02-102 2778 remy tnote.

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electrophoresis tech note 2778

Focusing Strategy and Influence of Conductivity on

Isoelectric Focusing in Immobilized pH Gradients
Arnaud Rémy 1, Naima Imam-Sghiouar 2, Florence Poirier 2, and Raymonde run allows evaluation of the entire run and analysis of the
Joubert-Caron 2 resulting electrical profile. This analysis is useful to determine
1 Bio-RadFrance, 92430 Marnes la Coquette, France the optimal migration parameters and for troubleshooting.
2 Université
Paris 13, Laboratoire de Biochimie des Protéines et Protéomique, Although numerous protocols are available in the literature
EA 2361, UFR SMBH Léonard de Vinci, 93017 Bobigny, France
(Gorg et al. 2000, Laboratoire de Biochimie des Protéines et
Correspondence: Dr Arnaud Rémy, Bio-Rad, 3 boulevard Raymond Poincaré, Protéomique server at http://www-smbh.univ-paris13.fr/
92430 Marnes la Coquette, France, arnaud_remy@bio-rad.com
lbtp/index.htm) as well as in electrophoresis equipment
Summary manufacturers’ instructions, it is difficult to identify the best
Sample preparation is a key component of successful two- protocol for a new sample.
dimensional (2-D) gel electrophoresis of proteins. No buffer
IEF protocols include three major steps: a step at low voltage
is universally suitable for the extraction and solubilization of
to initiate desalting of the sample; a progressive gradient to
protein samples. On one hand, it is necessary to extract the
high voltage to mobilize the ions, polypeptides, and proteins;
maximum number of proteins and maintain their solubility
and a final high-voltage step to complete the electrofocusing.
during 2-D electrophoresis. This is commonly done by adding
agents like detergents and chaotropic agents to solubilization Establishment of a voltage gradient between two electrodes
cocktails. On the other hand, isoelectric focusing (IEF) is most results in mass transfer between them. Three processes
successful with samples containing few ionic compounds, contribute to mass transfer: 1) electrical migration, where
although the contribution of ions to IEF is not often recognized. charged species move in response to the electrical field;
We therefore studied the effects of water quality, DTT, and 2) diffusion, where species move under a chemical potential
salt-containing buffers on IEF. We also investigated different gradient (for example a concentration gradient); and
factors that affect the applied current during IEF, such as the 3) convection, which occurs as a result of diffusion or
method of rehydration with sample and different modes of hydrodynamic transport (Garfin 2000). The movement of
voltage ramping. Our initial observations indicate that the total charged matter is observable as an electrical current (I) and
number of spots per gel is almost the same under different follows Ohm’s Law, V = I x R, where V is the voltage and R is
run conditions. However, variations in spot count were the resistance. The three mass transfer processes, namely the
observed outside the 25 –100 kD and pl 5 – 8 ranges under moving current, the equilibrium current, and the convection
different running conditions. current, contribute to the observable (net) current.

Introduction During electrophoresis in a pH gradient, as in IEF, the moving

2-D gel electrophoresis is a critical technique for proteomic current is due to amphoteric molecules that move in response
research, since it is the only method currently available that to the field until they reach their steady-state positions.
is capable of simultaneously separating thousands of In addition, ionic species move through the gradient to the
polypeptides for quantitative studies (Hochstrasser 1998). oppositely charged electrode. The transport of ionic species
One of the most important improvements in 2-D electrophoresis through, and out of, the focusing matrix can take considerable
was the introduction of immobilized pH gradients, or IPGs time — a fact that is not often recognized. As each species
(Bjellqvist et al. 1982, 1993). Various types of apparatus that reaches an electrode or the point in the IPG at which it is
allow direct incorporation of proteins into the IPG during uncharged, its contribution to the net current decreases.
rehydration have contributed to higher resolution and higher
protein loading capacity. Nevertheless, methods for proper
control of electrical parameters during IEF are not often fully
appreciated. Recording electrical parameters during an IEF
02-102 2778 remy tnote.qxd 5/1/02 5:00 PM Page 4

