Indirubin: Nature finding efficient light-activated protective molecular mechanisms. (April 2023)

• Record Type: Journal Article
• Authors: Nobre, Danîela C.; Delgado-Pinar, Estefanía; Cunha, Carla; Galvão, Adelino M.; Seixas de Melo, J. Sérgio
• Is Part Of: Dyes and pigments. Volume 212(2023)
• Publication Date: 2023-04
• Subjects: Indirubin -- Indigo -- H-chromophore -- Indirubin derivatives -- TDDFT -- Photochemistry -- Ultrafast spectroscopy -- Proton transfer -- Keto -- Enol -- Syn-rotamer; Dyes and dyeing -- Periodicals; Pigments -- Periodicals; 667.2
• DOI: 10.1016/j.dyepig.2023.111116
• Languages: English
• ISSNs: 0143-7208

Abstract

Abstract: Indirubin (INR ), a red structural isomer of the blue indigo (IND ) shows, for an organic dye, a high stability towards light. The nature of this photostability is unknown and is here proposed to be linked to efficient dark processes dominating the excited state deactivation. As with IND, an excited state proton transfer (ESPT) process, together with the possibility of rotation around the central carbon-carbon bond (with formation of a syn -rotamer) are, solvent dependent, active molecular mechanisms of the decay pathways in INR . To equate this behaviour, INR, together with N -methyl-(MINR ) and N, N ′-dimethyl-indirubin (DMINR ) have been synthetized and fully investigated from a combination of steady-state, fast-transient absorption and fluorescence techniques together with TDDFT electronic calculations. To rationalize the behaviour of INR in water, the water-soluble sulfonated indirubin (5SINR ) and the tri-sulfonated indigo (3SIND ) were further investigated. For INR in viscous solvents (e.g. glycerol) ESPT is present. With the studied indirubin derivatives (exception made to DMINR ), an additional pathway, to ESPT, exists involving rotation between the two indole-like moieties, with a syn -conformer, leading to a more efficient radiationless deactivation pathway, when compared to indigo. The mechanism of syn -conformer formation is found to involve a longer lifetime in the viscous solvent. This new deactivation process is not present in IND . ESPT, found to be enhanced in water via intermolecular water assisted proton transfer, with a kinetic isotopic effect (KIE = kH /kD ), equal to 2 for 3SIND, is also found present for 5SINR with a KIE = 1.5. The common structural origin of the N–H⋯OC bonds in the DNA nucleotides and the indigoid dyes (INR and IND ), introduces a new perspective for the study of the molecular protective mechanisms in these molecules. Graphical abstract: Image 1 Highlights: Indirubin (INR) the red isomer of the blue indigo is fully characterized. Different substituted INR derivatives were synthesized and studied. ESPT and a syn -rotamer are involved in the mechanisms of the decay pathways in INR. INR retains the donor-acceptor nature of indigo (the H-chromophore). INR and INR are photostable due to highly efficient radiationless deactivation processes.