Inhalation of silica particles can induce inflammatory lung reactions that lead to silicosis and/or lung cancer when the particles are biopersistent. This toxic activity of silica dusts is extremely variable depending on their source and preparation methods. The exact molecular moiety that explains and predicts this variable toxicity of silica remains elusive. Here, we have identified a unique subfamily of silanols as the major determinant of silica particle toxicity. This population of “nearly free silanols” (NFS) appears on the surface of quartz particles upon fracture and can be modulated by thermal treatments. Density functional theory calculations indicates that NFS locate at an intersilanol distance of 4.00 to 6.00 Å and form weak mutual interactions. Thus, NFS could act as an energetically favorable moiety at the surface of silica for establishing interactions with cell membrane components to initiate toxicity. With ad hoc prepared model quartz particles enriched or depleted in NFS, we demonstrate that NFS drive toxicity, including membranolysis, in vitro proinflammatory activity, and lung inflammation. The toxic activity of NFS is confirmed with pyrogenic and vitreous amorphous silica particles, and industrial quartz samples with noncontrolled surfaces. Our results identify the missing key molecular moieties of the silica surface that initiate interactions with cell membranes, leading to pathological outcomes. NFS may explain other important interfacial processes involving silica particles.
In situ IR and mass spectrometry evidence for the catalytic formation on SiO2 and TiO2 surfaces of glycine oligomers (poly‐Gly) up to 16 units long by successive feeding with monomers from the vapor phase is presented. Parallel experiments carried out on hydroxyapatite resulted in the unreactive adsorption of Gly, thus indicating that the oligomerization was specifically catalyzed by the surfaces of SiO2 and TiO2. Furthermore, the poly‐Gly moved on the surface when contacted with H2O vapor and formed self‐assembled aggregates containing both helical and β‐sheet‐like structural motifs. These results indicate that polypeptides formed by the condensation of amino acids adsorbed on a mineral surface can evolve into structured supramolecular assemblies.
The
mechanism of the amide bond formation between nonactivated
carboxylic acids and amines catalyzed by the surface of amorphous
silica under dry conditions is elucidated by combining spectroscopic
measurements and quantum chemical simulations. The results suggest
a plausible explanation of the catalytic role of silica in the reaction.
Both experiment and theory identify very weakly interacting SiOH surface
group pairs (ca. 5 Å apart) as key specific sites for simultaneously
hosting, in the proper orientation, ionic and canonical pairs of the
reactants. An atomistic interpretation of the experiments indicates
that this coexistence is crucial for the occurrence of the reaction,
since the components of the canonical pair are those undergoing the
amidation reaction while the ionic pair directly participates in the
final dehydration step. Transition state theory based on quantum mechanical
free energy potential energy shows the silica-catalyzed amide formation
as being relatively fast. The work also points out that the presence
of the specific SiOH group pairs is not exclusive of the adopted silica
sample, as they can also be present in natural forms of silica, for
instance as hydroxylation defects on α-quartz, so that they
could exhibit similar catalytic activity toward the amide bond formation.
Amide synthesis: In situ IR spectroscopy and HRMS provided evidence of the activation of surface carboxylates at mild temperatures (about 323 K) for the direct synthesis of amides from carboxylic acids and amines.
The polymerization of amino acids (AAs) to peptides on
oxide surfaces
has attracted interest owing to its high importance in biotechnology,
prebiotic chemistry, and origin of life theories. However, its mechanism
is still poorly understood. We tried to elucidate the reactivity of
glycine (Gly) from the vapor phase on the surface of amorphous silica
under controlled atmosphere at 160 °C. Infrared (IR) spectroscopy
reveals that Gly functionalizes the silica surface through the formation
of ester species, which represent, together with the weakly interacting
silanols, crucial elements for monomers activation and polymerization.
Once activated, β-turns start to form as initiators for the
growth of long linear polypeptides (poly-Gly) chains, which elongate
into ordered structures containing both β-sheet and helical
conformations. The work also points to the role of water vapor in
the formation of further self-assembled β-sheet structures that
are highly resistant to hydrolysis.
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