Centre for Molecular and Materials Science , TRIUMF , 4004 Wesbrook Mall , Vancouver , British Columbia , Canada V6T 2A3.
Department of Chemistry , Simon Fraser University , 8888 University Drive , Burnaby , British Columbia , Canada V5A 1S6.
J Phys Chem B. 2019 May 30;123(21):4540-4549. doi: 10.1021/acs.jpcb.9b02930. Epub 2019 May 16.
The radicals formed by muonium (Mu) addition to four nucleobases (adenine, guanine, cytosine, and thymine) have been characterized by avoided level-crossing muon spin resonance (ALC-μSR). Mu is considered to be a light isotope of the hydrogen atom, and the muoniated radicals observed by ALC-μSR are isotopomers of the radicals initially produced by H addition to the nucleobases. The observed radicals have been assigned by considering the relative energies of the possible radicals reported in the literature and comparing the experimental muon and proton hyperfine coupling constants with values from previously reported electron paramagnetic resonance and ab initio calculations that have been scaled to account for the larger magnetic moment of the muon and its lighter mass compared with the proton. Mu addition is observed to occur only at secondary carbons of the purine rings in adenine and guanine. Mu adds to C8 and C2 of adenine with the relative amount being ∼70:30%, and Mu adds exclusively to C8 of guanine. Mu addition is predominantly to the secondary carbons of the pyrimidine ring in cytosine (C5 and C6 with relative yields ∼80:20%) with a small amount of addition at N3. Mu adds to both the secondary C6 and tertiary C5 in thymine with approximately equal yields as well as the O4 adduct being a minor product.
通过 Mu 与四个碱基（腺嘌呤、鸟嘌呤、胞嘧啶和胸腺嘧啶）加成形成的自由基已通过避免交叉 Mu 自旋共振（ALC-μSR）进行了表征。Mu 被认为是氢原子的轻同位素，通过 ALC-μSR 观察到的 Mu 加合物自由基是最初由 H 加成到碱基形成的自由基的同位素异构体。通过考虑文献中报道的可能自由基的相对能量，并将实验中 Mu 和质子超精细耦合常数与以前报道的电子顺磁共振和从头计算的值进行比较，这些值已被缩放以考虑 Mu 较大的磁矩及其与质子相比较轻的质量，从而对观察到的自由基进行了分配。Mu 仅在腺嘌呤和鸟嘌呤的嘌呤环的次级碳原子上发生加成。Mu 与腺嘌呤的 C8 和 C2 加成，相对比例约为 70:30%，而 Mu 仅与鸟嘌呤的 C8 加成。Mu 加成主要发生在胞嘧啶的嘧啶环的次级碳原子上（C5 和 C6 的相对产率约为 80:20%），少量加成发生在 N3 上。Mu 在胸腺嘧啶的 C6 次位和 C5 叔位上的加成量大致相等，并且 O4 加合物是次要产物。
