Probing the molecular and electronic structure of capsaicin: A spectroscopic and quantum mechanical study
Probing the molecular and electronic structure of capsaicin: A spectroscopic and quantum mechanical study
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DOI：
10.1021/jp801890g
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发表时间：
2008-06-26
影响因子：
2.9
通讯作者：
Pichierri, F.
中科院分区：
文献类型：
--
作者：
Alberti, A.;Galasso, V.;Pichierri, F.
采用混合元密度泛函方法（MPWB1K）研究了辣椒素的构象偏好。其灵活的，下垂的侧链允许多种构象，只是在能量上略有不同。最稳定的构象具有明显的振动特征。为了阐明辣椒素清除自由基最有利的反应位点，还计算了各种均裂键离解能。在可能的自由基中间体中，烯丙基和苯基自由基在能量上是优选的。利用参考分子的光电子和电子透射光谱以及适当的量子力学计算，研究了辣椒素的填充和空电子结构。在此基础上，提出了辣椒素垂直电离能和电子附着能的可靠谱图。前沿分子轨道集中在香草基部分，中间脂肪链的影响不大。预测（负）第一垂直电子亲和与苯相似。辣椒素的吸收光谱及其转化为酚醛去质子阴离子（适度的变色位移）或苯氧基中性自由基（从无色到红色）的变化用时变DFT计算来解释。化学或电化学还原辣椒素后的ESR测量没有检测到相应的自由基阴离子。光谱显示了原始分子的断裂和多种自由基的形成，这些自由基被认为具有半醌结构。
The conformational preferences of capsaicin were investigated by using the hybrid meta density functional theory (DFT) method MPWB1K. Its flexible, pendant side chain allows for a multitude of conformations only slightly different in energy. The distinctive vibrational features of the most stable conformers were characterized. To elucidate the most favorable reaction sites of capsaicin for radical scavenging, various homolytic bond-dissociation energies were also calculated. Of the possible radical intermediates, the allyl and benzyl radicals are energetically preferred. The filled and empty electronic structures of capsaicin were investigated by exploiting the photoelectron and electron-transmission spectra also of reference molecules and suitable quantum-mechanical calculations. On this basis, a reliable pattern of the vertical ionization energies and electron-attachment energies of capsaicin was proposed. The frontier pi molecular orbitals are concentrated over the vanillyl moiety, with a modest influence of the amidic-aliphatic chain. The (negative) first vertical electron affinity is predicted to be similar to that of benzene. The absorption spectrum of capsaicin and its change by conversion into a phenolic deprotonated anion (modest bathochromic displacement) or a phenoxyl neutral radical (from colorless to red) were interpreted with time-dependent DFT calculations. ESR measurements following chemical or electrochemical reduction of capsaicin did not lead to detection of the corresponding radical anion. The spectra show fragmentation of the original molecule and formation of a variety of radical species which are believed to have a semiquinonic structure.