Abstract specific surface area 473.2 m2/g. An aqueous

Abstract

In
this study, nano hydroxyapatite (n-HAp)
with average crystallite size ~ 8.13 ± 3 nm and cauliflower-like microstructure
(hexagonal geometry with size ranging between 15-50 nm) was synthesized in
laboratory at room temperature by using suitable sources of calcium and
phosphate ions and using tri-ethanolamine (TEA). Mesoporous bioactive glass
(MBG) was synthesized by using cationic surfactant trimethyl ammonium bromide
(CTAB) from SiO2-CaO-P2O5 glass system. After
firing at 650 oC, MBG powders were having zeta potential of –16.5
mV, median particle size ~ 75 nm, specific surface area 473.2 m2/g.
An aqueous suspension of DNA was used to disperse both n-HAp and MBG and further subjected for analysis including
absorbance, Circular Dichromasm (CD) spectroscopy, UV-melting and isothermal
titration calorimetry (ITC). Absorbance spectroscopy indicated that an
equilibrium binding was obtained between n-HAp
and DNA in solution phase. But in case of MBG, no such isosbestic point was
observed. Due to addition of nanomaterial, molar ellipticity of DNA was changed
revealing that the materials were interacted with DNA. Under the experimental
conditions, DNA exhibited a melting temperature of 68.4 oC. In
presence of MBG and n-HAp, the
melting temperatures were shifted to 77.2 and 75.8 oC, respectively,
suggesting that the nanoparticles stabilized DNA helix to a considerable
extent. Finally, it can be seen that the affinity of DNA to MBG was higher than
that of n-HAp.

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Keywords:
Nano-hydroxyapatite;
Mesoporous bioactive glass; DNA; CD spectroscopy; UV-melting; isothermal
titration calorimetry

1. Introduction

 

Hierarchically
human bone is an excellent example of macro- to micro- to molecular structure
which starts from spongy bone from surface down to collagen fibrils on a length
scale, which is again composed of columnar collagen molecule and inorganic
component mainly hydroxyapatite (HAp) nano-crystals (Fig. 1). This structure
gives the bone its unique mechanical properties to support the body weight and
structure, producing different blood cells and storing various ions 1, 2.
Many authors have tried to explain the correlation between this hierarchic
structure up to molecular level and mechanical properties observed in different
parts of the body through different methods including FEM (finite element
modeling) 3, multi-scale modeling techniques 4, etc. Bhowmik et al. had
shown mechanical response of collagen molecule in proximity of mineral and on
the mechanics of protein molecule at organic-inorganic interfaces 5. At the
nano-scale, HAp crystals aligned by their c-axis parallel to collagen fibrils
6 and collagen interacts them through telopeptides non-helically available at
the end of these proteins 7. Natural bone tissue is a composite composed of
nano-hydroxyapatite (n-HAp)
crystals dispersed in collagen matrix. 
Biomaterials such as, HAp, bioactive glasses (BG) and glass-ceramics
materials can chemically bond to living bone tissue and brings out a specific
biological active response by forming an interfacial bond of
hydroxycarbonate-apatite (HCA, main inorganic part of human bone) layer on its
surfaces 8, 9. Due to this ability, these materials are used as bone repair,
bone tissue regeneration materials, bioactive coatings on inert or metallic
implants, as drug delivery vehicle, tumor treatment, biomimetics, etc. 10.
The apatite crystallites in human bone, enamel, dentin and cementum are all
extremely small in size 11 and can be considered as nano-structured
materials. n-HAp may possess some special properties due to its large specific
surface area and has applications in composites with organic polymers 12-16.
However, another aspect of this observation has not been studied in detail
regarding the interactions of this n-HAp crystals and DNA molecules available,
although the material has been used extensively for development of bone
scaffolds, localized drug delivery system, etc. Another new material called
mesoporous bioactive glass (MBG) has also recently gained considerable
attention which has shown its efficacy towards delivering biomoieties locally
especially for bone drug delivery and bone restoration and bone-in growth
bioactivities. MBG is favorably known for its bioactivity and ordered
mesoporous channels, considered as the third-generation of bioactive glasses. Increasing the specific surface area and pore
volume of bioactive glasses greatly accelerates the HCA formation and therefore
enhances the bioactive behavior 17. This HCA layer
has the same composition and structure as the mineral phase of bone 18. MBG
lead to superior bone-forming bioactivities in vitro compared to normal BGs
derived from sol-gels. Mesoporous structure allows cell migration, mass
transfer, nutrients supply and diffusion of nutrients and oxygen together with
elimination of metabolic wastes that are necessary for new tissue formation
19. MBGs are important in both mesoporous material and biomaterial research
20. Main characteristics of MBG are excellent surface textural properties and
porosity with high surface area, pore volume, tunable pore size and
well-defined mesoporous structure 17.

 

In
the present investigation thus we are reporting and also tried to establish
these two materials primarily in nano form towards their expression in contact
with DNA which will not only explain the hitherto complexity explaining the
bone ultra-structure upto molecular level but also interactions at the
interface could be justified when subjected to load.

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