Spotlights pubs.acs.org/JPCL
Spotlights: Volume 8, Issue 18
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OPTICAL MEASUREMENT OF RADIOCARBON BELOW UNITY FRACTION MODERN BY LINEAR ABSORPTION SPECTROSCOPY
ULTRAFAST CHARGE-TRANSFER DYNAMICS IN THE IRON−SULFUR COMPLEX OF RHODOBACTER CAPSULATUS FERREDOXIN VI The ability to couple light absorption with productive outputs effectively and cheaply will be critical in the coming years. Nitrogen fixation and hydrogen generation have been extensively explored, and there is now growing interest in the photoinduced chemical processes involving iron−sulfur (FeS) proteins that underlie these chemistries in biological systems. For example, the involvement of FeS clusters in photosensitization in living cells and photoinduced electron transfer in the purple phototroph Rhodoferax fermentans has been reported. The incorporation of external photosensitizers to hydrogenases and nitrogenases enables photoactivated hydrogen production and nitrogen fixation, which opens the possibility of using FeS proteins and model compounds as photosensitizers for solar hydrogen production. While researchers have focused on the light-induced dynamics of charge insertion in FeS complexes, the photodynamics induced by the direct excitation of these important complexes is largely unknown. Mao et al. (10.1021/acs.jpclett.7b02026) report on the critical charge-transfer dynamics in these systems. For their study, the authors chose a representative protein that binds a 2Fe−2S cluster: the sixth ferredoxin (Rc6) discovered from Rhodobacter capsulatus, which is involved in the synthesis of FeS clusters. They characterized the ultrafast electronic and vibration dynamics of the Rc6 protein, and the transient absorption signals revealed that multiple ligand-to-metal charge-transfer populations were induced by laser excitation to evolve into low-lying states. Two long-lived states were identified, with the longer one attributed to a long-range “external” charge transfer, suggesting the existence of a photoinduced long-range electron transfer pathway in FeS proteins.
When Indiana Jones was not busy fighting Nazis, the fictional archeologist would have been locating ancient artifacts and studying them for clues about the past. Of course, he would not have had one important tool at his disposal in the 1930s: carbon dating. Developed in the late 1940s, radiocarbon dating is used by scientists to determine the age of ancient artifacts, elucidate the global carbon cycle, and pinpoint disruptive events in paleoclimatology. Precision and worldwide consistency are crucial for these measurements, which presents a challenge. Fleisher et al. (10.1021/acs.jpclett.7b02105) describe the optical detection of radiocarbon (14C) at submodern isotopic via linear-absorption spectroscopy. Using a benchtop cavity ring-down spectrometer, they achieve 14C sensitivity comparable to that of commercial 14C measurement facilities. The authors also quantitatively analyzed CO2 from the combustion of either biogenic or petrogenic carbon, and their findings may be useful in many areas of study, from atmospheric chemistry (CO2 source apportionment) to physical organic chemistry (biobased products, biofuels, bioplastics, etc.) to forensic chemistry (bomb dating, isotopic signatures) and beyond. The authors propose a roadmap for the measurement of radiocarbon whereby the 14C scale is linked directly to the International System of Units rather than to a physical artifact, allowing more accurate, precise, and traceable radiocarbon metrology.
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ROLE OF SALT, PRESSURE, AND WATER ACTIVITY ON HOMOGENEOUS ICE NUCLEATION
The formation of ice is one of the most important freezing transitions on Earth, but this phase transition is still not completely understood. For example, the rate and mechanism by which ice nucleates in supercooled water remain topics of debate. Understanding how water freezes may not seem important to most, but it is crucial for modeling climate change (the content of ice in clouds has a strong impact on the Earth’s albedo and thus on climate change) and for improving cryopreservation (successful cryopreservation crucially depends on avoiding water freezing). Using improved and newly developed simulation techniques, Espinosa et al. (10.1021/ acs.jpclett.7b01551) investigated the analogy between the effects of pressure and salt on ice nucleation. Both factors are known to slow down ice nucleation, which is exploited in cryopreservation protocols. The authors found that pressure and salt delay ice nucleation in a qualitatively similar manner: by increasing the ice−liquid interfacial free energy. Despite this qualitative similarity, they found that it was not possible to quantitatively map ice nucleation rates of salty water onto those of compressed water through the activity of water. © 2017 American Chemical Society
Published: September 21, 2017 4645
DOI: 10.1021/acs.jpclett.7b02427 J. Phys. Chem. Lett. 2017, 8, 4645−4645
