Assaying and storing labelled analytes

Details Assaying and storing labelled analytes Assaying and storing labelled analytes

1999-01-08

2003-03-18

Nguyen, Bao-Thuy L. (Department: 1641)

Chemistry: analytical and immunological testing

Involving diffusion or migration of antigen or antibody

C204S450000, C204S456000, C204S465000, C204S466000, C204S600000, C422S050000, C435S968000, C436S516000, C436S528000

Reexamination Certificate

active

06534321

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to assays, and especially to gel electrophoresis, to achieve improved imaging and also storage.
BACKGROUND OF THE INVENTION
Separation of mixtures of entities, particularly bio-molecules such as proteins and nucleic acids, is commonly performed using electrophoresis in a gel medium, the gel being produced usually from polyacrylamide or agarose. Once separation has been performed, the separated elements are either detected while still in the gel or after transfer to a membrane support, using any of a variety of biochemical, chemical or spectroscopic methods. It is consistently taught by manufacturers of such gels and by the wider prior literature that gels should not be frozen or subjected to temperatures less than 0° C. Otherwise, loss of performance, of the gel's physical integrity, or of sample detection, will result. The current practice of gel electrophoresis therefore consists essentially of the following steps:
(a) gels are prepared or purchased which have not been subjected to a temperature below 0° C.;
(b) such gels are stored at a temperature greater than 0° C. until used;
(c) sample is applied to the gel and then electrophoresis is performed at a temperature greater than 0° C. for the desired period;
(d) the gel is recovered from the electrophoresis apparatus and the location of sample(s) in one or two dimensions of the gel is detected by any one of various standard procedures; and
(e) the gel is then fixed, using well-described chemicals or solvents, or alternatively the gel is “blotted” onto one of various membranes using any of the well-described procedures, thereby transferring to the membrane, some or a fraction of each sample without substantial change in the relative spatial distribution between samples. The gel itself is then usually discarded.
SUMMARY OF THE INVENTION
The present invention is based on the realisation that a reduction in temperature can be used to enhance the signal from fluorescent labels used on analytes in gel electrophoresis. This is useful to distinguish between analytes and to enhance distinction from background. This leads to a more general method in which, according to this invention, analytes are labelled with a detectable label, the label being chosen such that its detectability can be influenced by modifying the conditions of detection, the analytes are separated (before or after labelling), and the presence of the separated analytes is determined under the modified conditions. If desired, the presence of the analytes may also be determined under the unmodified conditions, e.g. as a control.
An important aspect of this invention is that gels may be cooled or frozen, apparently without adverse effect. The invention provides some applications of freezing gels to gel storage, e.g. in a sealed pack, and to the detection of samples contained within such gels.
Enhanced detection is particularly evident in use of an electrophoretic gel. Novel apparatus comprises a support for a gel, means for applying a current through the gel, means for detecting analytes in the gel, and also means for maintaining the temperature of the gel at below 10° C., preferably below 5° C., e.g. at 0° C. or below.
DESCRIPTION OF THE INVENTION
The modification to which this invention refers may be of any relevant parameter, as can readily be determined by one of ordinary skill in the art. Modification may be of physical conditions such as temperature, e.g. with respect to ambient conditions. For the purpose of illustration, this invention will be described with reference to modification by cooling.
The invention will be described, by way of example, with reference to detection using an electrophoretic gel. For the purpose of this specification, the term “electrophoretic gel” means a gel matrix, usually polymeric, impregnated with a medium, usually aqueous, in which salts or other conductive species are present; a current can be passed through the gel such that species of interest move through it at rates dependent on their respective characteristics, and those species of different characteristics are separated. Various formats may be used within the scope of this invention, including capillary, one-dimensional tubes, two-dimensional slabs with lanes, two-dimensional gels for two-dimensional separation, and also three-dimensional analysis.
It has been found that gels, e.g. consisting substantially of polyacrylamide, may be reversibly frozen without structural impairment. This is apparently independent of the dimensions of the gel or of its degree of cross-linking or its density. Further, this freezing can be achieved reproducibly and routinely according to a process which is capable of automation.
The freezing of electrophoretic gels allows longer-term storage of gels, without significant deterioration. Moreover, where the gels contain separated samples, freezing has the further benefit of reducing diffusion in the gels, thereby preserving the relative position of individual samples within the gel, thus allowing long-term storage of authentic gels without the need for chemical or solvent fixation or the need to prepare an artificial image. A further advantage of the invention is the dramatic enhancement of the fluorescent signal obtained from fluorescent materials contained within the gel, without significant loss of spatial resolution of the signal. Consequently, sample(s) can be detected with greater sensitivity and resolution in appropriately frozen gels than in unfrozen gels. This effect is apparently unaffected by the nature of the gel or the fluorescent substance.
While an unused electrophoretic gel may be frozen, particular advantages are associated with freezing after the application and electrophoresing of the sample, for storage and/or to enhance the signal from a fluorescent label. The use of fluorescent labels in the analysis of carbohydrates is described in U.S. Pat. No. 5,104,508 and WO-A-9428423. The latter publication discloses the use of 2-aminobenzamide, anthranilic acid and analogous compounds as particularly useful fluorescent markers. Their utility may be increased by means of the present invention.
Labelling may be done before or after separation. For instance, a label may be used which binds non-covalently to analytes after these have already been separated in a gel. The invention may involve modification of a known procedure of this type, such as that in which DNA fragments are separated in a gel, and the gel is then exposed to a label (ethidium bromide) which binds non-covalently to each separated fragment (but not to regions of the gel lacking DNA). The location of individual fragments is then found from the fluorescence of the label.
In a particularly preferred embodiment of the invention, an electrophoretic gel is used, in conventional manner, to separate respective species, to any desired degree, and the gel is then “snap-frozen”. This can be an automatic process, in which means for freezing the gel comes into operation at a predetermined time, e.g. when the most mobile species has passed a predetermined distance along the gel. Snap-freezing may be achieved, for example, by sandwiching the gel between two metal plates which have been previously cooled by immersion in, e.g. liquid nitrogen, or dry ice (frozen carbon dioxide), and will usually involve cooling the gel from ambient temperature to a temperature below 0° C., e.g. −10° C. or down to −40° C., over a period of less than 1 minute, preferably less than 30 seconds, in order to achieve the desired phase transition. The frozen gel, including separated species, can then be stored until required for analysis. Immediate analysis may benefit from the improved signal, as described above. When storage is the intention, the electrophoresis apparatus may not itself require detection means. If desired, the apparatus may include means for evacuating water and/or for introducing an inert atmosphere such as argon.
Separation of the analytes and modification of the signal may be simultaneous. In another embodiment of the invention, el

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