For decades, scientists
have wondered whether water bound to the vast amounts of surfaces of proteins
and membranes inside the cells could conduct electric charge in a very special
way. If the water molecules were aligned with their positive and negative
charges alternating in a chain, as would be the case if adjacent water molecules
were linked together by hydrogen-bonds (a kind of chemical bond involving
a hydrogen being shared between two oxygen atoms), then a ‘jump’ conduction
of positive electricity could, in theory, take place. This involves the positive
charge of the hydrogen nucleus - a proton – passing rapidly down the chain
by relay, without the proton actually moving down. The free proton takes
over bonding with the oxygen of the first water molecule in the chain, creating
a second free proton that displaces its neighbour down the chain until the
last proton comes off at the other end  (Fig. 1). Jump conduction is faster
than ordinary electricity passing through a metal wire, which involves electrons
actually moving, and much, much faster than conduction by charged ions diffusing
through water. But it needs to have chains of water in a sufficiently ordered
state and protein and membrane surfaces may impose that kind of order on water.
1. Jump conduction of protons along a chain of water molecules.
Within the past
10 years, evidence for jump conduction of protons via daisy chains of water
molecules has come from several sources.
Charging up the batteries of life
According to the story in biochemistry textbooks (and
you need a good one to even tell you that), living organisms are charged up
predominantly by accumulating protons on one side of a membrane, and discharged
by protons flowing back down to the other side. Protons are transported across
biological membranes by special membrane proteins called “proton pumps”. The
protons pumped uphill (to a higher energy state), using an external energy
source, such as the oxidation of foodstuff, or absorption of sunlight, is
returned downhill via another enzyme, ATP synthase embedded in the same membrane,
which uses the energy to make ATP, the universal energy intermediate that
powers all living activities. This “chemi-osmotic hypothesis” won a Nobel
Prize for British biochemist Peter Mitchell who first proposed it. The protons
are supposed to exist in bulk solution on either side of the membrane, and
it is the difference in concentration between the two compartments separated
by the membrane that drives the synthesis of ATP.
carried out on these proton pumps within the past ten years show that they
form a channel through the cell membrane that is threaded by a chain of hydrogen-bonding
water molecules from one side of the membrane to the other . Examples of
these proteins are the bacteriorhodopsin, the light-harvesting pigment of
the purple membrane belonging to a bacterium, and the cytochrome c oxidase
that catalyses the last stage in the oxidation of foodstuffs in the membrane
of the mitochondria (the powerhouses of the cell), in which oxygen is reduced
to water by combining with protons and electrons.
have noticed that the rate of some proton pumps, such as the cytochrome c
oxidase - which pumps more than 103
protons per second - is higher than the rate at which protons can be supplied
to the proton conducting channel via the bulk diffusion rate . And since
the chemiosmotic hypothesis was first proposed, it has been suggested by chemist
R.J.P. Williams in Oxford University , and others subsequently , that
the protons, rather than accumulating in solution in the bulk of the cell
compartment, actually diffuse along the membrane surface; perhaps directly
from proton pumps such as cytochrome c oxidase enzyme to the ATP synthase
embedded in the same membrane.
have suggested that proton conduction could indeed take place along the surface
of both natural and artificial membranes at the interface with water, and
more specifically in the water layer(s) immediately next to the membrane surface
. The long-distance migration of protons along membranes has been observed
in purple membranes and reconstituted bacteriorhodopsin, which demonstrated
a high rate of diffusion of protons along the membrane surface and a tendency
for protons to remain on the membrane surface as opposed to going into the
bulk of the cell compartment.
When protons diffuse
along the surface of membranes instead of through the bulk solution, the rates
of proton transport processes are significantly increased . This is due
to a fundamental difference of diffusion in two as opposed to three dimensions.
In three dimensions, a proton far away from its target - say, the entrance
to a proton pump embedded in the membrane - will have a very small probability
to be caught by the target. But in two dimensions, the probability of the
proton being caught is exactly 1; in other words, it will be caught sooner
or later. And if instead of random diffusion, protons are jump-conducted along
chains of interfacial water molecules aligned along the membrane surfaces,
then proton transport processes can indeed be quite fast.
