Multidisciplinary Research Journal

pdf_iconEvaluation of Hen Egg White as a Model of Cytoplasmic Pumping Mechanisms

Cameron, I1,*; Fullerton, G2

1 University of Texas Health Science Center, Department of Cellular and Structural Biology, San Antonio, Texas 78229
2 University of Colorado Denver, Department of Radiology, Aurora, Colorado 80045
* Correspondence: Tel.: (210) 567-3817; E-mail: cameron@uthscsa.edu

Key Words: Hen egg white, albumen structure, gel-sol, proton NMR, polarization microscopy, water

Received 16 May 2010; revised 29 July; accepted 14 August. Published 27 August 2010; available online 27 August 2010

doi: 10.14294/WATER.2010.7


Summary

The most widely taught explanation of non-homogeneous distributions of cellu­lar metabolites and ions relies on molecu­lar pumps residing in cellular membranes. This study uses fresh hen egg white to dem­onstrate the capacity of hen egg white to manipulate rheological properties that cor­relate with co-solute distribution changes previously attributed solely to active (en­ergy consuming) membrane pumps. When egg white was placed on a sieve a thin sol albumen fraction flowed through the sieve and a thick gel albumen fraction was ob­tained from the sieve surface. The thick gel, but not the thin albumen sol fraction, ex­cludes a low molecular weight dye, methy­lene blue. The two fractions were examined by polarization microscopy and their wa­ter proton NMR T1 and T2 relaxation times were measured. Thick gel was composed of elongated birefringent domains rich in mi­croscopic particles. The thin albumen sol had no such domains, fewer particles and does not exclude dye. Gel agitation frag­ments the gel domains and frees their par­ticles concomitant with conversion to a sol, and with loss of the dye exclusion property. The gel to sol conversion caused shortening of the NMR T1 relaxation time consistent with increased freedom of motion of water attached to proteins previously entrapped in the less mobile gel. Thus water in the gel differs from bulk water in both motional and dye exclusion properties.


Article Outline

Introduction

Hen egg white provides a readily available source of biological gel and sol state materi­al to test the capacity of protein interactions to change rheological properties that corre­late with non-homogenous concentration of particles, proteins, co-solute metabolites and ions. Hen egg white is easily separated into gel and sol fractions by a sieve method. The fraction remaining on the sieve sur­face is referred to as the thick-gel fraction (Cameron 2010) while the fraction passing through is the thin sol fraction. Cameron’s report also showed the thick gel fraction has a number of physical properties, similar to those ascribed to the cytoplasm of cells (Fels et al. 2009).

What makes egg white form a thick hy­drogel? According to the USDA database fresh whole hen egg white consists of: 88% water, 11% protein, 0.16% lipid and 0.7% carbohydrates. The protein fractions are: ovoalbumen 54%, ovotransferrin 12%, ovo­mucoid 11%, globulins 9%, lysozyme 3.4%, ovomucin 4.2%, others 7%. Of these pro­tein types ovomucin is the type thought responsible for the gel-like properties of thick egg albumen (Brooks and Hale 1959, 1961, Rabouille et al. 1990, Hiidenhovi et al. 2002, 2007, Laghi et al. 2005, Robinson and Monsey 1971, 1972a,b). Crude ovomu­cin is composed of at least two subunits: α-ovomucin and β-ovomucin both of which are high molecular weight glycoproteins. The carbohydrate content is 22.9% in thick egg white (Hiidenhovi et al. 2002). Analy­sis of the ovomucin molecule, purified from egg white for electron microscopic observa­tion reveals ovomucin to be a linear mol­ecule that can form a linear polymer chain up to 2000 or more nm long. The ends of such long polymers are apparently linked via disulphide bridges (Rabouille et al. 1990). This super long chain of polymers is proposed by Rabouille 1990 to be organized as a fibrillar network or hydrogel with tran­sient weak linkages that could confer non-Newtonian and viscoelastic properties to the thick egg white gel.

Assessment of differences in the amounts of ovomucin in thick vs. thin albumen re­veals thick to have 1.6 times more total ovo­mucin and 4 times more β-ovomucin than thin (Rabouille et al. 1989). Rabouille et al. also observed that sonication caused con­comitant loss of its gel fibrillar nature and of non-Newtonian properties of both thick egg-white and synthetic ovomucin gels. It seems likely that other protein types may interact with the fibrillar network of ovomu­cin to produce the thick egg-white hydrogel. One note of caution to the above description concerns the Rabouille et al. (1990) ovomu­cin purification procedure. The concern is with the first step in purification of ovomu­cin by a 1 to 10 fold dilution with distilled water. Such a dilution can and does cause unfolding and linearization of some folded (globular) proteins. Thus one can question if the linear molecular structure observed by electron microscopy is or is not the same linear structure thought responsible for the thick albumen hydrogel? It is clear, how­ever, that the fundamental mechanism in­volves direct protein-protein associations that cause non-homogenous protein distri­butions that could cause redistributions of salts, metabolites and a small test molecule such as methylene blue.

