Fluorescence kinetics of aqueous solutions of tetracycline and its complexes with Mg~2+ and Ca~2+

摘要

The fluorescence spectra of acidified aqueous solutins of tetracycline (tcH_3~+)exhibit three components with slightly different degrees of anisotropy:the 'blue"component (lambda_dapporox=475 nm)deccays on te timescale of a few picoseconds;the second ,most intense component (lambda_dapprox=530nm)shows decay times of about 25 (H_3O~+)and 70 ps (D_3O~+);the third component (lambda_d approx=650 nm)is longer lived (tau approx=200ps).All three fluorescence components appear quasiinstantaneouslym,thus providing evidence that the relaxation processes whihc give rise rto the unusualy large tokes shifts occur on a (sub-)picosecond timescale.The effect of H/D exchange suggests that these relaxations proceses involve exited-state intromolecular proton transfer (ESIPT) of OH10 and/or oh12,but does not exclude a change in the hydrogen-bonding pattern to the solvent molecules.The low overall fluorescence yield of the fully protonated form must be correlated to the presence of a very fast decaying species.In alkaline aqueosu solution,the fluorescence of the dianion (tc~2-)essentially comprises two components;the decary time of the stronger,shorter-lived component is about 30 ps,that of the weaker,longer-lived one about 160 ps.The relative amplitude of hte latter is larger at pH 11 than at pH 8.5,in accordance with the increase in the stady-state fluorescence intensity upon increasing the pH from 8.5 to 11.Complexation of the dianion with divalent methal ions like Mg~2+ or Ca~2+ leads to a strong enhancement of the steady-state fluorescence.In the time-resolved spectra,the decay time of the major fluorescence component exhibits approximately a five-fold increase in comparison to the major component of the dianion It is about 150 ps in both types of complexes.The decay times of the minor component are increased to about 500(Mg~2+)and 320 ps (Ca~2+).The absence of the ultra-fast component in the fluorexcence of the dianion and its metal complexes provides evidence that a reaction of OH12 must be responsible for the ultra-fast fluorescence component in tcH_3~+.The existance of a component with a lifetime of serveral tens of picoseconds in all samples suggests the involvement of hydrogen bonding at OH10 during the formation of the emitting species.DFT calculations for the isolated molecule provide evidence that ESIPT is indeed an energetically alowed relaxation process for those isomers that have only one interamolecular hydrogen both to O11.The esipt Process yields primary photoproducts that should emit at much longer wavelengths,thereby explaining the unusually large fluorescence Stokes shift.