Spotlights pubs.acs.org/JACS
Spotlights on Recent JACS Publications
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EXCITED BY THE SUN, RNA TRIES TO RELAX While DNA gets most of the attention when it comes to photodamage, UV light can also have a negative impact on the nucleobases in RNA. When UV light from the sun hits uracil embedded in RNA, the nucleobase is propelled into a photoexcited state, which has the capacity to form unwanted reaction products that can harm the cell. To protect itself from damage, photoexcited uracil rapidly unwinds, relaxing back to its ground state in less than 200 femtoseconds. However, it has been unclear exactly how uracil achieves its superfast recovery and whether it can be controlled. To unravel uracil’s secrets of relaxation, researchers led by Regina de Vivie-Riedle have run the first quantum dynamics simulations on unbound uracil, using shaped laser pulses to manipulate the excitation and relaxation of the molecule in silico (DOI: 10.1021/jacs.6b12033). In addition to interrogating the molecular relaxation pathways in uracil, the researchers figure out how to generate a long-living photoexcited state with shaped laser pulses. The theoretical simulations lay the groundwork for experimental laser studies on uracil that can further elucidate relaxation or photodamage pathways. Taken together, these findings could help shed light on how nature prevents RNA photodamage. Erika Gebel Berg, Ph.D.
moving field will likely lead to many innovative breakthroughs in the near future. Christen Brownlee
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DYNAMIC DANCE WHEN WATER MEETS PROTEIN Water surrounds the surface of proteins to form a hydration shell, influencing protein structure, function, and other properties. But these dynamics happen on such fast time scales that it is challenging to detect them and understand their effects in detail. Using time-resolved fluorescence spectroscopy, Dongping Zhong and colleagues uncover the dynamics of hydration on the surface of a β-barrel protein on a femtosecond time scale (DOI: 10.1021/jacs.6b12463). Zhong and co-workers shed new light on the issue by replacing individual residues in a protein with naturally fluorescent tryptophan. They then trace the motions and solvation dynamics of specific sites with femtosecond timeresolved fluorescence studies. By examining all 17 secondary structures of the β-barrel protein rat liver fatty-acid-binding protein, the researchers find three main relaxation dynamics due to hydration. The fastest, over hundreds of femtoseconds, involves water molecules on the outer layer of the hydration shell; in the second, over a few picoseconds, water molecules on the inner layer of the hydration shell rearrange themselves; and in the third, over tens of picoseconds, the whole water network restructures itself, driving local fluctuation of the protein. Notably, the authors conclude hydration dynamics are more sluggish around β sheets than α helices. Deirdre Lockwood, Ph.D.
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BRINGING PHOTON-TRIGGERED THERAPIES TO LIGHT Nanotechnology has led to a wide array of advances in biotechnology and biomedicine, including therapeutics with the potential to treat infections, cancer, and other serious diseases. Many of these systems can provide targeted delivery of therapeutic agents using stimuli from an external source, including light, which has shown particular promise. In a new Perspective, Michael Hamblin, Mahdi Karimi, and co-workers detail recent advances in this field, including the many mechanisms used thus far for light-controlled delivery of therapeutics and the challenges that still remain (DOI: 10.1021/jacs.6b08313). The authors discuss various possibilities for inducing lighttriggered release of therapeutic agents, including photocleavage, photoisomerization, photo-cross-linking, photoreduction, and photoinduced thermal triggers. Light-triggered therapies also encompass photothermal therapies, which convert light to heat, and photodynamic therapy, which uses light to generate reactive oxygen species. Combining any of these modalities into a single package might produce synergistic results with more clinical efficacy than a single modality. However, before many of these therapies can be translated to the clinic, researchers will need to overcome significant hurdles, such as choosing the most efficient nanomaterials to provide stability in biological environments without causing toxicity. The authors suggest that this fast© 2017 American Chemical Society
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PROGRESS TOWARD ARTIFICIAL MOLECULAR MACHINES FOR POLYMER SCIENCE When it comes to using molecular machines to facilitate the creation of materials on the macro-scale, the primary concern is whether the nanosized machines will be able to amplify their mechanical behavior to create a response in the bulk material. Now, researchers led by Nicolas Giuseppone show that one class of molecular machines can be incorporated into a polymer to induce a phase transition at the macroscopic level (DOI: 10.1021/jacs.7b00983). The team synthesizes a type of nanomachine known as a “molecular muscle”, composed of bistable [c2]daisy chain rotaxanes. The researchers link the molecules into a supramolecular polymer and show that by altering the pH of the solution, the molecular muscles can be prompted to induce a transition of the polymer from the liquid to the solid state (i.e., sol−gel transition). This is the first example of a mechanically controlled sol−gel transition induced by a nanomechanical actuation. The findings open up new possibilities of using molecular machines in smart responsive Published: April 12, 2017 4969
DOI: 10.1021/jacs.7b03306 J. Am. Chem. Soc. 2017, 139, 4969−4970
Journal of the American Chemical Society
Spotlights
materials and also showcase complex organic synthesis to yield new class of actuating molecular machines. Christine Herman, Ph.D.
