Process and apparatus for forming a continuous solution gradient

Abstract

 The invention concerns a process and apparatus for forming a continuous concentration gradient in a solution, e.g. of sucrose, in a tube for use in the separation of macromolecules. The process comprises:-

(a) introducing into a tube solutions of differing concentrations in a manner to layer said solutions therein;

(b) inclining said tube to an angle with respect to the vertical;

(c) rotating said tube while maintaining the contents thereof visually determinable thereby to form said continuous solution gradient; and

(d) discontinuing step (c) after formation of said continuous solution gradient.

Preferably the tube is secured in position magnetically.

Description

Field of the Invention

[0001] This invention relates to a process and apparatus for forming solution gradients, and more particularly to a novel process and apparatus for generating a continuous solution gradient.

Description of the Prior Art

[0002] Separation of macromolecules (proteins, DNA and RNA) and larger aggregates, such as viruses and cells, has been and continues to be one or the primary objectives in biochemical research. Perhaps the oldest and still most widely used separation technique is solution gradient density centrifugation comprising three basic steps forming a solution gradient, e.g., of sucrose in a tube; centrifuging a sample into the gradient, and recovering the now-separated samples from various positions in the gradient-containing tube, sometimes referred to an fractionation.

[0003] The term "gradient" implies a continuous variation in concentration from top to bottom, e.g., 5% to 45% sucrose. The gradient performs two critical functions. First and foremost, the gradient prevents mixing in a vertical direction. During acceleration and deceleration of the tube in the centrifuge, a mild degree of mixing is induced which, if unchecked by the gradient, would thoroughly mix the contents in the tube. The gradient, however, prevents such mixing of the density differential between adjacent layers. Secondly, heavier sucrose solutions are much more viscous than light sucrose solutions and consequently, there is established a viscosity gradient. Such a viscosity gradient is useful because "g" forces are greatest at the bottom of the tube (highest radius from the center of rotation) and the increased viscosity effectively cancels the increased "g" forces giving a nearly uniform rate of molecule or particle migration from top to bottom, and consequently one can predict the position of the desired molecules at the end of a run.

[0004] One of the most serious problems in the construction of sucrose gradients is reproducibility. It is apparent that the rate of migration of any molecular species through a gradient is subject to the cumulative effects of buoyancy and viscosity of the gradient. Since these two parameters are caused by the shape of the sucrose gradient itself, tube to tube variation in the gradient will produce tube to tube variation in the final position of any molecular species. Often, it is desired to determine whether subtle changes have occurred in the size or shape of call components, and with very reproducible gradients, such differences may be detected. By the same token, the absolute shape of the gradient is less important so long as the gradient is reproducible.

[0005] There has been a steady but slow evolution in the techniques used to form sucrose gradients, beginning with the laborious manual layering of one solution after another into a tube requiring a plurality of pipettes, a steady hand and mountains of patience and time. Such a technique was quickly supplanted with a technique similar to chromatographic technology wherein two solutions, in this case, the highest and lowest sucrose concentrations in a desired gradient, are measured into two adjacent chambers. The mixing chamber (heavy sucrose) is connected to a centrifuge tube on one side and the other chamber (light sucrose), on the other side. As the mixing chamber's contents empty into the centrifuge tube, the contents of the other chamber enter and gradually lower their sucrose concentration. As the chambers empty, the outflow approaches the light chamber's concentration. Such chromatography-like technology is the most commonly used technique and produces either linear or exponential gradients with minor modification, but has two major drawbacks, i.e., time and reproducibility. When more than one gradient is desired, the outflow must be partitioned, and nothing has yet been developed that will ensure exactly the same flow into each tube. Consequently, a user must watch the level in each tube, clamping off the fast ones until the slow ones catch up, etc. Additionally, there will be slight differences between the gradient in the various tubes because of constant flow adjustments.

[0006] Another technique currently in use is a freeze-thaw method, wherein a homogenous solution is introduced into a centrifuge tube and the tube is subjected to a plurality or freeze-thaw cycles. Such a freeze-thaw method suffers from a serious drawback in that, while the freezing and thawing produces a gradient (ice floats and excludes solute molecules from the pure water matrix), any buffer is subjected to the same forces and also ends up as a gradient, producing numerous potential artifacts. Reproducibility is poor because no two tubes thaw out exactly the same way, and also because the gradients decay with time.

