Biochemical clocks in fingerprints energy transfer between free tryptophan and NADH for potential estimation of postmortem interval, age, and sex.

NADH is an essential coenzyme involved in key metabolic processes. This study introduces a novel, non-invasive spectroscopic approach for analyzing age, sex, and time of death based on this coenzyme and other sweat-fatty substance components, while considering external factors such as drug exposure, acidic environments, and silver nanoparticles (AgNPs). The non-invasive mode involves the analysis of sweat-fatty substances collected from a fingerprint on a non-absorbent surface from both living and deceased individuals. The novel analysis considers excitation energy transfer (EET) in the form of Förster's resonance energy transfer and the complex mechanism of energy migration between other relevant components, such as tryptophan and free NADH in the fingerprint. The study also considered the intermediate dark state of electron transfer between NAD+, tryptophan, and NADH. The temporal resolution of the spectroscopy facilitated the determination of the temporal sequence of these processes, and the experimental approach adopted, in the form of a 'sandwich' analysis of the fluorescence processes of the tryptophan-NADH system, confirmed that photo-excited molecular vibrations enhance population transfer due to the strong coupling between the electron states of tryptophan and NADH. Model selection of the analytical environment - polar protic solvents like water and ethanol - enhanced the electron transfer process and EET. The analysis was based on the relative intensities of the different types of electron transitions occurring in tryptophan and NADH and the presentation of the correlation between the type of electron transition in the absorption and emission spectra, taking into account the maximum and total absorption coefficients. The analysis of the fluorescence lifetimes of the excited substances indicated as potential biomarkers of age, sex, and time of death allowed the derivation of a method based on the analysis of the EET from tryptophan to NADH in the mode of free molecules present in the sweat-fatty substance, i.e., in fingerprints. This research represents a significant advancement in the biochemical interpretation of fluorescent biomarkers and their medical and forensic relevance.