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COLLOIDAL PEROVSKITE NANOCRYSTALS: CHANGING, WHILE STAYING THE SAME Colloidal perovskite nanocrystals, such as CsPbBr3, have outstanding optical properties that have positioned them for use in a variety of applications, including lasers, highly efficient LEDs, and photovoltaic cells. Several studies have shown that their optical properties can be tuned through postsynthetic anion exchange without affecting the size and shape of the crystal lattice. However, similar cation exchange reactions have proven elusive. Celso de Mello Donega and colleagues demonstrate for the first time that cation exchange in CsPbBr3 is possible (DOI: 10.1021/jacs.6b13079). By reacting this material with SnBr2, CdBr2, or ZnBr2, the researchers ensure that only the cation would be exchanged since both the parent nanocrystals and these salts contain the same halide. These reactions produce new nanocrystals with blue-shifted photoluminescent bands that vary depending on which metal bromide salt is used and its concentration, while preserving the key spectral features of CsPbBr3. Further investigation with transmission electron microscopy shows that the size and shape of the parent and new nanocrystals are the same. However, other techniques demonstrate that the lattice contracts slightly due to incorporation of the smaller guest cations. The ability to postsynthetically modify CsPbBr3 through both anion and cation exchange could produce a bevy of new materials with unprecedented and unparalleled optoelectronic properties. Christen Brownlee
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A WOBBLE MISMATCH IN WATSON−CRICK CLOTHING The body is generally adept at identifying and fixing base-pair mismatches in DNA and RNA, using differences in the geometry of the canonical Watson−Crick pairings (G-C, A-T/ U) versus that of base-pair mismatches (“wobble base pairs”) to locate the errors. But every so often, a mismatch slips past, leading to a mutation that could potentially harm the organism or the next generation. Scientists have suspected that mismatches may go undetected and uncorrected by adopting Watson−Crick-like geometries, disguising their true identity and tricking the transcription/translation machinery, but have lacked definitive experimental proof to back up the theory. Hashim Al-Hashimi and his team, using nuclear magnetic resonance spectroscopy, find direct evidence for Watson− Crick patterns in G-T and G-U wobble base pairs (DOI: 10.1021/jacs.7b01156). The evidence is derived from two complementary types of NMR experiments focusing on the nitrogen atoms in NH and NH2 groups in the nucleic acids. The G-T and G-U pairings mimicked both the shape and hydrogen-bonding patterns of Watson−Crick base pairs. The mismatches accomplish this feat by transiently entering excited states that either are negatively charged or have atypical connectivity. The researchers determine that the occurrence of these excited states coincides with mutational rates, underlining the importance of high energy states in biology. Erika Gebel Berg, Ph.D. 4970
DOI: 10.1021/jacs.7b03306 J. Am. Chem. Soc. 2017, 139, 4969−4970