SUMMARY OF THE INVENTION

[0007] Using the preferred forms of the present invention, it is possible to provide a process and apparatus for simultaneously generating continuous solution gradients of faithful reproducibility and of good useful life prior to gradient decay in short periods of time without the necessity for personal supervision.

[0008] In the present invention solutions of differing concentrations are layered in a tube, and the tube is disposed at an angle with respect to the vertical and is rotated normally for a predetermined period of time, thereby to generate a continuous solution gradient, i.e., continuous variation in concentration between the concentration of the initial solutions.

[0009] According to one aspect of the present invention, there is provided a process for generating a continuous solution gradient which comprises the steps of: (a) introducing into a tube solutions of differing concentrations in a manner to layer the solutions therein: (b) inclining the tube to an angle with respect to the vertical; (c) rotating the tube thereby to form the continuous solution gradient; and (d) discontinuing step (c) after formation of the continuous solution gradient.

[0010] According to another aspect, there is provided an apparatus for generating a continuous solution gradient which comprises: a support member; means for rotating the support member about a vertical plane; a tube holder mounted for rotation on the support member; and means for rotating the tube holder.

[0011] In a specific form of one embodiment of the present invention, the apparatus includes a tube or container support which is rotatable with respect to the vertical and rotatable with respect to the horizontal. The tubes or containers are placed within a holder to be received on a support. The housing of the apparatus includes a keypad and an indicator, the keypad being linked to a program which allows control of the angle of inclination of the tube holder containing the tubes, the speed of rotation of the tubes, the duration of rotation, and as well, interrupt functions.

[0012] In another embodiment of the invention, the apparatus includes transparent tubes or containers which additionally include transparent stoppers. The tubes or containers incorporate a magnetic substance at their base and are thus adapted to be placed directly on the support. This allows differently sized tubes, jars or containers to be processed simultaneously while the contents thereof remain completely visible to the user of the apparatus. Further, the user may program the angle of inclination with extreme precision, i.e., within 0.1°. The housing of the apparatus in this embodiment is adapted to receive a separate memory cartridge, which allows multiple users to each have his or her own parameters preset without having to reprogram the apparatus after subsequent users.

[0013] In greater detail of certain embodiments of the present invention, the apparatus includes a tube holder in the form of a mounting member for mounting the tubes at or near their bases; such a tube holder is adapted to retain the tubes in a spaced apart position by magnetic or the like means to permit the tubes to be rotated with an angular inclination between the horizontal and the vertical. To this end, the tube holder may be a flat base having a metal surface or the like adapted to cooperate with magnetic means associated with the tubes whereby when a tube is mounted or placed on the tube holder, the magnetic means is sufficient to retain the tube on the tube holder.

[0014] In another embodiment, the tubes preferably comprise tubes having magnetic means associated with the tubes. To this end, the tubes may be provided with a magnetic film secured to the base and/or lower portion thereof to permit releasable mounting by the tube holder.

[0015] The use of the magnetic feature described above makes it very easy to change reagents while at the same accepting any bottle size without requiring a change of the supporting member or tube holder. In addition, by utilizing this arrangement, accurate control of a tilt angle during the rotation can be achieved with the resulting savings in reagents compared to other apparatus which utilizes, e.g., horizontal arrangements. Utilizing Applicant's process and apparatus of this embodiment, a blotting procedure may be carried out at any desired angle, e.g., 88° or 89°, such as when a filter may be smaller than the length of the tube or bottle. By tilting the bottle and placing a filter so that its bottom edge touches the bottom of the tube or bottle wall, less reagent is required. Thus, for example, a thin, one centimeter strip may only require 1 ml of reagent to cover the whole bottom of a tube or bottle rotating horizontally.

[0016] If desired, the present invention may be operated in conjunction with a programmable system utilizing conventional programming techniques and conventional controllers to provide a determined timing and tilt for a given blot and to provide an automatically reproducible procedure. As will be understood by those skilled in this art, a suitable program may be provided whereby the time, angle and speed of rotation for each step can be preset and the controller utilized to initiate the process utilizing very few keystrokes.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] A better understanding of the present invention as well as other advantages thereof will become apparent upon consideration of the following description of preferred embodiments thereof, especially when taken with the accompanying drawings wherein:

Figure 1 is an isometric view, partially cut away, of a continuous gradient generating assembly of the present invention,

Figure 2 is an isometric view of the continuous gradient generating assembly of Figure 1 in a rotating mode;

Figure 3 is an isometric view of an alternative embodiment of the present invention; and

Figure 4 is an isometric view of the embodiment of Figure 3 in a rotating mode.

