Spectroscopic
Characterization of SolventSensitive Foldamers
Debanti Sengupta
Advisor: Professor O’ Hara
Background
• S0 to S2 transition:
Absorption
• S1 to S0 transition:
Fluorescence
Diagram from:
http://www.shsu.edu/~chemistry/chemiluminescence
/JABLONSKI.html
Anisotropy
ISS technical notes
•Molecule excited with polarized light
•Measurement of level of polarization of fluorescence – indication of how fast
molecule rotates
•Low anisotropy – rotates fast; High anisotropy – rotates slowly
Foldamers
• Fold in one solvent, unfold
in another
• Practical applications: can
provide binding pockets
for rodlike chain
molecules; model
compounds to study how
structure and energetics
affect folding into
secondary structures
Prince, R.B., Saven, J.G., J. Am. Chem. Soc. 1999,
Vol 121.
Meta-substituted phenylene
ethynelenes
Nelson, J.C., Saven, J. G. Science, 1997 Vol 277.
• Meta-phenylene ethynelenes – extensively characterized
foldamers
• Display changes to fluorescence and UV-Visible spectra
upon folding in acetonitrile and unfolding in chloroform
M-PE Results
Absorption spectrum
chloroform
acetonitrile
• UV studies – ratio of one peak to another decreases
Cisoid vs. Transoid
Transoid
Cisoid
R
R
N3Et2
R
N3Et2
R
TMS
R
TMS
R
Transoid to cisoid shift responsible for absorbance
blue shift in acetonitrile from chloroform
M-PE results
Fluorescence Spectrum
• Fluorescence results:
adding foldamer to folding
solvent leads to a red shift
(due to excimer formation)
and dramatic quenching of
fluorescence intensity
• Solvent titrations: both
peaks present
• Excitation spectrum of redshifted peak appears to be
like UV spectrum of
foldamer in pure acetonitrile
chloroform
acetonitrile
M-PE Anisotropies
• For 10mer, 18mer, 24mer: anisotropies at 350
nm are ~10 mA higher than at 420 nm
• 420 nm peak – folded state; therefore, folded
state rotates more rapidly than unfolded.
• Folded state may be more compact than
unfolded – analogous to a ball/puck rolling
better rather than a rope
Ortho-substituted Phenylene
Ethynelene Foldamers
R
•
Fold at a much lower number
of monomers – 4 as opposed
to 8 for m-PE molecules
N3Et2
R
•
Center of helix formed is
smaller than m-PE foldamers
R
R
R
TMS
R:
R
R=
=
O
O
O
O
O-PE Foldamers
• Folding dependent on:
– Chain length
– Solvent
– Sidegroups (ester/ether/mixed)
– Temperature
Es6 (ester derivatized O-PE
hexamer)
Fluorescence Spectrum
chloroform
acetonitrile
Es6 Fluorescence Solvent Titration –
Acetonitrile to Chloroform
Two peaks correspond to two conformers: 420
nm conformer favoured in less polar solvent
like chloroform
Es6 Fluorescence Solvent
Titration – Acetonitrile to Water
360 nm conformer favoured in more polar
solvent like water
O-PE Anisotropies
• Preliminary studies indicate 360 nm peak has
lower anisotropy than 420 nm peak
• From M-PE case, possible conclusion – 360 nm
peak corresponds to folded state, 420 nm peak
corresponds to unfolded state
• More anisotropy work needed to prove this
hypothesis
Acknowledgements
• Prof. Gregory Tew, Ticora Jones and the Tew
research group at UMass Amherst for O-PE
work
• Prof. Jeffrey Moore at UIUC for providing MPEs
• Prof. O’ Hara for everything
• Amherst College Chemistry department