Catégorie d'actualités : PASTEUR

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

Carbonic acid H2CO3 (CA) is a key constituent of the universal CA/ bicarbonate/CO2 buffer maintaining the pH of both blood and the oceans. Here we demonstrate the ability of intact CA to quantita- tively protonate bases with biologically-relevant pKas and argue that CA has a previously unappreciated function as a major source of protons in blood plasma. We determine with high precision the temperature dependence of pKa(CA), pKa(T) = −373.604 + 16,500/T +56.478 ln T. At physiological-like conditions pKa(CA) = 3.45 (I =0.15 M, 37 °C), making CA stronger than lactic acid. We further demonstrate experimentally that CA decomposition to H2O and CO2 does not impair its ability to act as an ordinary carboxylic acid and to efficiently protonate physiological-like bases. The conse- quences of this conclusion are far reaching for human physiol- ogy and marine biology. While CA is somewhat less reactive than (H+)aq, it is more than 1 order of magnitude more abundant than (H+)aq in the blood plasma and in the oceans. In particular, CA is about 70× more abundant than (H+)aq in the blood plasma, where we argue that its overall protonation efficiency is 10 to 20× greater than that of (H+)aq, often considered to be the major protonating agent there. CA should thus function as a major source for fast in vivo acid–base reactivity in the blood plasma, possibly pene- trating intact into membranes and significantly helping to com- pensate for (H+)aq’s kinetic deficiency in sustaining the large proton fluxes that are vital for metabolic processes and rapid enzymatic reactions.

References:
Intact carbonic acid is a viable protonating agent for biological bases
Daniel Aminov, Dina Pines, Philip M. Kiefer, Snehasis Daschakraborty, James T. Hynes, and Ehud Pines
PNAS vol. 116 | no. 42 | 20837–20843
DOI: 10.1073/pnas.1909498116