C O M M U N I C A T I O N S
tional information from the peptide to the dendron. Accordingly,
an intense negative couplet centered at ca. 316 nm emerges for the
â-sheet forms of 4 and 8 in PBS. The couplet disappears in 60%
TFE/PBS when the peptides adopt R-helical structures and is absent
in PBS and 60% TFE/PBS for the other peptides (Figure 2C,D).
The presence of the negative couplet indicates an M-type helical
bias relating the anthranilate chromophores of the dendrons (Figure
2).8 The development of a helical bias requires a formal transfer of
chirality from the focal peptide sequence to the achiral anthranilate
termini. It is noteworthy that we have found this transfer of chirality
difficult in prior studies due to the lack of any apparent steric
interactions capable of communicating focal point chirality with
the dendron termini.8d The lower efficiency of chiral communication
in 4 compared with that in 8, as evidenced by a lower intensity
couplet, likely emanates from subtle differences in the nature and
extent of intermolecular dendron packing.
These studies support the supposition that chiral communication
between the peptide and dendron structural elements occurs most
efficiently in constructs that couple peptide folding with dendron
packing. In this case, dendron association perturbs the intrinsic
R-helical preference of alanine-rich sequences and enforces â-sheet
formation. Accordingly, the intended conformational synergism
occurs to a much greater extent within an aggregated â-sheet motif
than within an R-helical context.
Figure 3. (A) CD spectra of peptides 4 (i, i + 6), 8 (i, i + 10), and 1
(control) (50 µM in PBS). (B) Amide I region of FTIR spectra of 4 and 8
in D2O and TFE (10 mg/mL). (C) CD spectra of 4 and 8, and UV-vis
spectrum of 8 in 60% v/v TFE/PBS (50 µM). (D) CD spectra of 4 and 8,
and UV-vis spectrum of 8 in PBS (50 µM).
Acknowledgment. This work was supported by the National
Science Foundation (CHE-0239871).
Supporting Information Available: Experimental, FTIR, AFM,
CD, and MS data for the studied peptides. This material is available
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The intermolecular dendron association that stabilizes the â-sheet
structure simultaneously promotes the transfer of chiral conforma-
JA068154N
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J. AM. CHEM. SOC. VOL. 129, NO. 7, 2007 1885