Chemistry Reference
In-Depth Information
significantly to the stability of b-sheet structures (Griffiths-Jones and Searle 2000;
Syud et al. 2001; Phillips et al. 2005). Overall, these studies point to the importance
of controlled self-assembly (of b-strands into b-sheet fibrils) for chiral communi-
cation; conversely, structures lacking peptide
!
dendron chirality transfer do not
form stable b-sheet fibrils (Fig. 11.37). Therefore, the occurrence of strong peptide
!
dendron chirality transfer correlates strongly with b-sheet fibril stability.
11.5. LANGMUIR-BLODGETT MONOLAYERS
The formation of Langmuir -Blodgett monolayers from amphiphilic dendrimers at
the air-water interface generally confirms the highly flexible structure of most den-
drimers (Tully and Frechet 2001). In order to form monolayers on the surface of
water, a conformation that extends the hydrophilic regions into the water phase and
the hydrophobic groups upward from the interface must be accessible. For
example, early work demonstrated that G2-G4 Fr´chet-type poly(benzyl ether) den-
drons readily formed monolayers (Saville et al. 1993, 1995). Pressure-area (P-A)
measurements suggest that the dendrons adopt a prolate shape oriented on the
surface to point the focal benzyl alcohol group into the water phase and to project
the hydrophobic poly(benzyl
ether)
sequences upward from the
surface
(Fig. 11.38; Kampf et al. 1999).
The G5 and G6 dendrons did not assemble into monolayers because of the ten-
dency of the focal point to be encapsulated by the hydrophobic dendritic branches
at higher generations. Similarly, Liskamp's group reported a series of amino acid
based dendrons capped with Boc end groups and a focal fatty acid chain also
formed stable monolayers; however, the fatty acid group was not a significant
improvement over a shorter linkage (Mulders et al. 1998). In the condensed mono-
layer phase, the dendrons covered a smaller area than predicted for a fully spread
Figure 11.38 (Left) Fourth generation monodendron with a hexaethylene glycol tail.
(Right) Proposed conformations of poly(benzyl ethyl ether) monodendrons oriented at the
air-water interface.
