b.tech. Biotechnology Notes (3)

8/6/2019 b.tech. Biotechnology Notes (3)

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The Electrical Double Layer

IntroductionIn the discussion of electron transfer reactions so far there has been no

mention of the nature of the electrode/electrolyte interface. It is clear that any

interface will disrupt the electrolyte solution since the interactions betweenthe solid and the electrolyte will be considerably different to those in solution.

For electrodes which are under potentiostatic control there will also be the

additional influence of the charge held at the electrode. These different

factors result in strong interactions occurring between the ions/molecules insolution and the electrode surface. This gives rise to a region called the

electrical double layer. Many models have been put forward to explain the

behaviour observed when electrochemical measurements are performed in

electrolyte solutions. Below we introduce two of the models which have beenused to explain the effects occurring in this region.

The electrical double layer

The model which gave rise to the term 'electrical double layer' was first put

forward in the 1850's by Helmholtz. In this model he assumed that no

electron transfer reactions occur at the electrode and the solution is composed

only of electrolyte. The interactions between the ions in solution and theelectrode surface were asssumed to be electrostatic in nature and resulted

from the fact that the electrode holds a charge density (qm)which arises from

either an excess or deficiency of electrons at the electrode surface. In orderfor the interface to remain neutral the charge held on the electrode is balanced

by the redistribution of ions close to the electrode surface. Helmholtz's viewof this region is shown in the figure below


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The attracted ions are assumed to approach the electrode surface and form a

layer balancing the electrode charge, the distance of approach is assumed to

be limited to the radius of the ion and a single sphere of solvation round eachion. The overall result is two layers of charge (the double layer) and a

potential drop which is confined to only this region (termed the outer

Helmholtz Plane, OHP) in solution. The result is absolutely analogous to an

electrical capacitor which has two plates of charge separated by somedistance (d)


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with the potential drop occurring in a linear manner between the two plates. It

is perhaps no surprise that when impedance analysis is performed onelectrochemical systems the response due to the electrolyte redistribution is

modelled in terms of capacitative elements.

The model of Helmholtz while providing a basis for rationalising the

behaviour of this region does not account for many factors such as,diffusion/mixing in solution, the possibility of absorption on to the surface

and the interaction between solvent dipole moments and the electrode. A later

model put forward by Stern begins to address some of these limitations


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now the ions are assumed to be able to move in solution and so the

electrostatic interactions are in competition with Brownian motion. The result

is still a region close to the electrode surface (100x10-10 m) containing anexcess of one type of ion but now the potential drop occurs over the region

called the diffuse layer.

Many modifications and improvements have been made to these early models

with the latest approaches using numerical modelling to follow theredistribution effects as the electrode potential is varied.

Information provided by: http://www.bath.ac.uk


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Ion channelIon channels are pore-formingproteinsthat help to establish and control the smallvoltagegradient across theplasma membrane of all living cells (see cell potential) by

allowing the flow ofions down theirelectrochemical gradient. They are present in the

membranesthat surround allbiological cells.

Basic features

Ion channels regulate the flow of ions across the membrane in all cells. It is anintegral

membrane protein; or, more typically, an assembly of several proteins. Such "multi-

subunit" assemblies usually involve a circular arrangement of identical orhomologousproteins closely packed around a water-filled pore through the plane of the membrane or

lipid bilayer.[1] The pore-forming subunit(s) are called the subunit, while the auxiliary

subunits are denoted , , and so on. While some channels permit the passage of ions

based solely on charge, the archetypal channel pore is just one or two atoms wide at itsnarrowest point. It conducts a specific species of ion, such assodium orpotassium, and

conveys them through the membrane single file--nearly as quickly as the ions movethrough free fluid. In some ion channels, passage through the pore is governed by a

"gate," which may be opened or closed by chemical or electrical signals, temperature, or

mechanical force, depending on the variety of channel.