The equilibrium current is the current generated by all the IEF was carried out in the PROTEAN IEF cell. The 7 cm
species as they diffuse around their equilibrium position, pH 3–10 strips were subjected to the following IEF program:
including ions close to the electrode and proteins in the IPG 50 V for 15 min, 250 V for 15 min, linear gradient to 4,000 V
as they are converted between their neutral and ionic forms. over 2 hr, and 4,000 V for 2 hr. The longer 17 cm pH 3–10
The equilibrium current is proportional to the concentration of strips were subjected to three different IEF program strategies:
all ionized species in the sample. In contrast to the moving 1) a “direct progressive voltage” (50 V for 15 min, 250 V for
current that decreases with time at constant voltage, the 15 min, linear gradient to 10,000 V over 5 hr, and 10,000 V
equilibrium current is linearly proportional to the voltage. for 5 hr, or, for the protein-containing mixture, for a total of
40 kV-hr ); 2) a “desalting step and progressive voltage” (50 V
The convection current may be driven by electroendosmotic
for 9 hr, 200 V for 1 hr, linear gradient to 1,000 V over 1 hr,
movement, a component of hydrodynamic transport.
linear gradient to 10,000 V over 6 hr, and 10,000 V for a total
For safety from overheating, a limit of 50 µA per 3 mm wide of 40 kV-hr for the entire run) as described in Joubert-Caron et
strip is advised for commercial apparatus. If the current al. (2000); and 3) a “desalting step and rapid voltage” (50 V for
reaches 50 µA when the voltage is set to increase linearly, 9 hr, 10,000 V to 40 kV-hr). The current was limited to 50 µA
the voltage will then be regulated by the current limit, and the per strip, and the running temperature was 20°C. The strips
resulting voltage increase will no longer be linear. Once the were run in duplicate for samples that contained buffers in
current decreases below 50 µA, the voltage will again drive the absence of proteins, and in triplicate for in-gel protein
the linear progression of the gradient. One of the major incorporation. For protein-containing samples, the strips were
problems encountered by novices to 2-D electrophoresis is stored at -20°C until used for the second dimension.
the inability to reach the voltage they want. Connection of a PROTEAN IEF Cell to a Computer

We evaluated the contribution of various compounds to To monitor electrical properties during IEF, a modem cable
the electrical field. We show how recording the electrical was connected from the RS-232 port of the PROTEAN IEF
parameters helps to identify artifacts, allowing adjustment of cell to the serial port of a PC. The HyperTerminal software
the profile and the duration of each IEF step, and adaptation was opened from the Start menu of the Windows 95
of the protocol to the sample and the buffers used. operating system. The setup configuration was 9,600 bits/sec,
8 data bits, no parity, stop bit at 1, material control flow.
Methods The PROTEAN IEF cell exported run data every 5 min to
First-Dimension Electrophoresis
HyperTerminal software. At the end of the run, the data were
All reagents and materials were obtained from Bio-Rad unless
entered into an Excel spreadsheet for plotting.
otherwise indicated. The loading volumes were 125 µl and
300 µl for ReadyStrip™ 7 cm and 17 cm IPG strips, respectively. Second-Dimension Electrophoresis

Passive rehydration was carried out in the PROTEAN® IEF cell Prior to SDS-PAGE, IPG strips were equilibrated as previously
at 20°C. The rehydration step was 2 hr long for test samples described in Joubert-Caron et al. (2000). They were loaded on
that contained buffers in the absence of proteins, and 6 – 9 hr PROTEAN II Ready Gel® 8 –16%T precast gels and run for 1 hr
for in-gel protein incorporation. at 40 V followed by 15 hr at 150 V in a PROTEAN II XL cell.
The gels were silver stained and one gel per condition was
Three different premade reagents were tested: ReadyPrep™ scanned with a GS-700 scanner and quantitated as described
reagent 1, reagent 2, and reagent 3.* For protein in-gel by Joubert-Caron et al. (1999) and Poirier et al. (2001) with
incorporation, the salt-free rehydration solution was 7 M urea, Melanie 3 software (Genebio, Geneva, Switzerland).
2 M thiourea, 4% (w/v) CHAPS, 0.24% (v/v) Triton X-100,
0.2% (w/v) Bio-Lyte 3/10. Rehydration solution (280 µl) was
mixed with 20 µl (100 µg) of soluble proteins extracted from
the lymphoblastoid cell line PRI as previously described in
Joubert-Caron et al. (2000). To evaluate the effect of DTT on
the current, lyophilized reagent 2 was reconstituted with
deionized water and with either 10 mM or 20 mM DTT.

