Molecular clouds are the cold regions of the Milky Way where stars form. They are enriched by rather complex molecules. Many of these molecules are believed to be synthesized on the icy surfaces of the interstellar submicron-sized dust grains that permeate the Galaxy. At 10 K thermal desorption is inefficient and, therefore, why these molecules are found in the cold gas has tantalized astronomers for years. The assumption of the current models, called chemical desorption, is that the molecule formation energy released by the chemical reactions at the grain surface is partially absorbed by the grain and the remaining energy causes the ejection of the newly formed molecules into the gas. Here we report accurate ab initio molecular dynamics simulations aimed at studying the fate of the energy released by the first reaction of the H· addition chain to CO, H· + CO HCO·, occurring on a crystalline ice surface model. We show that about 90% of the HCO· formation energy is injected toward the ice in the first picosecond, leaving HCO· with an energy content (10–15 kJ mol−1) of less than half its binding energy (30 kJ mol−1). As a result, in agreement with laboratory experiments, we conclude that chemical desorption is inefficient for this specific system, namely H· + CO on crystalline ice. We suspect this behavior to be quite general when dealing with hydrogen bonds, which are responsible for both the cohesive energy of the ice mantle and the interaction with adsorbates, as HCO·, even though ad hoc simulations are needed to draw specific conclusions on other systems.

Understanding metal oxide MO 2 (M = Ti, Ru, and Ir)–water interfaces is essential to assess the catalytic behavior of these materials. The present study analyzes the H 2 O–MO 2 interactions at the most abundant (110) and (011) surfaces, at two different water coverages: isolated water molecules and full monolayer, by means of Perdew–Burke–Ernzerhof-D2 static calculations and ab initio molecular dynamics (AIMD) simulations. Results indicate that adsorption preferably occurs in its molecular form on (110)-TiO 2 and in its dissociative form on (110)-RuO 2 and (110)-IrO 2 . The opposite trend is observed at the (011) facet. This different behavior is related to the kind of octahedral distortion observed in the bulk of these materials (tetragonal elongation for TiO 2 and tetragonal compression for RuO 2 and IrO 2 ) and to the different nature of the vacant sites created, axial on (110) and equatorial on (011). For the monolayer, additional effects such as cooperative H-bond interactions and cooperative adsorption come into play in determining the degree of deprotonation. For TiO 2 , AIMD indicates that the water monolayer is fully undissociated at both (110) and (011) surfaces, whereas for RuO 2 , water monolayer exhibits a 50% dissociation, the formation of H 3 O 2 – motifs being essential. Finally, on (110)-IrO 2 , the main monolayer configuration is the fully dissociated one, whereas on (011)-IrO 2 , it exhibits a degree of dissociation that ranges between 50 and 75%. Overall, the present study shows that the degree of water dissociation results from a delicate balance between the H 2 O–MO 2 intrinsic interaction and cooperative hydrogen bonding and adsorption effects.

The interaction of 11 amino acids with the TiO2 (101) anatase surface was investigated by means of PBE-D2* periodic simulations, both from a static and dynamic points of view. Several adsorption states, with the amino acid in its canonical, zwitterionic, or deprotonated forms, were considered. The strongest interactions correspond to dative interactions between the electron pairs of N or O atoms of the amino acids and the Ti atoms of the surface. For glycine, the most stable configuration corresponds to the deprotonated (N,O) binding mode, at variance to that observed for the adsorption of glycine on rutile (110) surface, for which the dissociative (O,O) binding mode was determined to be the preferred one. For the remaining amino acids, the following general trend was identified: those amino acids with acidic or a basic lateral chain groups, except Arg, exhibit a deprotonated (N,O) binding mode as glycine, because the additional dative interactions with the side chain overcome the destabilizing effect induced by steric hindrances. However, for the remaining amino acids, with weaker side chain interactions, the zwitterionic state is the most stable conformer because the steric hindrances between the lateral chains and the surface are minimized. Overall, the present results indicate that the most stable amino acid adsorption state on the surface arises from a delicate balance between favorable interactions and steric hindrances of the amino acids with the surface, the latter ones becoming particularly relevant when lateral chains are considered.

Alzheimer's disease is a challenge of the utmost importance for contemporary society. An early diagnosis is essential for the development of treatments and for establishing a network of support for the patient. In this light, the deposition in the brain of amyloid-β fibrillar aggregates, which is a distinctive feature of Alzheimer, is key for an early detection of this disease. In this work we propose an atomistic study of the interaction of amyloid tracers with recently published polymorphic models of amyloid-β 1-40 and 1-42 fibrils, highlighting the relationship between marker architectures and binding affinity. This work uncovers the importance of quaternary structure, and in particular of junctions between amyloid-β protofilaments, as the key areas for marker binding.

Molecular hydrogen is the most abundant molecular species in the universe. While no doubts exist that it is mainly formed on the interstellar dust grain surfaces, many details of this process remain poorly known. In this work, we focus on the fate of the energy released by the H2 formation on the dust icy mantles: how it is partitioned between the substrate and the newly formed H2, a process that has a profound impact on the interstellar medium. We carried out state-of-the-art ab initio molecular dynamics simulations of H2 formation on periodic crystalline and amorphous ice surface models. Our calculations show that up to two-thirds of the energy liberated in the reaction (∼300 kJ mol−1 ∼3.1 eV) is absorbed by the ice in less than 1 ps. The remaining energy (∼140 kJ mol−1 ∼1.5 eV) is kept by the newly born H2. Since it is 10 times larger than the H2 binding energy on the ice, the new H2 molecule will eventually be released into the gas phase. The ice water molecules within ∼4 Å from the reaction site acquire enough energy, between 3 and 14 kJ mol−1 (360–1560 K), to potentially liberate other frozen H2 and, perhaps, frozen CO molecules. If confirmed, the latter process would solve the long standing conundrum of the presence of gaseous CO in molecular clouds. Finally, the vibrational state of the newly formed H2 drops from highly excited states (ν = 6) to low (ν ≤ 2) vibrational levels in a timescale of the order of picoseconds.