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2005
LSU-HHMI Summer Undergraduate Research Program |
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 Amanda
Steffens (U Wisconsin – River Falls) and Paul
Russo, Chemistry
Temperature Response of Poly- £`-carbobenzyloxy-L-lysine
Polarized light microscopy and polarimetry were used to characterize
the helix-coil transition in poly (e-carbobenzyloxy-L-lysine),
PCBL, at a molecular weight of 480,000. The polarimeter data
for a solution at 1% concentration by weight of PCBL in m-cresol
showed that the change from random coil to a-helix conformation
happened at 26.8 ? 0.2 °C, in agreement with Matsuoka et
al.1 Three solutions of PCBL, dissolved in m-cresol, each at
a different concentration, were observed through a range of
temperatures using polarized light microscopy. A solution at
25.07% by weight formed liquid crystals immediately at room
temperature—i.e., at a temperature where dilute PCBL exhibits
a random coil conformation. A solution at 15.04% required three
days at 35°C to form liquid crystals around the edge of
the sample. A 20.00% solution took less than 24 hours to form
liquid crystals along the edges of the sample at 35°C. All
samples exhibited the cholesteric “texture” that
resembles a person’s fingerprint. After being warmed,
the liquid crystal solutions were allowed to cool to room temperature,
where they did not revert to their previous states. This suggests
that liquid crystal formation in PCBL/m-cresol solutions exhibits
a strong history dependence, at least for the very high molecular
weight of 480,000.
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