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The pineal secretory product, melatonin, is a potent, endogenous hydroxyl radical (HO·) scavenger. When melatonin was incubated in different in vitro cell-free HO·-generating systems, a novel melatonin adduct was formed. The molecular weight of this new compound is 248. Its structure was found to be cyclic 3-hydroxymelatonin (3-OHM). A proposed reaction pathway suggests that 3-OHM is the footprint product of the interaction between melatonin with HO·. 3-OHM was also detected in the urine of both rats and humans. This urinary metabolite is identical to the compound generated in the in vitro chemical reaction between HO· and melatonin. This provides direct evidence that melatonin, under physiological conditions, functions as an antioxidant to detoxify the most reactive and cytotoxic endogenous HO·. When exogenous melatonin was administered to young rats, urinary 3-OHM levels increased significantly in the treated rats compared to those in controls. This indicates that even in young animals there is insufficient endogenously produced melatonin to detoxify the basal levels of the toxic HO·. The accumulated damage induced by the escaped HO· that results when the HO· avoids detoxification over the course of a life time may directly or indirectly accelerate aging and aging-related diseases.

The relationship between the calculated energy of the HOMO-LUMO gap, where (epsilonLUMO - epsilonHOMO)/2 is defined as DeltaHL, and of the longest wavelength transition in the UV-visible spectrum, DeltaE, was examined for a series of aromatic and antiaromatic cations and dications. TD-DFT calculations accurately modeled the energies of a series of dications including fluorenylidene dications whose UV-visible spectra are reported, as well as the energies of a series of aromatic and antiaromatic monocations whose spectra were previously reported. There is a linear correlation of the energy of the longest wavelength transition, DeltaEcalc, with DeltaHL. There is no linear relationship between DeltaEcalc and the sum of the NICS values, but there is a linear relationship between DeltaEcalc multiplied by the number of atoms in the conjugated system, DeltaEcalcT, and the sum of the NICS values. There is also an approximate linear relationship between the average 1H NMR shift and the sum of the NICS values. These relationships give further support to the suggestion that the magnetic and energetic criteria of aromaticity and antiaromaticity are related. Furthermore, the data suggest that species that have DeltaEcalcT < 20 are antiaromatic whereas those with DeltaEcalcT > 30 are aromatic.

A detailed study of the electronic behavior of a series of even nonalternant polycyclic aromatic hydrocarbons derived from fluoranthene is presented. Compounds were synthesized that contained substantial barriers to rotation of substituents into conjugation with the parent hydrocarbon. The heavy atom bromine was incorporated into several compounds to probe its ability to quench fluorescence by spin-orbital coupling. External heavy atom solvent effects on the quenching of fluorescence were also investigated. Fluorescence spectra are presented that clearly indicate that intramolecular heavy atom effects (HAEs) can be transmitted through phenyl substituents conjugated with a polycyclic aromatic hydrocarbon. The magnitude of the transmitted HAE is shown to vary as a function of the ability of the substituent to rotate into planarity with the polycyclic core. The steric strain present in the various fluoranthene derivatives is modeled using molecular mechanics and semiempirical molecular orbital calculations. Semiempirical molecular orbital calculations of the HOMO and LUMO energies are compared with data obtained experimentally by cyclic voltammetry.