modulation. a UV-Vis absorption spectra of cyclo-WW self-assemblies in the absence or presence of Zn(II) in MeOH. A peak at 515 nm appeared when complexing cyclo-WW and Zn(II). b Job Plot analysis of cyclo-WW with Zn(II) at different ratios, with the total molar concentration fixed at 15.0 mM. The red lines were added for guideline, showing the intersection point at cyclo-WW proportion of 0.7. c Chemical shifts of cyclo-WW hydrogen atoms upon coordination with Zn(II), compared to the peptide alone. The hydrogen atoms in different chemical environments are marked with italicized letters and labeled in e. d FTIR spectra of cyclo-WW self-assemblies in the absence or presence of metal ion/UV irradiation. The peaks of the active bonds are numerically marked. Note that the IR spectra were vertically moved for clarity. e Schematic presentation showing the possible molecular mechanism of cyclo-WW dimer coordination with Zn(II): the backbone diketopiperazine rings contribute to the complexation through nitrogen atoms, while the side-chain indole rings form aromatic interactions. f MS spectra of cyclo-WW + Cu(II) and cyclo-WW + Cu(II) + UV, showing the MW of oxidized cyclo-WW (marked in red) and reduced Cu(I), confirming the redox reactions in the solutions