Foldamer 3a (Figure 9) is consistent with 1a and2a , and the helical pitch is 3.52Å and 3.74 Å.Two three-center
hydrogen
bonds(N∗–H−NAand NA−H–NQ) were formed between the
terminal β -pinene-derived pyridyl carboxylic with quinoline
oligoamide, the hydrogen bonds length are 2.28 Å and 2.09 Å, the bond
angles are 107.9° and 106.8°, respectively. Interestingly, the
distribution of
NQ−CQ−C−NA shows that
angle peaks at ±10°, which is
consistent with 2 . That is, three-center hydrogen bonds do
improve coplanar of
∠NQ−CQ−C−NA. Next, the
investigation was locked at the character of dihedral angle ∠N*−C*−C=O
in 3a. Compare with ∠N*−C*−N−H dihedral angle in 1 and2 , rotation of ∠N*−C*−C=O is more difficult than that of
∠N*−C*−N−H. In addition, the electronic delocalization effect was found
between bond C=O and pyridine ring of terminal group. The delocalization
can be proved by three bonds length. As Figure 11 shows that 3aand 1a has the same bonds
NQ−CQ, CQ−C, and
C−NA, but the bonds length
of
NQ−CQ, CQ−C, and
C−NA are 1.3521 Å, 1.4972 Å and 1.3720 Å in 3a ,
respectively, which are shorter than those in 1a (1.3698,
1.5529, and 1.4330). Furthermore, the orbital properties of the terminal
group also show the delocalization properties of terminal group in3a . The terminal group has contributions in the HOMO orbital of3a , but terminal group has no contribution in the HOMO orbital
of 1a (Figure 12). It is worthwhile noting that a metastable
state EPP (Q1-Q2 is extended, Q2-Q3 and Q3-Q4 are P helix) has
been found in the FEP of 3a (Figure 10), the free-energy ofEPP is about 2.7kcalmol-1. That is, when the
P helix of 3a converts to M helix, it needs cross a metastable
state EPP. And the metastable state also results in the big free-energy
gap between P- 3a and M-3a , it is the main reason that3a has better helical selection than 1a . Based on the
above discussion, we conclude that the electronic delocalization effect
influences the selection of helical handedness, and the delivery of
chirality in the conjugated chains is easier than in unconjugated
system, which corresponds to some
experimental
results [12-15].
Figure 11 . The structure of
NQ−CQ −C −NA.Figure 12 The HOMO orbital distribution.