bulky and would change the conformational torsional motions along the backbone.
Conclusions could not therefore be drawn about the role of the helical reversals.
An answer to this quandary arises from the fact that a polymer chain with an
extended uninterrupted helical conformation will be highly cooperative, with any
per-unit influence favoring one helical sense being amplified by the number of
cooperating units. Might it therefore be possible that a minute chiral influence,
which would not affect the torsional motions along the backbone in each unit of the
chain, could substantially favor one helical sense because of this expected
amplification?
Enzymatic reduction of deuterated aldehydes led to poly(n-hexyl isocyanate) in
which the CH 2 group adjacent to the chain backbone was converted to a chiral
carbon atom by virtue of stereospecific deuterium substitution for one of the
hydrogen atoms, CHD. In this manner, for a single configuration of the deuterated
carbon, (R) or (S), the left- and right-handed helical states would no longer be
enantiomerically related and therefore, in principle, no longer of equal probability.
Although the energetic difference between the handed helical conformations
depended only on the presence of the deuterium, very large optical activities were
experimentally observed, which was especially remarkable considering that, as
expected, the monomer D-line optical activities were close to zero. Circular dichroism experiments demonstrated that the chiral optical properties of the polymer arose
from the chromophore of the helix. Apparently, the minute per-unit energy favoring
one helical sense was amplified, as expected, by the extended helical conformation.
Herman Mark encouraged international collaborations and this tradition at the
Polymer Research Institute led to critical collaborations that allowed full exploitation of these findings. Shneior Lifson of the Weizmann Institute was visiting New
York to lecture at the Courant Institute of NYU and to visit Herbert Morawetz,
whose wife Cathleen Synge Morawetz had headed this institute. On learning of the
observation of a large amplification occurring on deuterium substitution of a helical
polymer, Lifson noted that he had, some time ago, created a partition function
precisely describing, at the time, an unknown conformational situation in which an
interaction between a very small energy and a far larger energy controlled the
conformational properties of a cooperative system. The small energy, in the deuterated polymer would certainly be the favoring of one helical sense over the other,
per unit, arising from the chiral deuterium substitution. The far larger energy would
be the excess energy of the helical reversal along the chain backbone.
In order to attempt to fit Lifson’s partition function to the optical activity data
observed for the deuterated polyisocyanate, it was necessary to have a series of
samples of widely varying degrees of polymerization, each with a narrow
dispersity, and then to study these polymers as a function of temperature. Yoshi
Okamoto, another member of the Polymer Research Institute, suggested a Japanese
collaboration, which led to a connection with Osaka University in Japan. The
necessary samples were produced by Akio Teramoto’s group, who had the gel
permeation chromatography equipment and the experience to produce these narrow
dispersity polymers by controlled breakdown of a sample with a high degree of
polymerization and also to assign precise degrees of polymerization. Study, in
Stereochemical Studies at the Herman F. Mark Polymer Research Institute
267
Conclusions could not therefore be drawn about the role of the helical reversals.
An answer to this quandary arises from the fact that a polymer chain with an
extended uninterrupted helical conformation will be highly cooperative, with any
per-unit influence favoring one helical sense being amplified by the number of
cooperating units. Might it therefore be possible that a minute chiral influence,
which would not affect the torsional motions along the backbone in each unit of the
chain, could substantially favor one helical sense because of this expected
amplification?
Enzymatic reduction of deuterated aldehydes led to poly(n-hexyl isocyanate) in
which the CH 2 group adjacent to the chain backbone was converted to a chiral
carbon atom by virtue of stereospecific deuterium substitution for one of the
hydrogen atoms, CHD. In this manner, for a single configuration of the deuterated
carbon, (R) or (S), the left- and right-handed helical states would no longer be
enantiomerically related and therefore, in principle, no longer of equal probability.
Although the energetic difference between the handed helical conformations
depended only on the presence of the deuterium, very large optical activities were
experimentally observed, which was especially remarkable considering that, as
expected, the monomer D-line optical activities were close to zero. Circular dichroism experiments demonstrated that the chiral optical properties of the polymer arose
from the chromophore of the helix. Apparently, the minute per-unit energy favoring
one helical sense was amplified, as expected, by the extended helical conformation.
Herman Mark encouraged international collaborations and this tradition at the
Polymer Research Institute led to critical collaborations that allowed full exploitation of these findings. Shneior Lifson of the Weizmann Institute was visiting New
York to lecture at the Courant Institute of NYU and to visit Herbert Morawetz,
whose wife Cathleen Synge Morawetz had headed this institute. On learning of the
observation of a large amplification occurring on deuterium substitution of a helical
polymer, Lifson noted that he had, some time ago, created a partition function
precisely describing, at the time, an unknown conformational situation in which an
interaction between a very small energy and a far larger energy controlled the
conformational properties of a cooperative system. The small energy, in the deuterated polymer would certainly be the favoring of one helical sense over the other,
per unit, arising from the chiral deuterium substitution. The far larger energy would
be the excess energy of the helical reversal along the chain backbone.
In order to attempt to fit Lifson’s partition function to the optical activity data
observed for the deuterated polyisocyanate, it was necessary to have a series of
samples of widely varying degrees of polymerization, each with a narrow
dispersity, and then to study these polymers as a function of temperature. Yoshi
Okamoto, another member of the Polymer Research Institute, suggested a Japanese
collaboration, which led to a connection with Osaka University in Japan. The
necessary samples were produced by Akio Teramoto’s group, who had the gel
permeation chromatography equipment and the experience to produce these narrow
dispersity polymers by controlled breakdown of a sample with a high degree of
polymerization and also to assign precise degrees of polymerization. Study, in
Stereochemical Studies at the Herman F. Mark Polymer Research Institute
267