Biological role

Because "voltage-gated" channels underlie the nerve impulse and because "transmitter-

gated" channels mediate conduction across the synapses, channels are especially

prominent components of the nervous system. Indeed, most of the offensive anddefensive toxins that organisms have evolved for shutting down the nervous systems of

predators and prey (e.g., the venoms produced by spiders, scorpions, snakes, fish, bees,

sea snails and others) work by plugging ion channel pores. In addition, ion channels

figure in a wide variety of biological processes that involve rapid changes in cells, suchas cardiac, skeletal, and smooth musclecontraction, epithelial transport of nutrients and

ions, T-cell activation andpancreatic beta-cell insulin release. In the search for new

drugs, ion channels are a favorite target

Diversity

Voltage-gated sodium channels: Like othervoltage-gated channels, these

channels open and close in response to membrane potential. This family contains

at least 9 members and is largely responsible foraction potentialcreation andpropagation. The pore-forming subunits are very large (up to 4,000amino

acids) and consist of four homologous repeat domains (I-IV) each comprising six

transmembrane segments (S1-S6) for a total of 24 transmembrane segments. The
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members of this family also coassemble with auxiliary subunits, each spanning

the membrane once. Both and subunits are extensively glycosylated.

Voltage-gated calcium channels: As with the othervoltage-gated channels, theseopen and close according to themembrane potential. This family contains 10

members, though these members are known to coassemble with 2, , and subunits. These channels play an important role in both linking muscle excitation

with contraction as well as neuronal excitation with transmitter release. The subunits have an overall structural resemblance to those of the sodium channels

and are equally large.

Potassium channels: This superfamily is comprised of four families of channels,

which are grouped based on homology and activation. Potassium channels arenear ubiquitous in their expression and are primarily permeable to potassium over

other ions.

Voltage-gated potassium channels: Like othervoltage-gatedchannels, these KV channels open and close according to membrane

potential. This family contains almost 40 members, which are further

divided into 12 subfamilies. These channels are known mainly for their

role in repolarizing the cell membrane following action potentials. The subunits have six transmembrane segments, homologous to a single

domain of the sodium channels. Correspondingly, they assemble as

tetramers to produce a functioning channel.

Calcium-activated potassium channels: This family of channels is,for the most part, activated by intracellular Ca2+ and contains 8 members.

Inward-rectifier potassium channels: These channels allow

potassium to flow into the cell in an inwardly rectifying manner, i.e,potassium flows effectively into, but not out of, the cell. This family is

composed of 15 official and 1 unofficial members and is further

subdivided into 7 subfamilies based on homology. These channels are

affected by intracellularATP, PIP2, andG-protein subunits. They areinvolved in important physiological processes such as the pacemaker

activity in the heart, insulin release, and potassium uptake in glial cells.

They contain only two transmembrane segments, corresponding to thecore pore-forming segments of the KV and KCa channels. Their subunits

form tetramers.

Two-pore-domain potassium chann els: This family of 15 members

form what is known as leak channels, and they followGoldman-Hodgkin-Katz (open) rectification.

Chloride channels: This superfamily of poorly understood channels consists of

approximately 13 members.
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Transient receptor potential channels: This group of channels, normally referred

to simply as TRP channels, is named after their role in Drosophila

phototransduction. This family, containing at least 28 members, is incrediblydiverse in its method of activation. Some TRP channels seem to be constitutively

open, while others are gated by voltage, intracellular Ca2+, pH, redox state,

osmolarity, and mechanical stretch. These channels also vary according to theion(s) they pass, some being selective for Ca2+ while others are less selective,

acting as cation channels. This family is subdivided into 6 subfamilies based on

homology: classical (TRPC), vanilloid receptors (TRPV), melastatin (TRPM),polycystins (TRPP), mucolipins (TRPML), and ankyrin transmembrane protein 1

(TRPA).

Cyclic nucleotide-gated channels: This superfamily of channels contains two

families: the cyclic nucleotide-gated (CNG) channels and the hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels. It should be noted that this

grouping is functional rather than evolutionary.

Cyclic nucleotide-gated channels: This family of channels is

characterized by activation due to the binding of intracellularcAMP orcGMP, with specificity varying by member. These channels are primarily

permeable to monovalent cations such as K+ and Na+. They are also

permeable to Ca2+, though it acts to close them. There are 6 members ofthis family, which is divided into 2 subfamilies.

Hyperpolarization-activated, cyclic nucleotide-gated channels:

While these channels arevoltage-gated, their opening is due to

hyperpolarization rather than the depolarization required for other likechannels. These channels are also sensitive to the cyclic nucleotides

cAMP and cGMP, which alter the voltage sensitivity of the channelsopening. These channels are permeable to the monovalent cations K+ andNa+. There are 4 members of this family, all of which form tetramers of

six-transmembrane subunits. As these channels open under

hyperpolarizing conditions, they function aspacemaking channels in theheart, particularly the SA node.