Researchers in the
Max-Planck Institute of Biochemistry, Martinsried, Germany first showed that
very thin films of water (down to about one layer) adsorbed onto a solid surface
exhibits a “surprisingly high conductivity” while using a scanning tunnelling
microscope . The scanning tunnelling microscope depends on the flow of
an electrical current and thus cannot be used to directly image insulating
material. But in humid air, a thin film of water settles on the surface, and
is sufficient to provide sufficient electrical conductivity to allow imaging
at currents below 1 picoampere.
Nanotube, water transport and proton wire
A model of proton-conducting water chain or “proton-wire” has come from a further
unexpected source: studies on carbon nanotubes. A carbon nanotube is a new form
of carbon discovered in 1991 in which carbon atoms are joined up into the shape
of a long thin tube. Such tubes are typically of nanometre diameter, and could
be microns in length. These nanotubes are found to interact substantially with
Scientists from the
National Institutes of Health, Maryland, and the University
of Maine in the United States simulated experimental results on the computer
. They showed that a single-wall nanotube 1.34 nm long and .81nm in diameter
rapidly filled up with water from the surrounding reservoir, and remained
occupied by a chain of about 5 water molecules on average during the entire
66ns of simulation (a nanosecond is a billionth of a second, or 10-9s,
which is a long time in the life of a molecule).
This result was surprising,
because carbon does not have a high affinity for water. But it seems that
getting into tight places restricts the distribution of energies in the water
molecules, so they end up with a lower average energy than if they were in
bulk water, and hence it becomes energetically favourable for the water to
enter the nanotubes.
An analogy I can
offer is how, in a crowded underground carriage, people’s movements are restricted,
and hence the range of energy distribution is narrowed towards the lower end
of the scale.
between water molecules inside the nanotube are shielded from fluctuations
in the environment, and are much more stable. Within the nanotube, only 0.02
percent of pairs of water molecules in contact distance (0.35nm) are unbound,
compared with 15 percent in bulk water. H-bonds in the nanotube are highly
oriented, with less than 15 percent of the H-O….O angles between adjacent
water molecules exceeding 30o,
compared to 37 percent in bulk water. The average lifetime of a H-bond inside
the nanotube is 5.6 ps (picosecond, or10-12s), compared to
1 ps in bulk water. The H-bonds are nearly aligned with the nanotube axis,
collectively flipping direction from one side to the other every 2-3 ns on
Water molecules not only penetrate into the
nanotubes, but are also conducted through them. During the 66 ns, 1 119 molecules
of water entered the nanotube on one side and left on the other, about 17
molecules per ns. This rate is comparable to that measured through the twice
as long channel of the transmembrane water-conducting protein, aquaporin-1.
Water-conduction occurs in pulses, peaking at about 30 molecules per ns, again
reminiscent of single ion channel activity in the cell; and is a consequence
of the tight H-bond inside the tube.
There is a weak attractive
force between the water molecules and the carbon atoms, (‘van der Waals force’)
which is 0.114 kcal per mol. Reducing this by 0.05 kcal per mol (less than
5 percent) turns out to drastically change the number of water molecules inside
the nanotube, which fluctuates in sharp transitions between empty states (zero
water molecule) and filled states, suggesting that changes in the conformation
(shape) of enzyme protein molecules may control the transport of water from
one side to another in the cell membrane.
Do such water-filled
channels conduct protons? The answer is yes. If there is an excess of protons
on one side of the channel, positive electricity will spirit down fast, in
less than a picosecond, some 40 times faster than similar conduction of protons
in bulk water, according to Gerhard Hummer of the National Institutes of Health
in the United States, the leader of the team that carried out the nanotube
simulation studies .
If the nanotubes,
instead of swimming in free water solution, were immobilised in membranes,
they could be used for all kinds of applications, including light sensing,
field effect transistors for proton currents, and desalination of seawater.
What role does
interfacial water play in the life of an organism? Everything, it seems (See
previous “New age of water” series, SiS
23, SiS 24). Interfacial water
accounts for some 70 percent by weight of most organisms including human beings,
making organisms effectively liquid crystalline. I have proposed some years
ago  that proton-conduction through interfacial water may be how the body
intercommunicates at all levels, enabling it to function as a perfectly coordinated
whole. This idea is gaining ground  (See “The liquid crystalline organism
and biological water” http://www.i-sis.org.uk/onlinestore/papers1.php#section3).