Biomedical Engineering Reference
In-Depth Information
( a )
( b )
(004)
(002)
(211)
(202)
(300)
50 nm
2 nm
Figure 2.6 Conventional (a) and high-resolution TEM (b) micrographs of the
hierarchical organization of the synthetic mineralized collagen fi brils TEM image
of mineralized collagen fi brils. The two long arrows indicate the longitudinal
direction of the collagen fi brils. The two short arrows indicate two HA crystals.
Reprinted with permission from [55].
the collagen molecules combined with Ca 2+ to direct the nucleation of HA
by providing nucleation sites, while at the same time collagen molecules
adopt marked conformations changes in response to the formation of cal-
cium phosphate around them. Previous investigations of the nucleation
sites of HA crystals on collagen fi bers have suggested that the binding
of calcium ions with the negatively charged carboxyl groups (-COOH) in
the amino acid residues of collagen is one of the key factors for the fi rst-
step nucleation of HA crystals [56-59]. For the fi rst time, the Cui group
found that the carbonyl group (C
O) on collagen was another nucleation
site besides the -COOH. They examined the chemical interactions of cal-
cium ions and calcium phosphate crystals with collagen by using Fourier
transform infrared spectroscopy (FTIR) [60]. They observed that the peak
intensities of amides I, II, and III of collagen decreased signifi cantly and
the amide I peak underwent a red shift after mineralization, which indi-
cated that the chemical interaction between carboxyl groups and Ca 2+
ions formed in the mineralization and blocked the bond C
=
O stretch.
The behavior of the nucleation sites exerts an important infl uence on the
following crystal growth and the morphology of crystals. Beyond that,
circular dichroism (CD) analysis was performed to investigate the confor-
mation change of collagen during the initial process. An ultraviolet pho-
tometer was used for turbidity measurements of the mineralized system
in situ to study the kinetic process of mineralization. According to the CD
analysis results, they conjectured that the chelation between calcium ions
and carbonyls of collagen shorten the distances between amino acid resi-
dues and increase the triple-helical propensity of the structure (Figure 2.7).
=
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