Ultrafast dynamics of lumichrome in solution and in chemical and biological caging media

The ultrafast dynamics of lumichrome (Lc) in solutions and complexed by β-cyclodextrin (β-CD) and the human serum albumin (HSA) protein was studied by femtosecond (fs) time-resolved UV–visible emission and transient absorption spectroscopy. In organic solvents, we observed the emission components due to vibrational relaxation (VR)/cooling (up to 3.2 ps) and intramolecular vibrational-energy redistribution (IVR) (0.3–0.5 ps), as well as a relatively longer component (time up to 6 ps) assigned to H-bonding interactions (methanol) and leading to anionic forms in water/methanol mixture. The transient absorption experiment in water/methanol supports the fs-emission data. In presence of β-CD, the emission and transient absorption dynamics are similar to that observed in water. This suggests that cyclodextrin-caged Lc is accessible to water solvent, but the ps-rise time obtained in water at 750 nm is absent here showing that the complex absorbs at different region from that of free forms. Additionally, the transient absorption experiments show a rise of T1 → Tn triplet absorption which is faster in Lc/β-CD complex (∼0.5 ns) than that in water solution (>2 ns). This result suggests an increased efficiency of ISC process upon encapsulation. When complexed with the HSA protein, the Lc emission transients give decaying components of 0.3–0.45 and 5.6 ps at the higher-energy region, and in contrast with other media, no rising component is present at the red side of the emission spectrum. In turn, the transient absorption shows ps-decaying component (3.7–6.8 ps) and a ∼0.4 ps rise of non-emitting Lc structures interacting with the protein. These results suggested being due to fast electron transfer from HSA to Lc. They suggest a larger protection of Lc by the protein pocket from H-bonding with bulk water and reflect direct interactions between both entities. These findings give insight into Lc fs to ps behavior in solutions and when interacting with chemical and biological cavitants, and may be relevant to the behavior of flavin adenine dinucleotide (FAD).