* Reagent 1 contains 40 mM Tris. Reagent 2 contains 40 mM Tris, 8 M urea,

4% (w/v) CHAPS, and 0.2% (w/v) Bio-Lyte® 3/10 ampholyte. Reagent 3
contains 40 mM Tris, 5 M urea, 2 M thiourea, 2% (w/v) CHAPS, 2% (w/v)
sulfobetaine (SB 3-10), and 0.2% (w/v) Bio-Lyte 3/10.

2
02-102 2778 remy tnote.qxd 5/1/02 5:00 PM Page 5

Results and Discussion Effect of DTT on Electrical Properties

We evaluated the contribution to the current of different The effect of dithiothreitol (DTT) was determined as above.
compounds in the sample as well as in the rehydration buffers. DTT is an ionic compound (pKa 9.5) used to reduce disulfide
In all tests, the default current limit of 50 µA per strip of the bonds in proteins. Figure 1B illustrates the effect of the
PROTEAN IEF cell was used. concentration of DTT. The electrical field was maintained over
Contribution of Water Quality to Electrofocusing
25 hr for reagent 2 containing 20 mM DTT. After 8 hr, the
The effect of the quality of the distilled water on IEF was tested current, which had been stable at 32 µA, decreased
by reconstituting lyophilized ReadyPrep reagent 2 with either progressively to reach a new stable level at 24 µA, identical to
partially purified, lower quality demineralized water or deionized the equilibrium current value observed with the same buffer
(<0.6 µS/cm) water. Water (125 µl) was loaded on 7 cm strips without DTT (data not shown). Thus, DTT migrated in the field
as described in Methods. Figure 1A illustrates the impact of and became neutralized over long runs, which would allow the
the water quality on the conductivity of the buffers. The proteins to reoxidize. This effect can be counterbalanced by
equilibrium current when using lower quality water was almost laying an extra paper strip soaked with 20 mM DTT onto the
twice (41 µA versus 24 µA) that of deionized water. IPG gel surface near the cathode when running basic IPG
strips (Gorg et al. 1995). However, this has the disadvantage
A of providing a source of DTT ions that will continuously enter
4
the electrical field. Several protocols recommend tributyl-
phosphine (TBP) instead of DTT (Herbert et al. 1998). TBP is
50
neutral and does not migrate in the electrical field. Moreover,
3
it is used in smaller quantities (3 – 5 mM). Current profiles of
Current, µA/strip

40 41 µA reagent 2 with and without 2 mM TPB are similar (data not

Voltage, kV

shown). Thus, TBP offers a clear advantage over DTT.

30 2
Effect of Salts on IEF
24 µA At 50 µA per strip, the 40 mM Tris (reagent 1) required in
20
excess of 20 kV-hr over 5.5 hr to completely exit the 17 cm
1
IPG strips (Figure 2A, black lines). When 40 mM Tris was
10
added to a rehydration solution, resulting in a solution similar
0 0
to reagent 3, the current stayed at the limit of 50 µA for 8 hr
1 2 3 4 (Figure 2A, green lines). In contrast, for the rehydration solution
Run time, hr
alone (without Tris and protein), the current stayed at 50 µA for
B only 1 hr 50 min, and the moving current reached zero in
4
2.5 hr (Figure 2A, blue lines), whereas adding 100 µg of
50
proteins required only 30 min more for the moving current to
3
reach zero (Figure 2A, red lines). After that time, strips with
Current, µA/strip

40
and without protein both showed the same equilibrium current
Voltage, kV

32 µA
profile and values. The difference in migration time can be
30 2
attributed to the proteins, which are amphoteric molecules,
29 µA and to the presence of ions in the protein extract. Assuming
20
24 µA an average protein molecular weight of 40 kD, loading 100 µg
1
in 300 µl corresponds to an 8.3 µM concentration. The
10
associated conductivity is insignificant compared to the
contribution of any salts or ionic contaminants. The high current
0 0 contributed by a salt (such as Tris at only 40 mM) versus 100 µg
1 2 3 4 of protein, emphasizes the importance of desalting samples as
Run time, hr
extensively as possible.
Fig. 1. Current and voltage profiles of different solutions loaded on IPG strips Electrical Strategies
(current, thick lines; voltage, thin lines). Solutions (125 µl) were loaded onto
7 cm pH 3 –10 strips. A, lyophilized reagent 2 reconstituted with either Three different voltage regimens were tested. In the experiment
demineralized water (blue) or deionized water (red). B, reagent 2 illustrated in Figure 2A, the molecules were first mobilized at
supplemented with no DTT (blue), 10 mM DTT (red), or 20 mM DTT (black). low voltage for a short time. Then the voltage was increased
The IEF program was 50 V for 15 min, 250 V for 15 min, linear gradient to
4,000 V over 2 hr, and 4,000 V for 2 hr.
linearly to 10,000 V over 5 hr. Complete focusing was
achieved at 10,000 V for 5 hr or a total of 40 kV-hr for the
protein-containing solution. For both the rehydration solution
alone, and the protein-containing sample, it took only 3 hr for
the moving current to reach zero. The remaining current, due
to the equilibrium current, was then determined only by the
voltage according to Ohm’s Law. The graphical representation

