Stability and photodynamics of lumichrome structures in water at different pHs and in chemical and biological caging media.

We report on photophysical studies of lumichrome (Lc) in water at different pHs, and interacting with the human serum albumin (HSA) protein and β-cyclodextrin (β-CD) in neutral aqueous solutions. We used steady-state and picosecond time-resolved emission spectroscopy to investigate the structural changes of Lc at the ground and excited states, as well as the rotational dynamics of the complexes with HSA and β-CD. In neutral water, the predominant neutral alloxazine-type structure of Lc coexists with a small population of the anionic form. In the presence of HSA, we observed an increase in the absorption band intensity at 450 nm. This increase is due to a preferential complexation (1:1 stoichiometry, K=8600 M(-1)) of the Lc anion structures within the protein. This change is not observed when β-CD is added, in which the Lc neutral form is exclusively complexed, giving a 1:1 stoichiometry. The fluorescence lifetimes of Lc in neutral water solutions are 4.2 and 2.3 ns, assigned to anionic and neutral alloxazinic forms, respectively. Using β-CD, the lifetime of the 1:1 complexes is 0.74 ns, while in the case of HSA complexes we observed two lifetimes (0.83 and 0.14 ns), which we explained in terms of different interactions of the anions with the protein. The rotational relaxation time of free Lc in neutral water is 75 ps. For Lc:β-CD complexes this time is 0.44 ns, in full agreement with the expected value from the hydrodynamic theory. For HSA solutions, we obtained a distribution of values between ∼1 and 4.5 ns, suggesting a site heterogeneity of complexation and a different strength of binding for the involved Lc anionic forms. Our results give information about the different photorelaxation behavior of Lc within chemical and biological cavities, and might help in a better design of nanosystems for drug carriers and delivery.