Cation channels of sperm: This small family of channels, normally referred to as

Catsper channels, is related to the two-pore channels and distantly related to TRP

channels.

Two-pore channels: This small family of 2 members putatively forms cation-selective ion channels. They are predicted to contain two KV-style six-

transmembrane domains, suggesting they form a dimer in the membrane. These

channels are related tocatsper channels channels and, more distantly, TRPchannels.

Light-gated channels like channelrhodopsinare directly opened by the action of

light.
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Ligand-gatedchannels (LGICs): Also known as ionotropic receptors, this group

of channels open in response to specific ligand molecules binding to the

extracellular domain of the receptor protein. Ligand binding causes aconformational change in the structure of the channel protein that ultimately leads

to the opening of the channel gate and subsequent ion flux across the plasma

membrane. Examples of LGICs include the cation-permeable "nicotinic"Acetylcholine receptor,ionotropic glutamate-gated receptorsandATP-gated P2X

receptors, and the anion-permeable -aminobutyric acid-gated GABAA receptor.

Detailed structure

Channels differ with respect to the ion they let pass (for example, Na+, K+, Cl), the waysin which they may be regulated, the number of subunits of which they are composed and

other aspects of structure. Channels belonging to the largest class, which includes the

voltage-gated channels that underlie the nerve impulse, consists of four subunits with sixtransmembrane heliceseach. On activation, these helices move about and open the pore.

Two of these six helices are separated by a loop that lines the pore and is the primarydeterminant of ion selectivity and conductance in this channel class and some others. The

existence and mechanism for ion selectivity was first postulated in the 1960s by ClayArmstrong. The channel subunits of one such other class, for example, consist of just this

"P" loop and two transmembrane helices. The determination of their molecular structure

by Roderick MacKinnonusing X-ray crystallography won a share of the 2003NobelPrize in Chemistry.

Because of their small size and the difficulty of crystallizing integral membrane proteins

for X-ray analysis, it is only very recently that scientists have been able to directly

examine what channels "look like." Particularly in cases where the crystallography

required removing channels from their membranes with detergent, many researchersregard images that have been obtained as tentative. An example is the long-awaited

crystal structure of a voltage-gated potassium channel, which was reported in May 2003.The detailed 3D structure of the magnesium channel from bacteria can be seen here. One

inevitable ambiguity about these structures relates to the strong evidence that channels

change conformation as they operate (they open and close, for example), such that thestructure in the crystal could represent any one of these operational states. Most of what

researchers have deduced about channel operation so far they have established through

electrophysiology,biochemistry, genesequence comparison andmutagenesis

History

The existence of ion channels was hypothesized by the BritishbiophysicistsAlan

Hodgkin andAndrew Huxley as part of theirNobel Prize-winning theory of the nerve

impulse, published in 1952. The existence of ion channels was confirmed in the1970swith an electrical recording techniqueknown as the "patch clamp," which led to a Nobel
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Prize to Erwin Neherand Bert Sakmann, the technique's inventors. Hundreds if not

thousands of researchers continue to pursue a more detailed understanding of how these

proteins work. In recent years the development ofautomated patch clamp devices helpedto increase the throughput in ion channel screening significantly.

The Nobel Prize in Chemistry for 2003 was awarded to two American scientists:Roderick MacKinnonfor his studies on the physico-chemical properties of ion channel

function, including x-ray crystallographicstructure studies andPeter Agrefor his similarwork on aquaporins.

Ion Pump

In biology, an ion pump is atransmembrane protein that moves ions across a plasma membraneagainst their concentration gradient. Such ion pumps, sometimes also known as ion transporters,

can use energy from a variety of sources, includingATP or the concentration gradient of anotherion (sometimes called an "ion exchanger"). For a more detailed description of one particular kindof ion pump, see Na+/K+-ATPase.

Reduction potential

Standard Reduction potential (also known as redox potential, oxidation / reduction

potential orORP) is the tendency of a chemical species to acquireelectrons and thereby

bereduced. Each species has its own intrinsic reduction potential; the more positive the

potential, the greater the species' affinity for electrons and tendency to be reduced.

