GB2105750A

GB2105750A - Improvements in bioelectrocatalysis

Info

Publication number: GB2105750A
Application number: GB08137032A
Authority: GB
Inventors: Mark James Eddowes; Hugh Allen Oliver Hill; Irving John Higgins; Nikolai Konstantinovic Myshkin
Original assignee: National Research Development Corp UK
Current assignee: National Research Development Corp UK
Priority date: 1980-12-10
Filing date: 1981-12-09
Publication date: 1983-03-30
Also published as: NL8105573A; DE3148366A1; FR2495843A1; JPS57123662A; GB2105750B

Abstract

Bioelectrocatalysis is the use of materials derived from biological systems as catalysts for reactions occurring at electrodes. These materials are called electroactive biological materials. The applicants have found that the electroactive biological material can be bound to the electrode by providing within the electrically conductive substance of the electrode a binding species for orientation and chemical and reversible binding of the electroactive biological material to the electrode. The conductive substance may be particulate carbon, the binding species being provided by groups on the surface of the carbon particles by surface oxidation thereof. Alternatively the binding species may be provided by an additive which is dispersed throughout at least a portion of the conductive substance, optionally in the presence of a binder. The additive binding species may be non-ionic e.g. 4,4'-bipyridyl, or it may have an ionic functional group, for example it may be a fatty acid or fatty amine. The electroactive biological material is preferably cytochrome C. The bioelectrocatalytic processes of the invention include processes for the enzymatic oxidation or reduction of organic molecules, electroanalytical processes in which substances are monitored by means of the current or potential produced as a result of their interaction with an enzyme or cofactor and processes in biological fuel cells.

Description

SPECIFICATION Improvements in bioelectrocatalysis This invention relates to bioelectrocatalysis and electrodes useful for bioelectrocatalysis.

For the purpose of the present description bioelectrocatalysis is defined as the use of materials derived from biological systems as catalysts for reactions occurring at electrodes.

Thus bioelectrocatalysis includes in particular the electrolytic interaction of enzymes or cofactors with electrodes, by means of which reducing equivalents are either supplied to or given up by an enzyme in connection with a chemical reaction catalysed by that enzyme. Bioelectrocatalysis is useful in supplying energy requirements to biological molecules to catalyse chemical reactions and in biological fuel cells.

For example, published UK Patent Application No. 2033428 describes bioelectrocatalytic processes for carrying out enzymatic reactions, especially processes for the oxidative or reductive transformation of organic compounds catalyzed by enzymes, in which the enzymes require a continuing supply of reducing equivalents to regenerate reduced enzyme species in enzymatically active form, and in which the reducing equivalents are derived electrochemically. In the systems described in the above-mentioned prior patent application precious metal electrodes, e.g. gold 4,4'-bipyridyl electrodes, are used. In that invention electron transfer between electrode and electroactive biological material, e.g. enzyme, takes place rapidly and directly.However, the only electrodes, used for this purpose are precious metal electrodes normally in combination with 4,4to bipyridyl, an electron transfer promoter added to the electrolytes for absorption to the electrode.

Such precious metal electrodes, however, are very expensive and thus usually are not considered practical for use in large scale industrial applications.

It has now been discovered that electron transfer between electrode and electroactive biological material can be achieved using a different and cheaper form of electrode, arising from a novel concept.

Accordingly the present invention comprises a bioelectrocatalytic process in which electrons are transferred between, on the one hand, an electrode and, on the other hand, an electroactive biological material, said electrode comprising an electrically conductive substance which contains a binding species comprising (consisting of or including) a charged functional group for orientation and chemical and reversible binding of the electroactive biological material to the electrode.

The invention includes the use in a bioelectrocatalytic process of an electrode as defined above. it also includes per se a bioelectrocatalytic electrode made of an electrically conductive substance and having bonded thereto an electroactive biological material consisting of or including a cytochrome, especially cytochrome c, in which the electrically conductive substance contains therewithin a binding species as defined above.

The present invention relates to bioelectrocatalytic processes in general, including processes for the oxidative or reductive enzymatic transformation of substances, e.g. organic compounds, and whether occurring at anode or cathode. They also include electroanalytical processes including those in which substances are monitored by means of the current or potential produced as a result of their interaction with a biological material, such as an enzyme or cofactor. Thus the term electroactive biological material signifies any biological material which requires an electron transfer event to express its biological activity and which can undergo electron transfer with an electrode during the course of a biological process. In particular, the electroactive biological material may be a cofactor, e.g.NAD, NADH or FAD or especially, an enzyme or electron receptor component of an enzyme complex, e.g. a cytochrome, flavoprotein, or ferredoxin. Preferably the electroactive biological material consists of or includes a cytochrome, especially cytochrome c or p 450. It may take the form of a complex enzyme comprising a receptor component such as a cytochrome and a redox enzyme such as a monooxygenase. Other such complexes are described in our above-mentioned prior patent application.

