Closed-form solution and predictive simulations of photooxidation dynamics in tyrosine fluorescence under pulsed excitation

Intrinsic fluorescence of tyrosine residues provides a label-free approach for probing structural, dynamical and redox-dependent properties of biomolecular systems. However, the low fluorescence brightness of tyrosine together with the formation of long-lived photooxidized dark states complicates quantitative interpretation of fluorescence signals using conventional numerical and fitting-based approaches. In this work, an exact analytical framework is developed to describe excitation-driven photooxidation dynamics of intrinsically fluorescent tyrosine molecules under pulsed excitation conditions. A reduced two-state kinetic model consisting of an effective fluorescent state and a long-lived photooxidized dark state is employed to derive closed-form expressions for the temporal evolution of electronic-state populations. The analytical solution explicitly relates fluorescence dynamics to excitation irradiance, excitation pulse duration, excitation cross-section and photophysical transition rates without requiring numerical integration of the coupled rate equations. The derived formalism reveals two characteristic timescales governing the system dynamics, namely a photooxidation accumulation timescale, τ c , and a recovery timescale, τ red . These timescales define the transition between fluorescence-dominated and photooxidation-dominated excitation regimes and provide a quantitative criterion for distinguishing short and long pulse excitation conditions. The analytical framework further enables construction of dark-state accumulation regime maps and prediction of population dynamics under pulsed excitation sequences through piecewise propagation of the electronic state populations. Simulations demonstrate history-dependent population evolution arising from the interplay between excitation-driven photooxidation and dark-state recovery. The proposed framework provides a computationally efficient and physically transparent alternative to predominantly numerical photophysical analyses and may facilitate interpretation of transient-state fluorescence dynamics in weakly fluorescent biomolecular systems.