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Richard Kail/Science Photo Library
physicsworld.com
Breaking barriers Quantum tunnelling of hydrogen in DNA is one of many
quantum processes in nature being explored by physicists and biologists.
Heavier atoms will (classically) lead to lower vibration
frequencies or, viewed quantum mechanically, a
lower zero-point energy. More energy must therefore
be supplied to break the bond, which in turn lowers
the measured rate. Other quantum nuclear effects,
such as zero-point motion, are already known to
be important in hydrogen bonds, and if a proton
in a hydrogen bond is replaced by a deuteron then
the structure of the bond is altered (the Ubbelohde
effect), as will be the shape of the potential energy
surface and hence the tunnelling probability.
The bottom line is that absolute quantitative comparisons
between theory and experiment are always
very difficult in complex biological systems. While it
is known that an isolated nucleotide base in the lab
can exist in its rare tautomeric form between 0.1%
and 1% of the time, this will not be the same as the
rate inside living cells where polymerase enzymes
have error-correcting mechanisms that can achieve
fidelities up to 1 error in 108 or better. This errorcorrecting
process is of course not included in any
theoretical models. In any case, it is known that
mutations take place for a variety of reasons and disentangling
what fraction of these might be down to
quantum nuclear effects is a huge challenge.
Whether or not Löwdin’s hypothesis is confirmed,
his 1963 paper should at least be celebrated for a
statement he makes in the very first paragraph: “The
electronic and protonic structure of biologically
interesting molecules and systems has to be treated
by quantum chemistry. This has led to the opening
of a new field, which has been called sub-molecular
biology or ‘quantum biology’ ”.
To my knowledge, that was the first ever use of the
term, showing that even if we are reluctant to credit
Schrödinger with first establishing the field of quantum
biology, it is certainly half a century old. n
Physics World March 2013 45