Anastasia Vyalikh

An NMR Study of Biomimetic Fluorapatite : Gelatine Mesocrystals

The mesocrystal system fluoroapatite—gelatine grown by double-diffusion is characterized by hierarchical composite structure on a mesoscale. In the present work we apply solid state NMR to characterize its structure on the molecular level and provide a link between the structural organisation on the mesoscale and atomistic computer simulations. Thus, we find that the individual nanocrystals are composed of crystalline fluorapatite domains covered by a thin boundary apatite-like layer. The latter is in contact with an amorphous layer, which fills the interparticle space. The amorphous layer is comprised of the organic matrix impregnated by isolated phosphate groups, Ca3F motifs and water molecules. Our NMR data provide clear evidence for the existence of precursor complexes in the gelatine phase, which were not involved in the formation of apatite crystals, proving hence theoretical predictions on the structural pre-treatment of gelatine by ion impregnation. The interfacial interactions, which may be described as the glue holding the composite materials together, comprise hydrogen bond interactions with the apatite PO43− groups. The reported results are in a good agreement with molecular dynamics simulations, which address the mechanisms of a growth control by collagen fibers, and with experimental observations of an amorphous cover layer in biominerals.

Structural characterisation of aluminium layered double hydroxides by 27Al solid-state NMR

(27)Al solid-state NMR has been applied to study the local structure of pristine and chemically modified aluminium layered double hydroxides (LDH). The pristine LDH only shows six-fold coordinated, octahedral, aluminium, while the calcined and subsequently surfactant treated LDH sample shows a significant fraction of four-fold coordinated tetrahedral aluminium. The co-existence of two types of octahedral sites with different quadrupolar parameters is clearly observed in both samples. Quadrupolar coupling constants and isotropic chemical shifts have been measured from the (27)Al triple-quantum MAS NMR allowing to fit the (27)Al MAS spectra and quantify the different species in the samples. The quantitative analysis reveals that 30% of the aluminium is in four-fold coordination in the surfactant-modified LDH. We show that this chemical modification retains the two types of AlO(6) sites with a decreased intensity of the site showing the lowest quadrupolar coupling constant.

Hydrogen bonds and local symmetry in the crystal structure of gibbsite

First‐principles quantum mechanical calculations of NMR chemical shifts and quadrupolar parameters have been carried out to assign the 27Al MAS NMR resonances in gibbsite. The 27Al NMR spectrum shows two signals for octahedral aluminum revealing two aluminum sites coordinated by six hydroxyl groups each, although the crystallographic positions of the two Al sites show little difference. The presence of two distinguished 27Al NMR resonances characterized by rather similar chemical shifts but quadrupolar coupling constants differing by roughly a factor of two is explained by different character of the hydrogen bonds, in which the hydroxyls forming the corresponding octahedron around each aluminum site, are involved. The Al‐I site characterized by a CQ = 4.6 MHz is surrounded by OHgroups participating in four intralayer and two interlayer hydrogen bonds, while the Al‐II site with the smaller quadrupolar constant (2.2 MHz) is coordinated by hydroxides, of which two point toward the intralayer cavities and four OH‐bonds are aligned toward the interlayer gallery. In high‐resolution solid‐state 1H CRAMPS (combination of rotation and multiple‐pulse spectroscopy) four signals with an intensity ratio of 1:2:2:1 are resolved which allow to distinguish six nonequivalent hydrogen sites reported in the gibbsite crystal structure and to ascribe them to two types of structural OH groups associated with intralayer and interlayer hydrogen bonds. This study can be applied to characterize the gibbsite‐like layer—intergallery interactions associated with hydrogen bonding in the more complex systems, such as synthetic aluminum layered double hydroxides. Copyright

Intergrowth and Interfacial Structure of Biomimetic Fluorapatite-Gelatin Nanocomposite : A Solid-State NMR Study

The model system fluorapatite-gelatin allows mimicking the formation conditions on a lower level of complexity compared to natural dental and bone tissues. Here, we report on solid-state NMR investigations to examine the structure of fluorapatite-gelatin nanocomposites on a molecular level with particular focus on organic-inorganic interactions. Using (31)P, (19)F, and (1)H MAS NMR and heteronuclear correlations, we found the nanocomposite to consist of crystalline apatite-like regions (fluorapatite and hydroxyfluorapatite) in close contact with a more dissolved (amorphous) layer containing first motifs of the apatite crystal structure as well as the organic component. A scheme of the intergrowth region in the fluorapatite-gelatin nanocomposite, where mineral domains interact with organic matrix, is presented.

OH − deficiency in dental enamel, crown and root dentine as studied by 1H CRAMPS

High-resolution solid-state NMR based on combined rotation and multipulse spectroscopy (CRAMPS) has been applied to study chemical structures of dental tissues. The samples of human enamel, crown dentine and root dentin studied in this work were used without chemical pre-treatment. The quantitative ¹H NMR spectra permit an assignment to different structures and a quantification of the content of hydroxyl groups. While there is 40% hydroxyl content in the enamel, there is significantly less in the dentin, 14% in the crown and 9% in the root. Thus this study provides the direct evidence of OH⁻ ion deficiency in all dental tissues supporting earlier findings that bone and dental mineral apatite is poorly hydroxylated.