Life on Earth exits in the constant flux of both ionizing and non-ionizing electromagnetic radiation. Consequently, various strategies to protect living organisms against radiation insults have emerged during evolution. Melanin, a high molecular weight pigment that is ubiquitous in nature, has been described to function as a free radical scavenger and has characteristics of an amorphous semiconductor 
. Many microorganisms constitutively synthesize melanin including human pathogenic fungi Cryptococcus neoformans
and Histoplasma capsulatum
, and this pigment is known to protect these fungi against oxidants, extremes in temperature, UV light, chemotherapeutic drugs, and microbicidal peptides 
. The ability of free-living microorganisms to make melanin is likely to be associated with a survival advantage in the environment 
that includes protection against solar 
and ionizing radiation 
. Dramatic examples of such radiation protection are provided by the reports that melanized microorganisms are colonizing the highly radioactive environment inside the damaged nuclear reactor in Chernobyl 
and cooling pool water in nuclear reactors 
One in two cancer patients is treated with radiation therapy at some point in the course of their disease. The availability of radioprotective materials suitable for internal administration which would protect normal organs without protecting the tumor would greatly enhance the efficacy of radiation therapy by permitting higher tumoricidal doses while protecting normal organs. Radioprotective materials could also be extremely useful for protection against terrorist actions using radiological devices and for protecting astronauts in space. This highlights the enormous need for novel nature-inspired radioprotectors.
Although there is an ample body of literature describing the optical, condensed phase electric, electron exchange, paramagnetic, UV-visible absorbance, and ion exchange properties of melanin 
, the mechanism of melanin's interaction with ionizing radiation remains relatively unexplored. We recently provided evidence for the capacity of melanins to function in transducing the energy of gamma radiation in living cells 
and that the radioprotective efficacy of fungal melanins is dependent on their chemical composition 
. Our goal is to rationally design, and evaluate by a battery of physico-chemical methods the melanins as novel radioprotectors for internal administration. For that purpose, we needed well-controlled conditions of chemical synthesis to yield pure melanins, as fungal melanins contain cell wall polysaccharides in their structure 
, and thus might be harmful or immunogenic if used for radiation shielding in humans.
Free radical scavenging, primarily of the highly destructive, short-lived radicals generated by the radiolysis of water, has been assumed to be the mechanism of melanin radioprotection 
. However, the actual mechanism of melanin's interaction with ionizing radiation remains unexplored. We hypothesize that the physical interaction between melanin and the recoil electrons generated by Compton scattering of incident photons in melanin itself or transferred to melanin by other molecules and radicals is another significant contributor to the mechanism of radioprotection. A single Compton recoil electron traveling through tissue can result in multiple points of damage to DNA or other cell structures via the generation and propagation of free radical species 
. We hypothesize that, due to melanin's numerous aromatic oligomers containing multiple π-electron systems, a generated Compton recoil electron gradually loses energy while passing through the pigment, until its energy is sufficiently low that it can be trapped by stable free radicals present in the pigment. Controlled dissipation of high-energy recoil electrons by melanin would prevent secondary ionizations and the generation of free radical species.
To test this hypothesis we synthesized chemically diverse melanins using precursors with different functional groups. To select melanins with the best radioprotective properties from the panel of synthesized melanins, we determined their physico-chemical characteristics to identify those that correlated with favorable radioprotective properties. Consequently, the melanins were subjected to: (i) elemental analysis, (ii) high performance liquid chromatography (HPLC), and (iii) electron paramagnetic resonance (EPR). Surrogate measurements of radioprotective properties were obtained by performing: (i) quantitative EPR to estimate the number of stable free radicals, (ii) determination of mass attenuation coefficients from the cross section used in Monte Carlo simulation; and (iii) clonogenic survival assay to determine melanin's ability to protect mammalian cells.