Catching and controlling a quantum biomolecular wave
Wave properties of matter observed manipulated in a protein
Like a superhero with a secret identity and hidden powers, ordinary
matter has normally unseen properties. For example, all matter is both particle
and wave — a consequence of quantum mechanics. However, matter’s wavelike
nature is hard to detect for anything as big as a molecule because the mass-dependent
wavelength is so short.
Now a team from the University of Toronto, the
University of Guelph in Ontario, and the University of Connecticut in Storrs has
used shaped laser pulses to enhance or suppress the photoisomerization yield of
the retinal molecule bacteriorhodopsin as it transforms from one state to another.
University of Toronto professor of
chemistry and of physics R.J. Dwayne Miller explained that the experiments tested
for quantum effects such as wave coherence in biological functions. “We have
shown that quantum coherences can play a role to the point that we can even manipulate
them,” he said.
At first glance, there would not seem
to be any role for quantum effects in biology. The wavelength for a massive particle
like a protein is less than a thousandth of a single bond length between two carbon
atoms and, therefore, is much smaller than the functionally relevant protein motions.
Likewise, the coherence time of the wavefunction describing a protein is much less
than the time scale of a protein’s biological motions. So quantum effects
average out and disappear.
However, biological reactions ultimately
often depend on the localized movement of a few active site carbon atoms. That fact
allows quantum effects to play a part.
The researchers picked the photoisomerization
of retinal bacteriorhodopsin for this study for a number of reasons. It is a fast
reaction — taking place within 500 fs — and the structural change, the
photoisomerization, occurring between the 13th and 14th carbon atoms of the retinal’s
carbon backbone. The bond between the atoms softens or stretches, followed by twist
about the bond axis. The reaction time is comparable to the coherence time of the
excited wave function, and the reaction involves only key carbon atoms, not the
Moreover, Miller noted that there are
many retinal photoisomers in solution, each with only a few percent quantum yield.
In a protein environment, on the other hand, retinal has only one photoisomer and
it has a quantum yield of 65 percent. The protein structure, therefore, limits possible
motions, suggesting that quantum effects won’t be overwhelmed. “This
system in some sense is the acid test for any quantum coherence effects involved
in a biological function,” Miller said.
In carrying out their investigation, the researchers
required some technological advances. They needed very short pulses spanning the
wavelengths used by bacteriorhodopsin, and they needed the ability to shape those
pulses. Most of their key equipment was custom-built, with the exception of a programmable
acousto-optic modulator from Fastlite of Paris that was used for phase and amplitude
manipulation of the pulses.
Vital to the success of the research
was the energy stability of the beam. It had to supply an excitation energy of nanojoules
while maintaining stability of about half a percent. The researchers achieved this
with a diode-pumped solid-state Nd:YLF laser pumping a regenerative Ti:sapphire
amplifier delivering 150-fs pulses at 775 nm. These pulses pumped a noncollinear
optical parametric amplifier to produce final output pulses centered at 575 nm with
a full width at half maximum of 65 nm. The researchers shaped these pulses using
the acousto-optic modulator and kept them at a constant, or actinic, energy.
They flowed bacteriorhodopsin in solution
through a flow cell past the actinic pulse, which they focused down to 150 μm.
Some 20 ps later, they measured its effect by targeting a 630-nm absorption feature
of the photoisomerization product.
By measuring the absorption difference
and weighting that by the number of photons absorbed, they had a direct readout
of the isomerization yield. They used a white-light continuum generated by the laser
pulse for a light source and a calibrated photodiode/spectrometer combination detector
for these readings.
They began with a starting set of 30
actinic pulses, with a random distribution of spectral phase and amplitude. They
fired these into the flow cell, read out the isomerization yield and applied a genetic
algorithm to the pulses so that they evolved through breeding and mutation toward
higher yield. They repeated the process multiple times, eventually running more
than 20 optimization experiments. They then selected for lower yield. Again, they
did multiple runs.
During all of these experiments, they
kept the excitation energy constant to within 1.5 percent and ensured that the energy
was too low (15 nJ) to allow different excited electronic states to be reached.
The pulse intensities were such that the molecule could access the same states as
those attained naturally.
As described in the Sept. 1 issue of
Science, the researchers used these methods to boost the yield 23 percent
or decrease it by a like amount. According to Miller, the changes were consistent
with constructive and destructive interference effects along the reaction coordinate
of the large molecule, and the final shape of the optimization pulses wasn’t
random. “The optimal pulse has a very distinctive feature that corresponds
exactly to the frequency of the torsional motion that is key to isomerization,”
He noted that the optimal pulse drove
a polarization that resonantly coupled to this motion. The optimal pulse was centered
at 557 nm with regular structures spaced 6 nm apart. In contrast, the antioptimal
pulse lacked these features, was centered at 577 nm and was relatively broad. Because
of its characteristics, the antioptimal pulse was out of resonance with this key
The researchers also found that the
protein’s isomerization efficiency was slightly higher for a pulse of random
phase and amplitude than for one that was smoothly varying. This they attributed
to a significant built-in bias for the reaction due to a protein structure. This
bias, Miller said, is indicative of a protein structure that fully exploits random
spectral content fluctuations in low light levels to couple into the motions involved
in photoisomerization and thereby fend off the relaxation of the excited molecule
back to the ground state.
The scientists plan to use pulse shaping
methodologies to selectively excite different types of vibrational motions. That
will allow them to effectively explore the landscape of the protein and understand
how the protein directs events so efficiently to one particular desired reaction
out of all the many other possibilities.
“Biology will teach us a great
deal about how to control chemistry,” Miller said.
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