3
02-102 2778 remy tnote.qxd 5/1/02 5:00 PM Page 6

A Direct progressive voltage

50 43 µA 14 To evaluate the impact of so-called active rehydration, which
12 is proposed to facilitate the entry of some proteins into the gel
40
(Gorg et al. 1999), an initial extended step at 50 V was inserted.
Current, µA/strip

10

Voltage, kV
30 This step has also been described as a preliminary desalting
8
19 µA
step (Righetti and Bossi 1997). This second strategy was
6
20 used in the “desalting step and progressive voltage” program
4 described in the Methods section.
10 5 µA
2
As shown in Figure 2B, the global current profile looked very
0 0 different from the previous profile shown in Figure 2A. A
2 4 6 8 10
Run time, hr desalting process was apparent in the first step at 50 V for
B Desalting step and progressive voltage almost 7 hr, after which the current stabilized. This step could
50 14 hence be reduced from 9 hr to 7 hr. After this initial step, all
47 µA
12 tested mixtures except reagent 3 followed the voltage ramp
40
without current limitation. The rehydration solutions, with or
Current, µA/strip

10
without proteins, showed similar profiles, with a return to
Voltage, kV

30 8
equilibrium after 2 hr at the last gradient step. These data
18 µA 6
20 suggest that the duration of the gradient could be reduced
4 from 6 hr to 2 – 3 hr. Reagent 1 (40 mM Tris) reacted to the
10 gradient step with a high but short peak of current, due to
2
8 µA
the previous extended desalting step at 50 V. Even reagent 3
0 0
2 4 6 8 10 12 14 16 (the mixture with Tris) showed a zero moving current before
Run time, hr
the end of the gradient step, which could therefore be
C Desalting step and rapid voltage
45 µA
reduced from 6 hr to 4 hr. It is noteworthy that in both
50 14 electrical strategies, the values of the equilibrium current
12 were the same.
40
Current, µA per strip

10 In the first “direct progressive voltage” strategy (Figure 2A),

Voltage, kV

30 8 part of the run was under the control of the 50 µA limit.

23 µA Adding an initial long desalting step at low voltage in the
20 6
19 µA
second “desalting step and progressive voltage” strategy
4
10 8 µA 2
(Figure 2B) allowed the voltage to follow subsequent
programmed steps. However, 50 µA is only a compromise
0 0 value, which maximizes the possible current delivered without
2 4 6 8 10 12 14 16
Run time, hr disturbing the final reproducibility of the pattern by overheating
Fig. 2. Current and voltage profiles of solutions subjected to three electrical
(the Joule effect) and while preventing strip burning. The third
strategies (current, thick lines; voltage, thin lines). Blue, rehydration solution; red, electrical strategy, “desalting step and rapid voltage”
rehydration solution with 100 µg protein; black, reagent 1; green, reagent 3. (Figure 2C), was designed to examine the effect of a 50 µA
Solutions (300 µl) were loaded on 17 cm pH 3–10 IPG strips and then subjected
to different IEF electrical strategies.
limit regulation after an active rehydration and desalting step:
50 V for 9 hr, 10,000 V to 40 kV-hr. After the desalting step,
allowed us to deduce that the gradient duration might be the current reached the 50 µA limit immediately after the high
reduced in this case for optimization of the IEF. The current voltage was applied (Figure 2C). The current then decreased
profiles of the two samples (with and without proteins) were progressively, finally reaching the same values as in the two
the same for almost 2 hr. After 2 hr, the voltage increased previous electrical strategies.
rapidly for the sample without protein, whereas the protein-
containing solution remained under the 50 µA limit for a longer
time. This data illustrates the order in which the molecules are
mobilized, first the high-velocity ions, then the proteins.
Reagent 3 without the addition of potentially ionic constituents
(that is, without protein) was current limited at the end of the
voltage-ramp step. An 8 hr run was required for the current to
drop below the 50 µA limit, and 40 kV-hr were reached in 9 hr
20 min at a current of 46 µA. To reduce the current significantly
below the 50 µA limit for this particular case, it may be
necessary either to decrease the mixture’s salt composition,
or to increase the duration of the gradient step to allow the
moving current to drop near zero.

