Theoretical and experimental investigations of native bone are carried out to understand relationships between its hierarchical organization and local electronic and atomic structure of the mineralized phase. The 3D superlattice model of a coplanar assembly of the hydroxyapatite (HAP) nanocrystallites separated by the hydrated nanolayers is introduced to account the interplay of short-, long- and super-range order parameters in bone tissue. The model is applied to (i) predict and rationalize the HAP-to-bone spectral changes in the electronic structure and (ii) describe the mechanisms ensuring the link of the hierarchical organization with the electronic structure of the mineralized phase in bone. To check the predictions the near-edge x-ray absorption fine structure (NEXAFS) at the Ca 2p, P 2p and O 1s thresholds is measured for native bone and compared with NEXAFS for reference compounds. The NEXAFS analysis has demonstrated the essential hierarchy induced HAP-to-bone red shifts of the Ca and P 2p-to-valence transitions. The lowest O 1s excitation line at 532.2 eV in bone is assigned with superposition of core transitions in the hydroxide OH(HO) anions, Ca(HO) cations, the carboxyl groups inside the collagen and [PO] and [PO] anions with unsaturated P-O bonds.

Basing on scientific publications and original research the authors specified the effect of incorporation and adsorption of different ions and water molecules on physical, chemical and mechanical properties of bioapatite and determined new directions for investigations of intercrystallite interactions in nanoscale. Inner structure of the apatite crystallites more adaptable to chemical substitutions in comparison with other minerals controls their important characteristics such as a size, solubility, hardness, fragility, formability and thermal stability. The water molecules incorporated in crystallites and adsorbed on their surfaces stabilize them. In case the distances between crystallites become shorter than 10 nm the water molecules adsorbed on their surface play dominant role in bonding between the crystallites. This bond determines the main mechanical properties of bones. We bring forward a suggestion that theoretical model developed on the basis of near edge X-ray spectroscopic studies of bones using the contemporary high brilliant sources of X-ray radiation (synchrotrons and X-ray free electrons lasers) will allow to receive new quantitative data on local electronic and atomic structure (coordination numbers, ionic charges, interatomic distances interatomic and intercrystallite forces) of nanoelements in osseous tissue. The investigation results must bring to construction of new morphologically correct model providing deeper understanding of processes occurring in mineral matrix and mechanical properties of bones.

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Thermal dependence of mineralized bone structure is examined both experimentally by measuring the near edge X-ray absorption fine structure (NEXAFS) and theoretically by applying the 3DSL model to clarify relationship between the local electronic and atomic structure and hierarchical organization of skeleton. The high energy resolution NEXAFS spectra are acquired near the Ca 2p edges in native bone heated from RT up to 450C and hydroxyapatite (Ca10(PO4)6(OH)2, OH-Ap) to understand the interplay of short-, long-and super-range order parameters of bone matter. Our focus is on the thermal changes of spectral distribution of oscillator strength for Ca 2p 1 2, ⁄ 3 2 ⁄ → 3d transition in bone and OH-Ap. The investigations have confirmed the assignment of the OH-Ap-to-bone spectral changes to the predicted hierarchy effect on electronic and atomic structure of mineralized bone. At RT the OH-Ap-to-bone red shift 3d of the transition energy is found equal ≈ 0.2 eV and ≈ 0.1 eV for mature and young bone respectively. We stated that the shift behaves irregular and its magnitude varies from 0.1 eV up to 0.3 eV when the heating temperature grows. Two mechanisms associated with the thermal-induced dehydration of the inter-nanocrystallite spaces and with the subsequent atomic restructuring of the nanocrystallite interface in mineralized phase are revealed. We have detected that the OH-Ap-to-bone red shifts of the Ca 2p → 3d transition in young bone are smaller than those of in mature bone. The origin of the age differences is discussed.

Реферат Исходя из представлений о структурно-функциональной взаимосвязи соседних уровней иерархической организации минерального матрикса, предложена модель трехмерной сверхрешетки (3D-СР), описывающая влияние копланарнарных объединений нанокристаллитов гидроксиапатита (НКГА) на энергетические состояния ионов в костной ткани. В рамках этой 3D-СР-модели предсказаны низкоэнергетический конгломерат-кристаллический сдвиг незаполненных электронных состояний, расположенных вблизи дна зоны проводимости в кристалле ГА, и зависимость сдвига от отношения ширины гидратного слоя к характерному размеру нанокристаллита. Для проверки данного предсказания проведены экспериментальные исследования ближней тонкой структуры Ca2р 1/2,3/2-, P2р 1/2,3/2-и O1s-спектров поглощения в нативной кости. Сопоставление полученных спектров с ближней тонкой структурой рентгеновских спектров поглощения реперных соединений выявило отчетливый низкоэнергетический сдвиг рентгеновских переходов в костной ткани. Выявленный эффект служит основой для разработки новых методов диагностики состояния костной ткани с использованием ультрамягкой рентгеновской спектроскопии, а также визуализации изменений локальной электронной структуры костной ткани посредством регистрации конгломерат-кристаллических сдвигов рентгеновских переходов. подтверждена применимость 3D-СР-модели для понимания особенностей иерархической организации минеральных структур скелета на наноуровне. Ключевые слова: организация минерального матрикса, трехмерное моделирование, гидроксиапатит кости, структура костной ткани.