Biomedical Engineering Reference
In-Depth Information
Fig. 2.2
A tetragonal
lysozyme single crystal
c
b
(110)
a
The indentation and compression analysis of three-dimensional crystallized form
of the tetragonal lysozyme protein crystals reveal that they are relatively fragile and
soft materials and their mechanical properties are highly sensitive to both environ-
mental conditions and the type of the protein molecule [
19
], and size dependent
[
12
,
19
]. The compression testing of crystal leads to nonlinear elastic deforma-
tion leading to fracture, whereas, during microindentation, the microcrystals exhibit
elastic-plastic deformation. The temperature and amount of intracrystalline water
have significant effects on the elastic and plastic properties of the crystals. At lower
temperature and water content, the crystal is more brittle while it is more ductile at
higher temperature and humidity [
12
].
The elastic constants of tetragonal lysozyme crystal are highly sensitive to
both temperature and humidity [
20
-
24
]. The Young's modulus of lysozyme crys-
tal decreases with increasing temperature according to the following relationship
[
20
]:
E
=−
C
T
E
ⓦ
T
(2.16)
where
T
are increments in the Young's modulus and temperature, respec-
tively,
E
ⓦ
is the Young's modulus at 300K and
C
T
is a constant equal to 2
E
and
10
−
3
K
−
1
for lysozyme crystals [
20
]. The Young's modulus increases with increasing amount
of the intracrystalline water molecules [
21
,
22
]as
×
E
=
C
w
E
ⓦ
t
(2.17)
where
t
is the evaporation time and
C
w
is a constant whose value depends on
environmental parameters such as temperature. For natural evaporation of water
from lysozyme crystal surface at room temperature,
C
w
was calculated to be 0.0396
(1/min) [
17
].
The plastic flow in the lysozyme crystals is induced by the dislocation glide along
the preferred slip-systems [
25
]. The tetragonal lysozyme crystal has two sets of slip
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