Catégorie d'actualités : PASTEUR

For proteins in solvent mixtures, the relative abundances of each solvent in their solvation shell have a critical impact on their properties. Preferential solvation of a series of proteins in water–glycerol mixtures is studied here over a broad range of solvent compositions via classical molecular dynamics simulations. Our simulation results reveal that the differences between shell and bulk compositions exhibit dramatic changes with solvent composition, temperature, and protein nature. In contrast with the simple and widely used picture where glycerol is completely excluded from the protein interface, we show that for aqueous solutions with less than 50% glycerol in volume, protein solvation shells have approximately the same composition as the bulk solvent and proteins are in direct contact with glycerol. We further demonstrate that at high glycerol concentration, glycerol depletion from the solvation shell is due to an entropic factor arising from the reduced accessibility of bulky glycerol molecules in protein cavities. The resulting molecular picture is important to understand protein activity and cryopreservation in mixed aqueous solvents.

References:
Protein Preferential Solvation in Water:Glycerol Mixtures
Nicolas Chéron, Margaux Naepels, Eva Pluhařová and Damien Laage
J. Phys. Chem. B 2020, 124, 8, 1424-1437
doi: 10.1021/acs.jpcb.9b11190

Congratulations to Marie LABEYE, newly recruited as assistant professor at our department since September 2019.

Attosecond pulses are central to the investigation of valence- and core-electron dynamics on their natural timescales. The reproducible generation and characterization of attosecond waveforms has been demonstrated so far only through the process of high-order harmonic generation. Several methods for shaping attosecond waveforms have been proposed, including the use of metallic filters, multilayer mirrors and manipulation of the driving field. However, none of these approaches allows the flexible manipulation of the temporal characteristics of the attosecond waveforms, and they suffer from the low conversion efficiency of the high-order harmonic generation process. Free-electron lasers, by contrast, deliver femtosecond, extreme-ultraviolet and X-ray pulses with energies ranging from tens of microjoules to a few millijoules. Recent experiments have shown that they can generate subfemtosecond spikes, but with temporal characteristics that change shot- to-shot. Here we report reproducible generation of high-energy (microjoule level) attosecond waveforms using a seeded free-electron laser. We demonstrate amplitude and phase manipulation of the harmonic components of an attosecond pulse train in combination with an approach for its temporal reconstruction. The results presented here open the way to performing attosecond time-resolved experiments with free-electron lasers.

References:
Attosecond pulse shaping using a seeded free-electron laser
Praveen Kumar Maroju, Cesare Grazioli, Michele Di Fraia, Matteo Moioli, Dominik Ertel1, Hamed Ahmadi, Oksana Plekan, Paola Finetti, Enrico Allaria, Luca Giannessi, Giovanni De Ninno, Carlo Spezzani, Giuseppe Penco, Simone Spampinati, Alexander Demidovich, Miltcho B. Danailov, Roberto Borghes, George Kourousias, Carlos Eduardo Sanches Dos Reis, Fulvio Billé, Alberto A. Lutman, Richard J. Squibb, Raimund Feifel, Paolo Carpeggiani, Maurizio Reduzzi, Tommaso Mazza, Michael Meyer, Samuel Bengtsson, Neven Ibrakovic, Emma Rose Simpson, Johan Mauritsson,
Tamás Csizmadia, Mathieu Dumergue, Sergei Kühn, Harshitha Nandiga Gopalakrishna, Daehyun You, Kiyoshi Ueda, Marie Labeye, Jens Egebjerg Bækhøj, Kenneth J. Schafer, Elena V. Gryzlova, Alexei N. Grum-Grzhimailo, Kevin C. Prince, Carlo Callegari & Giuseppe Sansone
Nature volume 578, pages386–391(2020)
doi: 10.1038/s41586-020-2005-6

We report the on-surface synthesis of a nonmetal porphyrin, namely, silicon tetraphenylporphyrin (Si-TPP), by the deposition of atomic silicon onto a free-base TPP layer on a Ag(100) surface under ultrahigh vacuum (UHV) conditions. Scanning tunneling microscopy provides insights into the self-assembly of the TPP molecules before and after Si insertion. Silicon coordinates with all four nitrogen atoms of the TPP macrocycle and interacts with a silver atom of the substrate as confirmed by scanning tunneling spectroscopy, X-ray photoelectron spectroscopy, and complementary density functional theory calculations. The Si-TPP complex presents a saddle-shaped conformation that is stable under STM manipulation. Our study shows how protocols established for the on-surface metalation of tetrapyrroles can be adopted to achieve nonmetal porphyrins. Complementary experiments yielding Si-TPP and Ge-TPP on Ag(111) highlight the applicability to different main group elements and supports. The success of our nonmetal porphyrin synthesis procedure is further corroborated by a temperature- programmed desorption experiment, revealing the desorption of Ge-TPP. This extension of interfacial complex formation beyond metal elements opens promising prospects for new tetrapyrrole architectures with distinct properties and functionalities.

References:
On-Surface Synthesis of Nonmetal Porphyrins
Aleksandr Baklanov, Manuela Garnica, Anton Robert, Marie-Laure Bocquet, Knud Seufert, Johannes T. Küchle, Paul T. P. Ryan, Felix Haag, Reza Kakavandi, Francesco Allegretti, and Willi Auwar̈ter
J. Am. Chem. Soc. 2020, 142, 4, 1871-1881
DOI: 10.1021/jacs.9b10711

Recent advances in scanning probe techniques rely on the chemical functionalization of the probe-tip termination by a single molecule. The success of this approach opens the prospect of introducing spin sensitivity through functionalization by a magnetic molecule. We used a nickelocene-terminated tip (Nc-tip), which offered the possibility of producing spin excitations on the tip apex of a scanning tunneling microscope (STM). When the Nc-tip was 100 picometers away from point contact with a surface-supported object, magnetic effects could be probed through changes in the spin excitation spectrum of nickelocene. We used this detection scheme to simultaneously determine the exchange field and the spin polarization of iron atoms and cobalt films on a copper surface with atomic-scale resolution.

Retrouvez le communiqué de presse, proposé par l’Institut National de Physique : Une molécule à la pointe pour cartographier le magnétisme à l’échelle atomique

References:
Atomic-scale spin sensing with a single molecule at the apex of a scanning tunneling microscope.
B. Verlhac, N. Bachellier, L. Garnier, M. Ormaza, P. Abufager, R. Robles, M.-L. Bocquet, M. Ternes, N. Lorente, L. Limot.
Science, Vol. 366, Issue 6465, pp. 623-627 2019
DOI: 10.1126/science.aax8222