Insights Into the Micelle-Induced ß-Hairpin-to-α-Helix Transition of a LytA-Derived Peptide by Photo-CIDNP Spectroscopy.

A choline-binding module from pneumococcal LytA autolysin, LytA239-252, was reported to have a highly stable nativelike ß-hairpin in aqueous solution, which turns into a stable amphipathic α-helix in the presence of micelles. Here, we aim to obtain insights into this DPC-micelle triggered ß-hairpin-to-α-helix conformational transition using photo-CIDNP NMR experiments. Our results illustrate the dependency between photo-CIDNP phenomena and the light intensity in the sample volume, showing that the use of smaller-diameter (2.5 mm) NMR tubes instead of the conventional 5 mm ones enables more efficient illumination for our laser-diode light setup. Photo-CIDNP experiments reveal different solvent accessibility for the two tyrosine residues, Y249 and Y250, the latter being less accessible to the solvent. The cross-polarization effects of these two tyrosine residues of LytA239-252 allow for deeper insights and evidence their different behavior, showing that the Y250 aromatic side chain is involved in a stronger interaction with DPC micelles than Y249 is. These results can be interpreted in terms of the DPC micelle disrupting the aromatic stacking between W241 and Y250 present in the nativelike ß-hairpin, hence initiating conversion towards the α-helix structure. Our photo-CIDNP methodology represents a powerful tool for observing residue-level information in switch peptides that is difficult to obtain by other spectroscopic techniques.

Versatile Rh- and Ir-Based Catalysts for CO2 Hydrogenation, Formic Acid Dehydrogenation, and Transfer Hydrogenation of Quinolines.

Considering the interest in processes related to hydrogen storage such as CO2 hydrogenation and formic acid (FA) decomposition, we have synthesized a set of Ir, Rh, or Ru complexes to be tested as versatile precatalysts in these reactions. In relation with the formation of H2 from FA, the possible applicability of these complexes in the transfer hydrogenation (TH) of challenging substrates as quinoline derivatives using FA/formate as hydrogen donor has also been addressed. Bearing in mind the importance of secondary coordination sphere interactions, N,N' ligands containing NH2 groups, coordinated or not to the metal center, were used. The general formula of the new complexes are [( p-cymene)RuCl(N,N')]X, X = Cl-, BF4- and [Cp*MCl(N,N')]Cl, M = Rh, Ir, where the N,N' ligands are 8-aminoquinoline (HL1), 6-pyridyl-2,4-diamine-1,3,5-triazine (L2) and 5-amino-1,10-phenanthroline (L3). Some complexes are not active or catalyze only one process. However, the complexes [Cp*MCl(HL1)]Cl with M = Rh, Ir are versatile catalysts that are active in hydrogenation of quinolines, FA decomposition, and also in CO2 hydrogenation with the iridium derivative being more active and robust. The CO2 hydrogenation takes place in mild conditions using only 5 bar of pressure of each gas (CO2 and H2). The behavior of some precatalysts in D2O and after the addition of 9 equiv of HCO2Na (pseudocatalytic conditions) has been studied in detail and mechanisms for the FA decomposition and the hydrogenation of CO2 have been proposed. For the Ru, Ir, or Rh complexes with ligand HL1, the amido species with the deprotonated ligand are observed. In the case of ruthenium, the formate complex is also detected. For the iridium derivative, both the amido intermediate and the hydrido species have been observed. This hydrido complex undergoes a process of umpolung D+↔ Ir-D. All in all, the results of this work reflect the active role of -NH2 in the transfer of H+.