The optical properties of eumelanin (from Sepia officinalis) are found to vary with particle size. The absorption spectrum for small eumelanin particles agrees quantitatively with the reported action spectra for photoinduced oxygen consumption and free radical generation by eumelanin. These small particles, unlike the large ones, generate long-lived reactive intermediates upon absorption of UV light. The data presented suggest that the small eumelanin particles may be involved in UV-A-induced photochemical processes believed to lead to DNA damage in skin cells, whereas the large particles efficiently dispose of UV-A energy through rapid nonradiative decay processes. These results provide new insight into the dichotomy that eumelanin is both photoprotective and photosensitizing. This size-dependent photoreactivity may be one of the contributing factors to the observed variations in skin cancer rates among different skin types.

Scanning electron microscopy (SEM) is used to examine the structure of natural and synthetic melanins. Eumelanin from Sepia officinalis and synthetic eumelanin are found to be structurally dissimilar. The natural sample has a significant structural order with subunits that have a lateral dimension of approximately 15 nm. The synthetic samples, on the other hand, appear to be amorphous solids. These results lend support for the existence of fundamental structural units proposed from the analyses of wide-angle X-ray diffraction measurements and previous mass-spectrometry results. These findings also provide insight into the disparate photophysical behavior of Sepia and synthetic eumelanin.

The emission properties of eumelanin from Sepia officinalis are examined following UV-A excitation. The emission decay is nonexponential, exhibiting decay components on the tens of picosecond to several nanosecond time scales. The corresponding depolarization dynamics are also nonexponential and reveal that the emission becomes totally depolarized with an average time constant of ∼80 ps at 20 °C. The depolarization of the emission is found to be activated; a simple Arrhenius fit to the depolarization rate data gives an activation barrier of 21 ± 3 kJ mol-1. The nonexponential emission decay is concluded to be a reflection of the structural disorder of eumelanin. The rapid and nonexponential depolarization dynamics are attributed to energy transfer processes that occur within “spherical” subunits that comprise the eumelanin aggregates.

Photoacoustic calorimetry is used to examine the energy dissipation in melanin under physiological conditions (pH 7.2) following irradiation by UV and visible (VIS) light. Four different excitation wavelengths were examined: 264 nm, representative of UVC radiation, 351 nm and 400 nm (UVA-I radiation) and 527 nm, representative of VIS radiation. Following absorption at 527 nm, essentially all of the photon energy is released nonradiatively on a sub-nanosecond of excitation. Similar results are observed at 400 nm. At 351 nm, most of the energy was released as heat; a small amount of energy was retained (5 +/- 5%). When melanin is excited at 264 nm, 29 +/- 7% of the photon energy is retained by the molecule for a time period longer than a few hundred nanoseconds. These results show that a long-lived excited state or reactive intermediate is generated upon UV irradiation, whereas all of the excitation energy is dissipated nonradiatively in the visible portion of the spectrum. These results establish that the photochemistry of melanin is wavelength dependent.

Photoacoustic calorimetry is used to examine the energy dissipation in melanin under physiological conditions (pH 7.2) following irradiation by UV and visible (VIS) light. Four different excitation wavelengths were examined: 264 nm, representative of UVC radiation, 351 nm and 400 nm (UVA-I radiation) and 527 nm, representative of VIS radiation. Following absorption at 527 nm, essentially all of the photon energy is released nonradiatively on a sub-nanosecond of excitation. Similar results are observed at 400 nm. At 351 nm, most of the energy was released as heat; a small amount of energy was retained (5 +/- 5%). When melanin is excited at 264 nm, 29 +/- 7% of the photon energy is retained by the molecule for a time period longer than a few hundred nanoseconds. These results show that a long-lived excited state or reactive intermediate is generated upon UV irradiation, whereas all of the excitation energy is dissipated nonradiatively in the visible portion of the spectrum. These results establish that the photochemistry of melanin is wavelength dependent.

Transient absorption spectroscopy has been used to elucidate the UV-A induced photodynamics of quinacrine. Following excitation of quinacrine in a pH 7.2 buffer solution at 395 nm, the excited molecule ejects an electron into the surrounding aqueous solution. The quantum efficiency for solvated electron formation at this excitation wavelength is determined to be in the range of 0.20−0.40. These results provide a simple mechanism that accounts for the generation of superoxide (O2 -) following UV excitation of quinacrine in aqueous solution.

The microwave spectrum of the cyclopropane-ammonia (CsHs* 14NHs) complex has been observed using a pulsed nozzle, Fourier-transform microwave spectrometer. The spectrum is characteristic of a symmetric top, IS,,= 2668.7 16 l(4), with free internal rotation of the NHs subunit. The spectra of the CsHs*r5NHs, CsDs*r5NHs, and CSHe*'4NDs isotopomers were also measured. This gives a structure in which the nitrogen of the ammonia interacts with the top of the cyclopropane ring, resulting in a stacked structure with R ,.,,=3.657(3). Thequadrupolecouplingconstant ofthe nitrogen nucleus is eQq=-2.509(2) MHz.

The microwave spectrum of the methylcyclopropane-HC1 (MCP.HC1) complex has been observed using a pulsed nozzle, molecular beam Fourier transform microwave spectrometer. Transitions were assigned with the aid of the quadrupole hyperfine structure from the chlorine nucleus. The rotational constants of the C~H T H~~C~ speciesareA =6408.1(2) M H z , B = 1410.486(1) M H z , a n d C = 1224.155(1) MHz. Inadditiontothisspecies, the spectra of the MCP-H3'C1 and MCP.D3T1 isotopomers have also been studied. The data are consistent with a structure in which the HCl interacts with the C -C bond of the MCP ring that is adjacent to the methyl-substituted carbon.