Department of Nuclear Medicine, Albert Einstein College of Medicine, New York, New York, United States of America.
PLoS One. 2009 Sep 30;4(9):e7229. doi: 10.1371/journal.pone.0007229.
Melanin, a high-molecular weight pigment that is ubiquitous in nature, protects melanized microorganisms against high doses of ionizing radiation. However, the physics of melanin interaction with ionizing radiation is unknown.
METHODOLOGY/PRINCIPAL FINDINGS: We rationally designed melanins from either 5-S-cysteinyl-DOPA, L-cysteine/L-DOPA, or L-DOPA with diverse structures as shown by elemental analysis and HPLC. Sulfur-containing melanins had higher predicted attenuation coefficients than non-sulfur-containing melanins. All synthetic melanins displayed strong electron paramagnetic resonance (2.14.10(18), 7.09.10(18), and 9.05.10(17) spins/g, respectively), with sulfur-containing melanins demonstrating more complex spectra and higher numbers of stable free radicals. There was no change in the quality or quantity of the stable free radicals after high-dose (30,000 cGy), high-energy ((137)Cs, 661.6 keV) irradiation, indicating a high degree of radical stability as well as a robust resistance to the ionizing effects of gamma irradiation. The rationally designed melanins protected mammalian cells against ionizing radiation of different energies.
CONCLUSIONS/SIGNIFICANCE: We propose that due to melanin's numerous aromatic oligomers containing multiple pi-electron system, 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 prevents secondary ionizations and the generation of damaging free radical species.
黑色素是一种高分子量的色素，在自然界中普遍存在，它能保护黑色素微生物免受高剂量电离辐射的伤害。然而，黑色素与电离辐射相互作用的物理机制尚不清楚。
方法/主要发现：我们从 5-S-半胱氨酰-DOPA、L-半胱氨酸/L-DOPA 或 L-DOPA 合理设计了具有不同结构的黑色素，这可通过元素分析和 HPLC 来证明。含硫黑色素的预测衰减系数高于不含硫黑色素。所有合成的黑色素都显示出很强的电子顺磁共振（2.14.10(18)、7.09.10(18)和 9.05.10(17)自旋/g），其中含硫黑色素的光谱更复杂，稳定自由基的数量更多。在高剂量（30,000 cGy）、高能（(137)Cs，661.6 keV）辐照后，稳定自由基的质量或数量没有变化，这表明自由基的稳定性很高，对伽马辐照的电离效应具有很强的抵抗力。合理设计的黑色素能保护哺乳动物细胞免受不同能量的电离辐射。
结论/意义：我们提出，由于黑色素含有多个芳香族低聚物，其中包含多个π电子体系，产生的康普顿反冲电子在通过色素时逐渐失去能量，直到其能量足够低，以至于可以被色素中存在的稳定自由基捕获。黑色素对高能反冲电子的控制耗散可防止二次电离和产生破坏性的自由基。
