Department of Chemistry, University of Hawaii at Manoa, Honolulu, Hawaii, 96822, USA.
Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.
Phys Chem Chem Phys. 2022 Oct 27;24(41):25077-25087. doi: 10.1039/d2cp03084e.
The molecular origins of homochirality on Earth is not understood well, particularly how enantiomerically enriched molecules of astrobiological significance like sugars and amino acids might have been synthesized on icy grains in space preceding their delivery to Earth. Polycyclic aromatic hydrocarbons (PAHs) identified in carbonaceous chondrites could have been processed in molecular clouds by circularly polarized light prior to the depletion of enantiomerically enriched helicenes onto carbonaceous grains resulting in chiral islands. However, the fundamental low temperature reaction mechanisms leading to racemic helicenes are still unknown. Here, by exploiting synchrotron based molecular beam photoionization mass spectrometry combined with electronic structure calculations, we provide compelling testimony on barrierless, low temperature pathways leading to racemates of [5] and [6]helicene. Astrochemical modeling advocates that gas-phase reactions in molecular clouds lead to racemates of helicenes suggesting a pathway for future astronomical observation and providing a fundamental understanding for the origin of homochirality on early Earth.
地球上手性起源的分子机制尚未得到很好的理解，特别是在这些具有生物意义的对映体富集分子（如糖和氨基酸）被输送到地球之前，它们是如何在太空冰粒上合成的。在对映体富集的螺旋烯耗尽到含碳颗粒上之前，在分子云中可能已经通过圆偏振光对碳质球粒陨石中鉴定出的多环芳烃（PAHs）进行了处理，从而形成了手性岛。然而，导致外消旋螺旋烯的基本低温反应机制仍然未知。在这里，我们通过利用基于同步加速器的分子束光致电离质谱结合电子结构计算，为通往[5]和[6]螺旋烯外消旋体的无势垒、低温途径提供了令人信服的证据。天体化学模型认为，分子云中的气相反应导致螺旋烯外消旋体，这为未来的天文观测提供了一种途径，并为早期地球上手性起源提供了基本的理解。