DETAILED DESCRIPTION OF THE DRAWINGS

[0018] Referring now to the drawings, there is illustrated a continuous gradient generating assembly, generally indicated at 10, for effecting the process of the present invention and comprised of a rectangularly-shaped base member 12 having an upper surface 14 and a lower surface 16. On the lower surface 16 of the rectangularly-shaped base member 12, there are positioned leg members 18 (one shown) disposed proximate one side thereof. On the other side of the rectangularly-shaped base member 12, there are provided thumb-type screw members 20 disposed in threaded orifices (not shown) formed in the rectangularly-shaped base member 12 for levelling of the continuous gradient generating assembly 10.

[0019] Extending upwardly from the rectangularly-shaped base member 12 on a side thereof proximate the thumb-type screw members 20, there is positioned a vertically disposed cylindrically-shaped support member 22 having a channel 24 formed in an upper end portion therein perpendicularly disposed to the axis of the vertically disposed cylindrically-shaped support member 22. On the other side of the rectangularly-shaped base member 12, there is positioned an operating console member, generally indicated at 26, having a keyboard 28, digital read-out members 30 and concomitant electronics (not shown) including a central processing unit (CPU) and related microprocessors. Within the operating console member 26 there is mounted an electric motor 32 having a shaft 34 and mounted on a side wall of the operating console member 26. The shaft 34 extends horizontally towards and is disposed in the channel 24 of the vertically disposed cylindrically-shaped support member 22 and is mounted for rotation within the channel 24 of the vertically disposed cylindrically-shaped support member 22.

[0020] On the shaft 34 intermediate the vertically disposed cylindrically-shaped support member 22 and the side wall of the operating console member 26, there is mounted a support plate member 36 for rotation with the shaft 34 about the horizontal in response to rotation of the commutator of the electric motor 32, as more fully hereinafter discussed. On a bottom surface of the support plate member 36 there is mounted an electric motor 38 including a shaft 40 mounted to a commutator thereof and, journaled for rotation, is a bearing member (not shown) disposed in the support plate member 36. On the shaft 40 extending above the support plate member 36, there is mounted a tube holder 42 having a plurality of cylindrically-shaped channels 44 disposed radially about a center axis of the tube holder 42. In the position illustrated in Figure 1, the support plate member 36 is disposed in the horizontal plane with the shaft 40 of the electric motor 38 and the tube holder 42 mounted on the shaft 40 being perpendicularly disposed thereto, i.e., in the vertical. Rotation of the shaft 34 of the electric motor 32 causes the support plate member 36 to rotate about the horizontal, thereby causing the tube holder 42 to rotate in a vertical plane perpendicular to the horizontal, as more fully hereinafter discussed. Rotation of the shaft 40 of the electric motor 38 causes the tube holder 42 to rotate in an axis in the vertical plane determined by the preselected rotation of the shaft 34 of the electric motor 32.

[0021] To facilitate an understanding of the process of the present invention, the process of the present invention will be described with reference to the formation of a continuous sucrose gradient. It will be understood by one skilled in the art that the process of the invention may use any tube size, as well as the fact that continuous solution gradients may be formed of solutions other than sucrose, e.g., Ficoll, Percoll, Ficoll-PAQUE (registered trademarks of Pharmacia) Laboratories).

[0022] Equal volumes of solutions representing a solution of low concentration (S₁) and a solution of high concentration (S_h) are layered in a tube 46 generally with the tube 46 in a vertical position. It has been observed that pre-mixing of the solutions may be minimized by first introducing the solution of the lower concentration (S₁) into the tube and then introducing into the lower end portion of the tube 46 by a cannula syringe the solution of the higher concentration (S_h), thereby floating the solution of the lower concentration onto the solution of the higher concentration. To initially establish optimum processing conditions for a particular continuous solution gradient, a dye is admixed in the solution of higher concentration to permit visual observation of gradient formation for any such gradient system, as more fully hereinafter discussed.

