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was orange owing to charge
transfer from the indole rings to the electron-deficient triazine
ligand of
solution color of the complex
62•64
62
. The association constant (
K
) obtained on the analysis
a
×
5
−
1
of this charge transfer was 1.5
10
M
, which was slightly smaller
−
6
1
than the palladium analogue
63
(
K
> 10
M
). These cages were
a
also capable of binding Ac-Trp-Ala-Trp-NH
(
65
) over
Ac-Ala-Trp-
2
). Further, the cages
can recognize hexapeptides. For example,
Trp-NH
2
(
66
) and
Ac-Trp-Trp-Gly-NH
2
(
67
Ac-Ser-Gly-Ala-Trp-
Trp-Ala-NH
2
(
68
) possessing an additional
Ser-Gly-Ala
sequence
at the N-terminal of
64
bound and resided in the cage of
61
with
5
−
1
high affinity (
K
> 10
M
). However,
Ac-Ala-Trp-Trp-Ala-Gly-
a
Ser-NH
2
(
69
) having additional sequence at both terminals of
64
−
showed poorer affinity than
68
(
K
= 7.2
×
10
4
M
1
); namely, cage
62
a
discriminated between two similar hexapeptides consisting of the
same residues in different sequences. This selectivity is explained
by the fact that the stable conformation of
is disturbed by
steric repulsion between the cage and the elongated sequence at
the C-terminal. Owing to the highly cationic nature of the cage
62•64
62
(12+), charged peptides were strictly discriminated by electrostatic
attraction and repulsion in various pH conditions. For example, the
binding affinity of the nonprotected tripeptide
H-Trp-Trp-Ala-OH
(
) was dramatically affected by pH. In an acetate buffer (pH 4.8),
tripeptide
70
−
70
showed poorer affinity than
64
(
K
= 4.7
×
10
4
M
1
)
a
6
−
1
while under basic conditions showed strong binding (
K
> 10
M
).
a
were
not affected by pH conditions. Furthermore, it was also found that
the neutral and anionic peptides were bound by cage
In contrast to
70
, the affinities of the protected peptide
64
62
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