Positive Electricity Zaps Through Water Chains

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27 Oct 2005 16:00:32 -0000

Positive Electricity Zaps Through Water Chains

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ISIS Press Release 27/10/05

 

Positive Electricity Zaps Through Water Chains

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Water bound on surfaces of proteins and membranes conducts

positive electricity, and could enable cells and tissues to

intercommunicate rapidly and efficiently. Dr. Mae-Wan Ho

 

A version of this article containing the sources and diagram

are posted on ISIS members' website

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Details here

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Jump conduction down a daisy chain?

 

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 [1] (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.

 

Figure 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.

 

Structural studies 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 [2]. 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.

 

However, biochemists 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 [3]. And

since the chemiosmotic hypothesis was first proposed, it has

been suggested by chemist R.J.P. Williams in Oxford

University [4], and others subsequently [5], 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.

 

Experimental observations 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 [6]. 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 [3]. 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 [7]. 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 water.

 

Scientists from the National Institutes of Health, Maryland,

and the University of Maine in the United States simulated

experimental results on the computer [8]. 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.

 

Hydrogen bonds 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 average.

 

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 [9].

 

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 [5] 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

[10] (See " The liquid crystalline organism and biological

water "

http://www.i-sis.org.uk/onlinestore/papers1.php#section3).

 

 

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