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Colloquium | Nanoscience and Technology Division

Skin in the Game: Photoproperties of DNA and Melanin Biopolymers Revealed by Ultrafast Laser Spectroscopy

NST Colloquium

Abstract: An Achilles heel of many organic materials is photochemical degradation; yet, living organisms have thrived for eons in a world bathed in ultraviolet and visible radiation. Biopolymers, with their long history of evolutionary optimization, can inspire new strategies for designing photoactive materials. Our bottom-up studies of DNA excited states began 20 years ago with single nucleotides and have progressed to experiments on single- and double-stranded oligonucleotides. Nucleobase monomers act as absorbing sunscreens with excited states that decay in less than 1 ps. Ultraviolet excitation of single strands creates self-trapped excitons whenever two or more nucleobases are stacked. In double-stranded DNA, femtosecond time-resolved infrared spectroscopy reveals photoinduced interstrand proton transfer. Our growing understanding of DNA photophysics is now being used to study self-assembled DNA-metal nanoassemblies.

In human skin, the biopolymer melanin is present as carbonaceous nanoparticles, but melanin is found throughout the kingdom of life. Paramagnetic melanin is not only photoprotective, but it may also act as a scavenger of radicals. The structure of melanin is unknown, impeding efforts to understand this multifunctional material. The structureless absorption spectrum of this black pigment is believed to result from a heterogeneous distribution of chromophores. Our latest experiments demonstrate selective bleaching of groups of chromophores, but reveal a wavelength-independent response. Striking similarities between the photoproperties of melanin and disordered, two-dimensional carbon nanomaterials will be discussed.