In aqueous solutions, the reduction potential is the tendency of the solution to either gainor lose electrons when it is subject to change by introduction of a new species. A solution

with a higher reduction potential will have a tendency to gain electrons from new species

(i.e. oxidize them) and a solution with a lower reduction potential will have a tendency tolose electrons to new species (i.e. reduce them). Just as the transfer of hydrogen ions

between chemical species determines the pH of an aqueous solution, the transfer of

electrons between chemical species determines the reduction potential of an aqueoussolution. Like pH, the reduction potential represents an intensity factor. It does not

characterise the capacity of the system for oxidation or reduction, in much the same way

that pH does not characterise the buffering capacity.

Reduction potential is measured involts (V), millivolts (mV), or Eh (1Eh = 1mV).Because the true or absolute potentials are difficult to accurately measure, reduction

potentials are defined relative to the standard hydrogen electrode(SHE) which is

arbitrarily given a potential of 0.00 volts. Standard reduction potential (E0), is

measured understandard conditions: 25C, a 1Mconcentration for each ion participatingin the reaction, apartial pressure of 1 atm for each gas that is part of the reaction, and
http://en.wikipedia.org/wiki/Erwin_Neherhttp://en.wikipedia.org/wiki/Bert_Sakmannhttp://en.wikipedia.org/wiki/Bert_Sakmannhttp://en.wikipedia.org/wiki/Electrophysiology#Planar_patch_clamphttp://en.wikipedia.org/wiki/Electrophysiology#Planar_patch_clamphttp://en.wikipedia.org/wiki/Roderick_MacKinnonhttp://en.wikipedia.org/wiki/Roderick_MacKinnonhttp://en.wikipedia.org/wiki/X-ray_crystallographyhttp://en.wikipedia.org/wiki/Protein_structurehttp://en.wikipedia.org/wiki/Peter_Agrehttp://en.wikipedia.org/wiki/Peter_Agrehttp://en.wikipedia.org/wiki/Peter_Agrehttp://en.wikipedia.org/wiki/Aquaporinhttp://en.wikipedia.org/wiki/Aquaporinhttp://encyclopedia.thefreedictionary.com/transmembrane+proteinhttp://encyclopedia.thefreedictionary.com/transmembrane+proteinhttp://encyclopedia.thefreedictionary.com/ATPhttp://encyclopedia.thefreedictionary.com/ATPhttp://en.wikipedia.org/wiki/Chemical_specieshttp://en.wikipedia.org/wiki/Electronhttp://en.wikipedia.org/wiki/Electronhttp://en.wikipedia.org/wiki/Redoxhttp://en.wikipedia.org/wiki/Redoxhttp://en.wikipedia.org/wiki/Volthttp://en.wikipedia.org/wiki/Volthttp://en.wikipedia.org/wiki/Standard_hydrogen_electrodehttp://en.wikipedia.org/wiki/Standard_hydrogen_electrodehttp://en.wikipedia.org/wiki/Standard_conditionshttp://en.wikipedia.org/wiki/Celsiushttp://en.wikipedia.org/wiki/Concentration#Molarityhttp://en.wikipedia.org/wiki/Concentration#Molarityhttp://en.wikipedia.org/wiki/Concentrationhttp://en.wikipedia.org/wiki/Ionhttp://en.wikipedia.org/wiki/Chemical_reactionhttp://en.wikipedia.org/wiki/Partial_pressurehttp://en.wikipedia.org/wiki/Atmosphere_(unit)http://en.wikipedia.org/wiki/Gashttp://en.wikipedia.org/wiki/Erwin_Neherhttp://en.wikipedia.org/wiki/Bert_Sakmannhttp://en.wikipedia.org/wiki/Electrophysiology#Planar_patch_clamphttp://en.wikipedia.org/wiki/Roderick_MacKinnonhttp://en.wikipedia.org/wiki/X-ray_crystallographyhttp://en.wikipedia.org/wiki/Protein_structurehttp://en.wikipedia.org/wiki/Peter_Agrehttp://en.wikipedia.org/wiki/Aquaporinhttp://encyclopedia.thefreedictionary.com/transmembrane+proteinhttp://encyclopedia.thefreedictionary.com/ATPhttp://en.wikipedia.org/wiki/Chemical_specieshttp://en.wikipedia.org/wiki/Electronhttp://en.wikipedia.org/wiki/Redoxhttp://en.wikipedia.org/wiki/Volthttp://en.wikipedia.org/wiki/Standard_hydrogen_electrodehttp://en.wikipedia.org/wiki/Standard_conditionshttp://en.wikipedia.org/wiki/Celsiushttp://en.wikipedia.org/wiki/Concentration#Molarityhttp://en.wikipedia.org/wiki/Concentrationhttp://en.wikipedia.org/wiki/Ionhttp://en.wikipedia.org/wiki/Chemical_reactionhttp://en.wikipedia.org/wiki/Partial_pressurehttp://en.wikipedia.org/wiki/Atmosphere_(unit)http://en.wikipedia.org/wiki/Gas


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metalsin their pure state. Historically, many countries, including the United States, used

standard oxidation potentials rather than reduction potentials in their calculations.

