Biomedical Engineering Reference
In-Depth Information
quaternary ammonium salt at the peptide N-terminal, and hydrophobic alkyl chain at its C-terminal,
were used as template for Proteosilica preparation that can be used without calcinations. Sol-gel
reaction under selected conditions provides Proteosilica in both transparent fi lm and powder forms.
The TEM images of the fi lms displayed in Figure 12.13B indicate highly ordered regular pore
arrays. Peptide assemblies hybridized in mesopores provide asymmetric environment. Therefore,
photochromic dye, spiropyran, was doped in the chiral environments of the Proteosilica fi lms, and
asymmetric photoreaction was demonstrated [94]. Alanylalanine-type amphiphiles, named LL
and DD, referred to chirality of the peptide moiety, were used as the host peptides. Isomerization
between the spiropyran form and the merocyanine form can be repeated upon alternate irradiation
of the visible light (420 nm) and UV light (280 nm) to the fi lms, respectively. Only negligible cir-
cular dichroism (CD) signals that originates from the guest were observed for the fi lm containing
the merocyanine form. In contrast, the fi lm with the spiropyran form showed clear CD activity in
the region from 250 to 400 nm, where the host surfactant does not have any absorbance. Alternate
irradiation with the UV light and the visible light also induced repeated changes in the CD spectra,
with a small degradation in the intensity. In addition, a complete mirror image of the CD spectra
was obtained between LL-type and DD-type Proteosilica fi lms. The presented biohybrid materials
are expected to be applied to memory device with nondestructive read-out capability.
Practical applications under harsh conditions often require strong covalent bonding between
framework materials and biocomponents. Recently, a new synthetic method that copes with both
dense functionalization of the pore inside and high accessibility of external guests has been devel-
oped by Ariga, Aida, and coworkers (Figure 12.14) [95,96]. The template amphiphile was covalently
attached to the silica framework upon sol-gel reactions with tetraethyl orthosilicate, resulting in
mesoporous silica channels, , which were fi lled with an organic group of the template. Cleavage and
removal of the alkyl tail by selective hydrolysis of the ester at the C-terminal resulted in open pores
with a surface covalently functionalized by the alanine residue. This method was named as “lizard
templating method,” because the template behaves like a lizard, whose head bites the silica wall
and whose tail can be cleaved off. The TEM images observed for the hydrolyzed material ensured
preservation of the hexagonal porous structure after hydrolytic treatment. Pore structure formation
on hydrolysis was also demonstrated by nitrogen adsorption-desorption measurement. Selective
hydrolysis of the template ester was clearly demonstrated by FT-IR measurement of the obtained
silica materials. In the spectrum of the as-synthesized silica, peaks of alkyl chains were clearly
detected at 2924 cm
-
1
(
ν
as
[CH
2
]) and 2854 cm
-
1
(
ν
s
[CH
2
]) together with peaks characteristic of the
O at 1742 cm
-
1
, amide I at 1685 cm
-
1
, and amide II at 1555 cm
-
1
).
The former two peaks for the alkyl chain completely disappeared after hydrolysis, although amide
I at 1685 cm
-
1
and amide II at 1555 cm
-
1
remained intact, and the ester peak was converted to the
peak of free carboxylic acid at 1733 cm
-
1
. Temperature-programmed desorption (TPD) analysis
with NH
3
as a basic guest confi rmed the exposure of the alanine C-terminal in the silicate channel.
The lizard templating method is highly expected to be used for the fabrication of mesoporous silica
with a variety of bioorganic functional groups and would be a powerful method for the preparation
of novel types of biohybrid nanomaterials.
alanine ester residue (ester C
=
12.4 HYBRID NANOMATERIALS WITH PROTEINS
Proteins are highly functionalized biopolymers that express various functions such as material
conversion and information transmission. Therefore, hybridization of proteins to the other nano-
structured materials has a great potential in the development of novel functional systems. Since
mesoporous materials have pores with diameter appropriate for protein accommodation, adsorption
of proteins onto mesoporous materials is one of the simplest and most realistic approaches toward
mechanically stable nanomaterials possessing biofunctions. Therefore, immobilization of proteins
to mesoporous materials, especially to mesoporous silica, has been paid rapid attention [97-100].
As shown in the following discussion, Vinu et al. performed systematic physicochemical research