The electrically conductive substance of the electrode may comprise a conductive metal powder, e.g. silver or copper powder, dispersed in a suitable mechanical binder, such as a polymeriq binder, e.g. an epoxy resin or polyester binder.

Preferably, however, the electrically conductive substance comprises a non-metallic conductor, especially carbon, e.g. pyrolytic graphite or carbon paste.

The chemical binding species incorporated in the conductive substance may be any capable of chemically and reversibly binding the electroactive biological material to facilitate electron transfer between the material and the electrode. Thus the chemical binding species may comprise a non-ionic species, giving rise to a permanent or induced dipole e.g. 4,4'-bipyridyl or 1 ,2(bis-4-pyridyl) ethylene. Although in the above-mentioned prior patent application these bipyridyl type compounds were used as electron transfer promoters, it was not then realised that they could be used as chemical binding species to be anchored within an electrically conductive substance.Preferably, however, the binding species comprises a species having one or more ionic functional groups to provide charged groups for orientation and binding of the biological material at the electrode surface. For example, the binding species may comprise any species capable of ionising to provide a negatively charged functional group e.g. a sulphonic or carboxylic acid group, or a positively charged functional group, e.g. a tertiary amino group or guanidinium group.

The choice of binding species will be influenced firstly by the sign of the charge on the electroactive biological material at a relevant site thereof. For example, cytochrome c has a positively charged site close to its haem prosthetic group involved in electron transfer. For cytochrome c, therefore, a negatively charged species is used. Generally stated, the same applies to other cytochromes and other electroactive biological materials of isoelectric point greater than about 4. Conversely, a protein having a negatively charged site close to an electron transfer portion of the molecule will require a positively charged binding species. The objective is to cause the electron transfer portion of the molecule to "face" the electrode and thereby cause rapid direct transfer of electrons to or from the electroactive biological material, without the necessary use of a mediator.

The chemical binding species may be comprised by the conductive entity of the conductive substance itself. Thus in a preferred embodiment the conductive substance comprises particulate carbon and the chemical binding species are provided as groups on the surface of the carbon particles by surface oxidation thereof.

Alternatively, the chemical binding species contained within the conductive material are separate from the conductive entity thereof. Thus the chemical binding species may be provided by the mechanical binder used with a conductive substance comprising a powdered metal or other conductor; for instance, by functional groups present in or introduced into the polymeric binder used.

In further preferred embodiments however, the chemical binding species are provided by an additive which is physically combined with the conductive entity of the conductive substance.

Preferred negatively charged chemical binding species additives include organic acids, in particular long chain fatty acids, e.g. C,O~C30 or preferably C16-C24 fatty acids, such as sulphonic acids and especially carboxylic acids, e.g. stearic acid and similar acids, and also organic amines, in particular fatty amines, e.g. dodecylamine.

It will be appreciated that the chemical binding species are not present purely as layers coated on an electrode, but are contained within the conductive substance, dispersed throughout at least a portion thereof.

A mechanical binder may be added to the electrode composition if desired. For example, a liquid binder, such as Nujol or other suitable liquid may be used, or a solid binder, especially a polymeric binder, e.g. PTFE may also be used.

Generally also the electrode composition may be formed into an electrode as desired, including any manner known per se. For instance, the composition may be compressed to form an electrode pellet.

The relative proportions of conductive substance, chemical binding species and mechanical binder (if used) may be varied widely as desired. Generally, however, the ratio by weight of the conductive substance to the physically combined chemical binding species is in the range from 1:1 to 100:1, preferably about 10-1 5:1. For example, electrode compositions comprising 800 mg of graphite with 5 mg of stearic acid in 0.5 ml of Nujol gave electrodes that worked, and compositions comprising 800 mg graphite with 50 mg of stearic acid in the same quantity of Nujol gave electrodes which appeared to function as well as any. In addition, for example, electrodes have been successfully constructed from compositions comprising ten parts by weight of graphite to one part by weight of stearic acid, containing overall 10% by weight of PTFE.

The invention is illustrated with reference to the accompanying sectional drawing of one form of electrode assembly.

With reference to the accompanying drawing one form of electrode assembly comprises a replaceable electrode holder 1 and insulated conductor 2. The conductor 2 is in the form of a conducting metal rod 3 having a screw-thread at one end 4, and an enlarged portion 5 at the other end of an axial screw-threaded recess, the whole encased in a coating of inert, insulating plastics material 6. The screw-threaded end 4 projects from the top end of the coating 6 to provide an attachment for attaching the electrode assembly to a monitoring circuit (not shown).