[0023] After layering of the solutions, the tube 46 is closed, preferably using a stopper 48 having a channel (not shown) configured to permit the removal of air from the tube during insertion of the stopper 48 into the open end of the tube 46 in a manner which eliminates any gaseous phase in the tube 46 at completion of the act of insertion of the stopper 48 into the tube 46. Any gaseous phase in the tube 46 is deleterious to gradient formation. The tube 46 is thereupon inserted into one of the cylindrically-shaped channels 44 of the tube holder 42, and the tube holder 42 is then inclined from the vertical in response to rotation of the shaft 34 by the electric motor 32 to an angle to the vertical generally or from about 50° to 89.9°. It has been found that a particular preferred angle is defined by the tube with respect to the vertical when the meniscus formed between the layers first contacts the lower end portion of that end of the stopper 48 inserted into the tube 46, i.e., the meniscus extends laterally across the tube 46 in contact with the lower end portion of the stopper 48.

[0024] Upon reaching a predetermined angle of inclination, the tube holder 42 is caused to be rotated in response to rotation of the shaft 40 of the electric motor 38 by energisation thereof. The tube 46 is caused to be rotated for a predetermined period or time, generally of from about 1.5 to 5 minutes at a rotational speed of from about 10 to about 25 RPM , during which time the solution of the higher concentration is caused to rise along the inner tube wall and by such contact with the solution of lower concentration flowing over the interface therebetween, there is formed a continuous solution gradient. Once the continuous gradient is established, the tube holder 42 is righted to the vertical and the tube 46 removed from the channel 44 for use in a subsequent protocol.

[0025] Referring to Figures 3 and 4, there is shown an alternative embodiment of the present invention. In this embodiment, the arrangement permits a clear view of the tubes or bottles mounted by the tube holder, whereby a user may adjust the tilt to cover only a part of the bottle where the blot lies and to permit a clear view of the blot as the colour develops.

[0026] In this arrangement, the device postitions tubes 46 directly onto a tube support 50. The support 50 in this embodiment may be a flat support of, e.g., metallic construction. The support 50 may be exchanged with the tube holder 42 by removing the holder from the shaft 40. The tubes or containers 46 preferably include a magnetic base on the lower ends 52 thereof, which attracts the support 50. To this end, the tube may include a magnetic film secured to the outside of the tube base as shown in Figure ...*

[0027] The stoppers 48 and the tubes 46 preferably are transparent to enable clear visibility of the contents, which additionally aids in determining the point at which an ideal meniscus is formed. The tubes 46 may be tubular or cylindrical in shape depending upon the desired application.

[0028] In a preferred embodiment, the above-described apparatus may operate in conjunction with a conventional controller which includes operating console 26, including the keyboard 28 , digital read-out 30 and electronics having a CPU and related microprocessors. Suitably, the program utilized in carrying out the process will provide the option of setting the angle of inclination of the tubes, the speed or rotation and duration of rotation. The console includes a slot 54, in communication with the internal electronics, which is adapted to receive a separate memory cartridge (not shown). The cartridge allows each user to have his or her own parameters preset, i.e., the angle of inclination, speed of rotation and duration of rotation without having to reset these parameters after consecutive users.

[0029] An important aspect of the present invention, in addition to establishing a preferred predetermined tilt angle, is a requirement to establish a predetermined time of rotation for any given rotational speed. For this aspect, visual reliance is placed upon the dye and its migration from the higher concentrated solution into the lower concentrated solution. The formation of a continuous solution gradient is widened by the dye extending to the top of the lower concentrated solution. Upon reaching the point at which the dye has completed initial migration into the solution of lower concentration, rotation of the tube should be discontinued. Extended rotation of the tube, e.g., for more than about 5 seconds after observation of migration, should be avoided, since degradation of the continuous solution gradient will thereafter begin and solution homogeneity result with uncontrolled time for discontinuing rotation of the tube.

[0030] It will be understood that at low rotational speeds the time of formation of the continuous solution gradient is longer than at higher rotational speeds, and that there are lower and upper limits of rotational speed at which a continuous solution gradient may not be efficaciously formed. Generally, at rotational speeds in excess of about 60 RPM, usable gradients may not be formed. While in the apparatus of the present invention, the tube 46, in which the continuous solution gradient is being formed, is positioned in a chamber displaced from the axis of rotation of the tube holder 42 , the tube 46 may be rotated about its axis with like results. Additionally, while the use of a dye has been described in initial determinations or tilt angle time periods for rotation of the tubes, etc., in a visual determination, it will be understood that other methods for determining such process condition, will be understood by one skilled in the art. It will also be understood that the configuration of the tube in which the continuous solution gradient is to be formed is also important in the determination of appropriate processing conditions. Once having established processing conditions for generating a specific continuous solution gradient from specific solutions using a dye, the resulting processing conditions, i.e., tilt angle, rotational speed and duration of rotation, etc., may be readily introduced as required into the central processing unit or may be preprogrammed and accessed by appropriate language to the CPU thereby to ensure faithful reproduction of specific solution gradients from specific initial solutions.

