Biology Reference
In-Depth Information
The combination of the ferrocene scaffold, as a central reverse-
turn unit, with the -L-alanyl-D-proline hetrochiral sequence, as a
dipeptide unit, is performed to induce the antiparallel
b
-sheet-like
and type II
-turn-like structures simultaneously [10]. On the contrary,
the combination with the -L-prolyl-L-alanine homochiral sequence
as a dipeptide unit is found to induce the simultaneous formation of
the inverse
b
g
b
-sheet-like structures [11].
The organization of host molecules by self-assembly is a useful
strategy to form active receptors [12]. Metal-templated organization
has been exploited to provide oriented binding sites, resulting in
the construction of artificial receptors for molecular recognition
[13]. Utilization of self-assembling properties of amino acids as
observed in proteins, which are organized into well-defined three-
dimensional structures, is considered to be a convenient approach to
the desired receptors. However, ferrocenes have been focused on as
an organometallic scaffold for molecular receptors [14]. Ferrocene-
peptide bioconjugates have been demonstrated to recognize anions
[15] and biomolecules [16]. In the ferrocene-peptide bioconjugate
3
-turn-like and antiparallel
bearing the dipeptide chains (-L-Ala-L-Pro-NH-2-PyMe), the
two amido pyridyl moieties as hydrogen bonding sites are well
arranged for dicarboxylic acids by the chirality organization through
two intramolecular hydrogen bondings (Fig. 3.5) [17]. In fact, the
ferrocene-peptide bioconjugate
forms a 1:1 complex with a series
of dicarboxylic acids, wherein the highest association constant is
observed with adipic acid (
3
×
4
-1
K
= 2.1
10
M
) [17]. The binding space
a
size of
can discriminate the size of dicarboxylic acids. Noteworthy
is that benzoyl-L-glutamic acid (
3
×
3
-1
K
= 5.5
10
M
) is bound
a
approximately fifteen times more tightly to
3
than benzoyl-D-glutamic
×
2
-1
acid (
K
= 3.7
10
M
) [17]. The chirality-organized binding site of
a
3
is capable of discriminating the chirality of guest molecules. The
size-selective and chiral recognition of dicarboxylic acids is achieved
by multipoint hydrogen bondings of the binding sites. Crystallization
of a 1:2 mixture of
3
and (1
R
,3
S
)-camphoric acid (
CA
) gives the 1:2
complex
as orange crystals by slow diffusion of hexane into
chloroform [18]. The single-crystal X-ray structure determination of
3•(CA)
2
3•(CA)
2
reveals a polymeric cocrystal composed of alternating units
of
, which are connected by continuous
intermolecular hydrogen bonds to form the double-helical-like
hydrogen-bonded molecular arrangement (Fig. 3.6). Each
3
and two molecules of
CA
is found
to serve as a hydrogen bonding bridge. As a result, each molecule of
3
CA
is bridged by two molecules of
CA
as depicted in Fig. 3.6. Another
Search WWH ::
Custom Search