Chemistry Reference
In-Depth Information
Figure 9.1 Quadruply H bonded duplexes 1†1 and 2†2. In spite of their different donor
(D)-acceptor (A) sequences, duplexes 1†1 and 2†2 have similar stabilities.
The structures and formation of homoduplexes 1†1 and 2†2 were confirmed by
one-dimensional (1-D), two-dimensional (2-D), and variable temperature
1
H-NMR;
vapor pressure osmometry (VPO) studies; and X-ray crystallography. The
1
H-NMR
experiments in CDCl
3
revealed significant downfield (.2 ppm) shifts of the
aniline NH signals of 1a and 2 compared to those of the one-ring compounds 1
0
and 2
0
, consistent with the formation of duplexes 1†1 and 2†2. The formation of
the duplexes was also indicated by
1
H-NMR binding studies carried out in CDCl
3
,
which revealed dimerization constants of
4.4
10
4
M
21
for 1a†1a and
6.5
10
4
M
21
for 2†2. Considering the error of the NMR binding experiments
(+10%), duplexes 1a†1a and 2†2 can be regarded as having similar stabilities. In
addition to the NMR binding experiments, the formation of dimers by 1a and 2
were also indicated by VPO studies.
The involvement of the aniline NH groups in forming intermolecular H bonding
was demonstrated by variable temperature
1
H-NMR, which showed that the amide H
atoms of both of the two aniline NH signals of 1a shifted upfield. In contrast, the two
glycine NH signals, which formed the S(6) intramolecular hydrogen bonds, showed
much smaller changes within the same concentration and temperature ranges.
The most conclusive evidence for the presence of duplexes 1a†1a and 2†2 came
from 2-D
1
H-NMR [nuclear Overhauser effect spectroscopy (NOESY)] and X-ray
crystallography. The NOESY spectrum of 1b in CDCl
3
contains interstrand NOEs
(Fig. 9.2a) between protons c and e, c and i, and c and j, which are consistent with
an H bonded dimer. The crystal structures of 1a and 2 both revealed the expected
dimeric structures held together by intermolecular H bonds (Fig. 9.2b).
It was found that a-amino acid linkers other than glycine, or different alkoxy side
chains did not affect the stability of each of the duplexes, which lies in the range of
10
4
M
21
. Because of its ready availability, the glycine residue has been adopted in
most of the subsequently designed duplexes. The similar stability of these duplexes