Claims

1. An apparatus for generating a continuous solution gradient which comprises:
   a support member;
   means for rotating said support member in a vertical plane;
   a tube holder mounted for rotation on said support member; and
   means for rotating said tube holder.

2. The apparatus for generating a continuous solution gradient as defined in claim 1 and further including control means for activating said means for rotating said support member to a predetermined angle.

3. The apparatus for generating a continuous solution gradient as defined in claim 1 or 2, wherein said means for rotating said support member is an electric motor.

4. The apparatus for generating a continuous solution gradient as defined in claim 1,2 or 3 and further including control means for activating said means for rotating said tube holder.

5. The apparatus for generating a continuous solution gradient as defined in claim 4, wherein said means for rotating said tube holder is an electric motor.

6. The apparatus for generating a continuous solution gradient as defined in claim 4 or 5, wherein said control means includes means to stop said means for rotating said tube holder after a predetermined time period.

7. The apparatus for generating a continuous solution gradient as defined in claim 4, 5 or 6, wherein said means is arranged to rotate said tube holder at from 10 to 15 revolutions per minute.

8. The apparatus for generating a continuous solution gradient as defined in any preceding claim, wherein said tube holder is provided with a plurality of cylindrically-shaped channels radially formed about an axis of said tube holder.

9. The apparatus for generating a continuous solution gradient as defined in claim 2 and further including control means for effecting movement of said means for rotating said support member to a vertical position after a predetermined time period.

10. The apparatus for generating a continuous solution gradient as defined in any preceding claim, wherein said tube holder is arranged to releasably mount at least one tube and to permit visual inspection of the contents of said tube.

11. The apparatus for generating a continuous solution gradient as defined in any preceding claim, wherein the tube holder is arranged to magnetically mount at least one tube.

12. The apparatus for generating a continuous solution gradient as defined in claim 11, wherein said tube holder is of a metallic construction such that a tube having a magnetic means associated therewith can be mounted on said tube holder.

13. The apparatus for generating a continuous solution gradient as defined in claim 12, wherein said tube holder comprises a substantially flat surface.

14. A process for generating a continuous solution gradient which comprises the steps of:

(a) introducing into a tube solutions of differing concentrations in a manner to layer said solutions therein;

(b) inclining said tube to an angle with respect to the vertical;

(c) rotating said tube while maintaining the contents thereof visually determinable thereby to form said continuous solution gradient; and

(d) discontinuing step (c) after formation of said continuous solution gradient.

15. The process for generating a continuous solution gradient as defined in claim 14, wherein said tube is inclined to an angle of from 50° to 89.9° from said vertical.

16. The process for generating a continuous solution gradient as defined in claim 14 or 15, wherein said tube is rotated at from 10 to 25 revolutions per minute.

17. The process for generating a continuous solution gradient as defined in claim 16, wherein said rotation is about 15 revolutions per minute.

18. The process for generating a continuous solution gradient as defined in any one of claims 14-17, wherein said tube is closed prior to step (b).

19. The process for generating a continuous solution gradient as defined in claim 18, wherein said tube is closed in a manner to eliminate a gaseous phase.

20. The process for generating a continuous solution gradient as defined in any one of claims 14-19, wherein said solutions of differing concentration are sequentially introduced into said tube whereby a solution of lower density is introduced prior to a solution of higher density.

21. The process for generating a continuous solution gradient as defined in claim 20, wherein said solution of higher density is introduced by a cannula syringe in a lower portion of said tube beneath said solution of lower density.

22. The process for generating a continuous solution gradient as defined in any one of claims 14-21, wherein said tube is closed by a stopper.

23. The process for generating a continuous solution gradient as defined in claim 22, wherein said tube is inclined to an angle whereat a meniscus formed between said solutions contacts a lower portion of said stopper.

24. The process for generating a continuous solution gradient as defined in any one of claims 14-23, wherein a dye is admixed with one of said solutions.

25. The process for generating a continuous solution gradient as defined in claim 24, wherein step (c) is effected for a predetermined time established by migration of said dye into the other solution.

26. The process for generating a continuous solution gradient as defined in claim 25, wherein said dye is admixed in a solution of higher density.

Drawing