Chemistry Reference
In-Depth Information
11.4.1. Conformational Switching in Water
The overall impact of water on the folding propensity of the dendrons emerges from
the interplay of several competing noncovalent interactions. The surface area exposed
to the aqueous media is minimal in the syn-syn conformational state, compared with
other unfolded conformations. However, water could also be expected to disrupt the
hydrogen bonding interactions that stabilize the syn-syn conformation and reduce
the energetic cost of the higher dipole moments of the syn-anti and anti -anti con-
formations of the pyridine-2,6-dicarboxamide repeat unit. Recent computational
studies suggested that dipole minimization effects and nonspecific solvophobic com-
pression were more important factors in stabilizing the syn-syn conformation of
these dendrons than intramolecular hydrogenizing interactions (Gabriel et al.
2006). A series of water-soluble dendrons with chiral pentaethylene glycol terminal
groups were constructed to explore the potential for these systems to fold in water
(Fig. 11.33; Hofacker and Parquette 2005).
Evidence for folding was obtained by CD spectroscopy, which displayed a nega-
tive excitonic couplet centered at 316 nm, demonstrating that the first-generation
dendron adopted an M-type helical conformation relating the anthranilate chromo-
phores in both tetrahydrofuran (THF) and water. The G2 dendron exhibited a negative
couplet in THF of comparable magnitude to that of the G1 dendron, also indicating an
M helical bias. The negative couplet transitioned to an equally intense positive
couplet in aqueous media, indicating that an M
!
P helical transition occurred in
water. The third-generation dendron exhibited identical behavior. IR spectroscopy
Figure 11.33 Structures of water-soluble dendrons with chiral pentaethylene glycols.