4
02-102 2778 remy tnote.qxd 5/1/02 5:00 PM Page 1

A, Direct progressive voltage, 7 hr run B, Desalting step and progressive voltage, 17.5 hr run C, Desalting step and rapid voltage, 14 hr run
50 10 50 10 50 10

40 8 40 8 40 8
Current, µA/strip

Voltage, kV
30 6 30 6 30 6

20 4 20 4 20 4

10 2 10 2 10 2

0 0 0 0 0 0
2 4 6 2 4 6 8 10 12 14 16 2 4 6 8 10 12 14
Run time, hr Run time, hr Run time, hr

995 spots 1,013 spots 1,009 spots

Fig. 3. 2-D analytical gels of proteins focused using different electrical strategies. Proteins were extracted from the lymphoblastoid cell line PRI in extraction buffer
containing 2 mM tributylphosphine. Aliquots (100 µg) of the extracts were loaded onto 17 cm pH 3–10 IPG strips. Top row, current (thick lines) and voltage (thin lines)
profiles obtained during IEF of the samples (data from Figure 2). Proteins focused in these runs were separated in the second dimension on 8–16%T Ready Gel
precast gels, with Precision Protein™ molecular weight markers on the left side of each gel. Proteins were detected by silver staining.

Analysis of 2-D gels allowed further comparison of the three The third strategy, however, with its sudden voltage increase,
protocols. Each of the 2-D gels of samples subjected to the showed fewer large protein spots and many more small
“direct progressive voltage”, the “desalting and progressive protein spots compared to the two other strategies. One can
voltage”, and the “desalting and rapid voltage” IEF runs hypothesize that the small, more mobile molecules in a
(Figure 3A, B, and C, respectively) appeared similar and complex mixture respond first to the suddenly applied field.
showed almost the same number of spots (1,005, SEM <1%). As the applied voltage increases, the molecules could be
The IEF run durations were 7 hr, 17.5 hr, and 14 hr, respectively. progressively subjected to the field and mobilized, and the
Image analysis of the gels showed that the distribution of movement of the larger ones would be better regulated by
protein spots, however, was not identical. The number of the applied current.
spots was determined in three molecular weight intervals:
small proteins <25 kD; proteins between 25 kD and 100 kD; Table. Number and distribution of protein spots focused using different
electrical strategies by mass intervals. Data from gels in Figure 3.
and large proteins >100 kD (see Table). The sum of the spots
Focusing Strategy
counted in the three intervals slightly differed from the total
Mass Direct Desalting Step Desalting Step
number of spots per gel; some spots could have been Range, kD Progressive and Progressive and Rapid
counted twice because they lie near the boundary between Voltage Voltage Voltage
intervals. The three electrical strategies gave similar numbers >100 119 132 76
of spots for the proteins between 25 kD and 100 kD (739, 25–100 740 745 732
SEM <1%). This corresponds to the mass range that is well 0–25 143 132 191
separated by 2-D electrophoresis. For these common proteins,
intense evaluation of different electrical strategies is not Looking at the distribution of spots in various pH intervals
necessary once the moving current reaches zero. The fastest (Figure 4), proteins with pIs in the ranges of 5 – 6 and 6 –7,
strategy (7 hr in this case) can be chosen. For both large and which are the most abundant in the sample (Joubert-Caron
small proteins, the first two strategies, which have a et al. 2000), were present in the same proportion at all of the
progressive voltage gradient in common, showed similar voltage regimens applied (SEM around 1%). The extremely
numbers of spots. The initial low-voltage step at 50 V did not acidic (pH 3.5 – 4) and alkaline (pH 9 – 9.5) regions also showed
give a significant increase in the number of large protein spots. similar numbers of spots but the small number of spots

5
02-102 2778 remy tnote.qxd 5/1/02 5:00 PM Page 2

350 The current through IPG strips is often limited to 50 µA per

Direct
progressive strip to minimize heating. This is the case with the most
voltage
300
popular commercial IPG cells, the PROTEAN IEF cell and the
Desalting step
and progressive IPGPhor. The 50 µA per strip limit helps to keep sample
250 voltage temperatures below 30°C. Above 30°C, urea in the sample
Number of spots