These are simply the negative of standard reduction potentials, so it is not a majorproblem in practice. However, because these can also be referred to as "redox potentials",

the terms "reduction potentials" and "oxidation potentials" are preferred by the IUPAC.

The two may be explicitly distinguished in symbols as Er0

and Eo0

.

The relative reactivitiesof differenthalf-cellscan be compared to predict the direction ofelectron flow. A higher E0 means there is a greater tendency for reduction to occur, while

a lower one means there is a greater tendency for oxidation to occur.

Any system or environment that accepts electrons from a normal hydrogen electrode is a

half cell that is defined as having a positive redox potential; any system donatingelectrons to the hydrogen electrode is defined as having a negative redox potential. Eh is

measured in millivolts (mV). A high positive Eh indicates an environment that favors

oxidation reaction such as freeoxygen. A low negative Eh indicates a strong reducing

environment, such as free metals.

Sometimes when electrolysis is carried out in an aqueous solution, water, rather than the

solute, is oxidized or reduced. For example, if an aqueous solution ofNaClis

electrolyzed, water may be reduced at thecathode to produce H2(g) andOH- ions, instead

of Na+ being reduced toNa(s), as occurs in the absence of water. It is the reduction

potential of each species present that will determine which species will be oxidized or

reduced.

Absolute reduction potentials can be determined if we find the actual potential betweenelectrode and electrolyte for any one reaction. Surface polarization interferes with

measurements, but various sources give an estimated potential for the standard hydrogenelectrode of 4.4V to 4.6V (the electrolyte being positive.)

Reduction potential in biochemistry

Many enzymaticreactions are oxidation-reduction reactions in which one compound is

oxidized and another compound is reduced. The ability of an organism to carry out

oxidation-reduction reactions depends on the oxidation-reduction state of theenvironment, or its reduction potential (Eh).

Strictly aerobic microorganisms can be active only at positive Eh values, whereas strict

anaerobes can be active only at negative Eh values. Redox affects the solubility ofnutrients, especially metal ions. Oxygen strongly affects redox potential.

Practical measurement of reduction potential

Although measurement of the reduction potential in aqueous samples is relatively

straightforward, many factors limit its interpretation, such as irreversible reactions, slow
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electrode kinetics, non-equilibrium, presence of multiple redox couples, electrode

poisoning, small exchange currents and inert redox couples. Consequently, practical

measurements seldom correlate with calculated values. Nevertheless, reduction potentialmeasurement has proven useful as an analytical tool in monitoring changes in a system

rather than determining their absolute value (e.g. process control and titrations).

Reduction potentials of aqueous solutions are determined by measuring the potential

difference between an inert indicator electrode in contact with the solution and a stablereference electrode connected to the solution by a salt bridge. The indicator electrode acts

as a platform for electron transfer to or from the reference half cell. It is typically

platinum, although gold and graphite can be used. The reference half cell consists of aredox standard of known potential. The standard hydrogen electrode (SHE) is the

reference from which all standard redox potentials are determined and has been assigned

an arbitrary half cell potential of 0.0 mV. However, it is fragile and impractical forroutine laboratory use. Therefore, Ag/AgCl and saturated calomel (SCE) reference

electrodes are commonly used. The voltage relationships for several different reference

electrodes at 25 degrees C can be interrelated as follows:

Reference

electrode

Electrode potential with

respect to SHE (mV)

Standard hydrogen

electrode(SHE)0

Saturated calomelelectrode(SCE)

+ 245

Ag/AgCl, 1 M KCl + 236

Ag/AgCl, 4 M KCl + 200

Ag/AgCl, sat. KCl +199

For example: If you had a reading of 100mV using a saturated KCl Ag/AgCl reference

and wanted to refer it back to an SHE you would add 199mV to obtain 299mV.

Alternatively, if you took a reading in the same solution using an SCE, you would obtain54mV (subtract 245mV from 299mV)
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b.tech. Biotechnology Notes (3)