pH-dependent absorption spectra of rhodopsin mutant E113Q: On the role of counterions and protein

Multi-scale simulation reveals that an amino acid substitution increases photosensitizing reaction inputs in Rhodopsins

Evaluating the availability of molecular oxygen (O₂ ) and energy of excited states in the retinal binding site of rhodopsin is a crucial challenging first step to understand photosensitizing reactions in wild-type (WT) and mutant rhodopsins by absorbing visible light. In the present work, energies of the ground and excited states related to 11-cis-retinal and the O₂ accessibility to the β-ionone ring are evaluated inside WT and human M207R mutant rhodopsins. Putative O₂ pathways within rhodopsins are identified by using molecular dynamics simulations, Voronoi-diagram analysis, and implicit ligand sampling while retinal energetic properties are investigated through density functional theory, and quantum mechanical/molecular mechanical methods. Here, the predictions reveal that an amino acid substitution can lead to enough energy and O₂ accessibility in the core hosting retinal of mutant rhodopsins to favor the photosensitized singlet oxygen generation, which can be useful in understanding retinal degeneration mechanisms and in designing blue-lighting-absorbing proteic photosensitizers.

The role played by solvent polarity in regulating the competitive mechanism between ESIPT and TICT of coumarin (E-8-((4-dimethylamino-phenylimino)-methyl)-7-hydroxy-4-methyl-2H-chromen-2-one)

Excited-state intramolecular proton transfer (ESIPT) and twist intramolecular charge transfer (TICT) are the two most fundamental dynamic processes, ubiquitous in biological and chemical reactions. The excited-state properties of (E-8-((4-dimethylamino-phenylimino)-methyl)-7-hydroxy-4-methyl-2H-chromen-2-one (CDPA) in various solvents with different polarities were investigated by using steady-state and femtosecond transient absorption spectroscopy combined with DFT/TDDFT calculations. The results demonstrated that CDPA exhibited low fluorescence in polar acetonitrile (ACN) due to ESIPT but high fluorescence in nonpolar n-Hexane was attributed to intramolecular rotation blocking ESIPT. TDDFT calculations confirmed that the dramatic phenyl group torsional of CDPA in Hexane, whereas a near planar conformation in ACN solvent. The ESIPT barrier decreases regularly with the increase of solvent polarity from n-Hexane, tetrahydrofuran to ACN solvent. These results demonstrated that the ESIPT and TICT processes of CDPA are competitive mechanisms. Our work revealed the effect solvent polarity on the emission behavior and excited-state deactivation mechanism of CDPA, which could help to design and develop new polarity probe in the microenvironments.