The electrode holder 1 comprises a carbon paste electrode in the form of a compressed discshaped pellet 10. The pellet is made by compressing a mixture comprising 0.8 g of graphite, 50 mg of stearic acid and 0.5 ml of Nujol in a hydraulic pressure. The carbon pellet electrode 10 is joined by a layer of a conducting, silver containing epoxy resin glue 12 to one face of a conducting metal disc 13 having a screwthreaded boss 14 extending axially from its other face. The electrode disc 10 and metal disc 13 have a coating of inert, insulating plastics material 11 around their circumferences.

As shown in the accompanying diagram, the electrode holder 1 is attached to the conductor 2 by screwing the boss 1 4 into the axial recess in the enlarged portion 5 at the bottom of the metal conductor 3. An annular, resilient, insulating washer 1 5 is used between the holder 1 and conductor 2 to seal conducting metal components from solutions, when the electrode assembly is in use.

Electrode assemblies, as above, where constructed using various different electrode compositions and tested as cathodes with solutions containing cytochrome c by DC cyclic voltammetry to determine their effectiveness as electrodes for bioelectrocatalysis.

Stearic acid An electrode composition comprising 0.8 9 of graphite and 50 mg of stearic acid in 0.5 ml of Nujol was used in another electrode assembly.

1 ,2-bis(4-pyridyl)-ethylene An electrode composition, comprising 0.8 g of graphite and 50 mg of 1 ,2-bis(4-pyridyl)ethylene in 0.5 ml of Nujol was used in one electrode assembly.

4,4'-bipyridyl In a similar manner to that for 1 ,2-bis(4pyridyl)ethylene an electrode was made from graphite and 4,4'-bipyridyl and tested.

Comparative electrode from carbon paste For the sake of comparison a simple carbon paste electrode composition comprising 0.8 g of graphite in 0.5 ml of Nujol was used in a further electrode assembly.

The DC cyclic voltammograms of these electrodes were determined both in the presence (~1 my, i.e. 5 mg per ml) and absence of cytochrome c in an aqueous supporting electrolyte containing sodium chlorate (0.1 M) and phosphate (0.02M) at pH7.

The cyclic voltammograms for both the bipyridyl type (1,2-bis(4-pyridyl)ethylene and 4,4'-bipyridyl) and stearic acid electrodes in the presence of cytochrome c showed pronounced peaks indicating rapid reversible electron transfer between these electrodes and cytochrome c. No such peaks were observed in the absence of cytochrome c for these electrodes, or for the simple carbon paste electrode, whether in the presence or absence of cytochrome c.

The scan rate used for the bipyridyl type electrode was 100 mV S-1 over a scan range of f400 mV against the Standard Calomel Electrode; whereas the stearic acid electrode was scanned over the range~260 mV to +333 mV at scan rates of 10, 20, 50 and 100 mV S-1. A slight spreading of the cytochrome c peaks with increasing scan rate was noted in the latter case.

Claims

1. A bioelectrocatalytic process in which electrons are transferred between, on the one hand, an electrode made of an electrically conductive substance and on the other hand, an electroactive biological material, characterised in that the electrically conductive substance contains therewithin a binding species comprising a charged functional group, for orientation and chemical and reversible binding of the electroactive biological material to the electrode.

2. A process according to Claim 1, characterized in that the binding species is provided by functional groups within the molecules of the electrically conductive substance.

3. A process according to Claim 1, characterised in that the binding species is an additive contained within the electroconductive substance.

4. A process according to Claim 1, 2 or 3, characterised in that the electroactive biological material has an isoelectric point greater than 4 and the binding species is negatively charged or has a dipole moment.

5. A process according to Claim 4 characterised in that the electroactive biological material is a cytochrome.

6. A process according to Claim 5 characterised in that the cytochrome is cytochrome c.

7. A process according to Claim 4, 5 or 6 characterised in that the electrically conductive substance is surface-oxidised carbon, the binding species being provided by the oxidised groups on the surface of the carbon.

8. A process according to Claim 4, 5 or 6 characterised in that the electrically conductive substance is carbon and the binding species is a fatty acid having from 10 to 30 carbon atoms.

9. A bioelectrocatalytic electrode made of an electrically conductive substance and having bonded thereto an electroactive biological material consisting of or including a cytochrome, characterised in that the electrically conductive substance contains therewith in a binding species comprising a charged functional group, for orientation and chemical and reversible binding of the electroactive biological material to the electrode.

10. An electrode according to Claim 9, characterised in that the cytochrome is cytochrome c.

1 An electrode according to Claim 9 or 10, characterised in that the electrically conductive substance is surface-oxidised carbon, the binding species being provided by the oxidised groups on the surface of the carbon.

12. An electrode according to Claim 9 or 10, characterised in that the electrically conductive substance is graphite and the binding species is a fatty acid having from 10 to 30 carbon atoms.