Desalting step solutions can begin to break down to cyanates, thereby

200 and rapid
voltage increasing the risk of protein carbamylation. The current limit
can be increased in cases where carbamylation is not an issue.
150
The best advice that we can give when running IEF on
100 conductive (high-salt) samples under current-limiting IPG
conditions is to be patient and allow sufficient time for ions to
50 clear from the IPG strips before expecting the voltage to reach
high levels.
0
3.5– 4 4–5 5–6 6–7 7–8 8– 9 9–9.5
Acknowledgments
pH range
We are deeply indebted to Chau Nguyen for helpful discussions
Fig. 4. Number and distribution of the spots by pI intervals focused using and thank Dave Garfin for critical reading of this manuscript.
different electrical strategies. Data from gels in Figure 3.
References
Bjellqvist B et al., Isoelectric focusing in immobilized pH gradients: principle,
counted may not allow detection of any significant discrepancy.
methodology and some applications, J Biochem Biophys Methods 6,
In contrast, the pH 4 – 5 range, and the pH 7 – 8 and 8 – 9 317-–339 (1982)
ranges showed a different number of spots depending on the Bjellqvist B et al., Micropreparative two-dimensional electrophoresis allowing
strategy (SEM around 20 – 30%). Interestingly, compared to the separation of samples containing milligram amounts of proteins,
Electrophoresis 14, 1375–1378 (1993)
the two other strategies, the “desalting and rapid voltage”
Garfin DE, Isoelectric Focusing, pp 263–298 in Ahuja S (ed) Handbook of
strategy yielded more proteins in these two alkaline ranges
Bioseparation, Academic Press, San Diego, 2000
and fewer proteins in the acidic range. Further investigations
Gorg A et al., Two-dimensional polyacrylamide gel electrophoresis with immobilized
with other samples are needed to confirm this observation pH gradients in the first dimension (IPG-Dalt): state of the art and the controversy
before proposing any explanation. of vertical versus horizontal systems, Electrophoresis 16, 1079–1086 (1995)
Gorg A et al., Recent developments in two-dimensional gel electrophoresis with
Conclusions immobilized pH gradients: wide pH gradients up to pH 12, longer separation
Recording the electrical parameters during the IEF process distances and simplified procedures, Electrophoresis 20, 712–717 (1999)
allows tracking of the electrical behavior of the samples. Gorg A et al., The current state of two-dimensional electrophoresis with immobilized
pH gradients, Electrophoresis 21, 1037–1053 (2000)
Tracking emphasizes the role of the small ions in the sample,
Herbert BR et al., Improved protein solubility in two-dimensional electrophoresis
which move in response to the applied voltage more readily
using tributyl phosphine as reducing agent, Electrophoresis 19, 845–851 (1998)
than the proteins. Protocols can also be adjusted based on
Hochstrasser DF, Proteome in perspective, Clin Chem Lab Med 36, 825–836 (1998)
these records (for instance to optimize step duration). Sample
Joubert-Caron R et al., A computer-assisted two-dimensional gel electrophoresis
conductivity influences IEF runs in IPG strips more than in approach for studying the variations in protein expression related to an induced
capillary IEF because larger amounts of sample solution can be functional repression of NFkappaB in lymphoblastoid cell lines, Electrophoresis 20,
1017–1026 (1999)
loaded into IPGs. Contrary to common perception, mobile
Joubert-Caron R et al., Protein analysis by mass spectrometry and sequence
ions in an IPG sample do not migrate rapidly to the electrodes
database searching: a proteomic approach to identify human lymphoblastoid
when voltage is applied, but may require a long time to clear cell line proteins, Electrophoresis 21, 2566–2575 (2000)
from the strips. The present data confirm the usefulness of a Poirier F et al., Two-dimensional database of a Burkitt lymphoma cell line (DG 75)
progressive voltage gradient with a duration sufficient to lower proteins: protein pattern changes following treatment with 5'-azycytidine,
Electrophoresis 22, 1867–1877 (2001)
the moving current to near zero.
Righetti PG and Bossi A, Isoelectric focusing in immobilized pH gradients:
recent analytical and preparative developments, Anal Biochem 247,1–10 (1997)

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