Among the properties of thick egg white are the capacities to exclude vital dyes, respond osmotically and transform from a gel to a non-dye excluding sol under the influence of physical stress and to transform back to a dye excluding gel upon rest (Cameron 2010). Based on the above observations one can logically ask what changes occur when the gel is transformed to a sol? The pres­ent report examines two aspects of the hen egg white gel and sol state. One aspect deals with the morphological structure using po­larization microscopy and the second as­pect deals with water motion using proton NMR relaxation time measures.

Methods and Materials

Specimen preparation for microsco­py

The hen eggs used in this study were pur­chased from a grocery store and were re­ported to have been stored at 40o C for between 2 to 4 days after being laid. The specimen preparation procedure was to col­lect thick and thin albumen fraction using the sieve filter separation method as illus­trated in Figure 1 (Cameron 2010). When 2 ml of 0.1% methylene blue dye was added to the whole egg white from an egg, prior to placement on the sieve, the thin sol albu­men fraction was blue while the gel fraction remained almost free of blue color (Figure 2).


Fig1

Figure 1: Photograph of the under surfaces of the egg white sieve taken during separation of thin sol and thick egg white albumen fractions. Thin drops through the sieve and thick is removed from the up­per sieve surface.


fig2

Figure 2: Photograph of thin and thick egg white fractions. Methylene blue dye was added to whole egg white prior to being added to the sieve surface. The thin fraction that passed through the sieve is blue (right tube) and the thick fraction from the sieve surface is almost free of blue color (left tube).

For microscopy the thick albumen fraction on the surface of the filter was gently placed into a 50 ml beaker. The beaker contents were tipped to the beaker pouring groove so that a blob of thick albumen gel hung over the lip of the groove. A microscopic slide was positioned just under the protruding albumen blob and scissors were used to cut off a small drop of the blob that touched the horizontal surface of the microscope slide. A large number one coverslip was placed gently on top of the small drop and capillary action caused the gel drop to flatten and to form a thin layer of albumen between the microscopic slide and the cover slip.

Proton NMR relaxation measure­ment procedure

Pulsed proton MR relaxation times were measured with a Praxis Model II, with a permanent 0.25-T magnet, sample coil, and radiofrequency (RF) pulser tuned to 10.7 MHz. This machine is interfaced to a mi­crocomputer for fast data acquisition and has built-in analytical software. The T1 or spin-lattice relaxation time was measured using the saturation recovery sequence (SR) 900 τ 900. The resultant free induc­tion decay (FID) curves included 30 mea­surements with a sequence of increasing interpulse delay times. All 30 were fit by means of single-component exponential least-squares regression analysis. The T2 or spin-spin relaxation time was measured by using the Hahn spin-echo method, 900 τ 1800 with 30 increasing values of τ.

Results

Visualization of the thick and thin hen egg white albumen gel and sol state structure by polarization microscopy

Viewing a sample of the thick albumen frac­tion slide preparation under bright field light conditions using both a 3.2 and 10 X objec­tive lens and a 10 X ocular lens, revealed no structure. However, reviewing the sample specimen field between cross polarizing fil­tered light at 32 X and 100 X magnification revealed large weakly birefringent elon­gated domains containing bright birefrin­gent particles. The photograph in Figure 3 illustrates the birefringent domains. Figure 3 shows three parallel elongated domains (arrows) with small particles and areas with fewer particles between the domains. The arrows in Figure 3 are 20 μm in length. Three large bright objects are also seen in one of the large domains.

fig3

Figure 3: Polarization microscopic image of the thick albumen gel fraction (arrows are 20μm in length). Three parallel elongated weakly birefingent domains with numerous particles are seen. The intervening non-domain areas have fewer particles than the domain.

The size and numbers of particles was de­termined using an ocular grid calibrated with a stage micrometer. The depth of the sample was determined using the depth of the focus scale on the focus level dial. The focus level scale was calibrated using his­tological slide sections of known thickness. To determine thickness of the specimen the distance between where particles were in focus at the highest and the lowest focal level was measured. The number of small particles in a 1 mm2 area was scored in 3 eggs. As illustrated in the photograph (Figure 3) the small particles varied in diameter from about 0.5 to about 5 micrometer. The particles in a domain were not observed to move about or to demonstrate Brownian motion. The number of particles per mm2 times the sample depth was used to deter­mine the number of particles per mm3. The numerical values are reported in Table 1.

table1

As illustrated in Figure 3, the number of par­ticles is greater within a domain volume vs. the total field volume. The scoring of par­ticles per domain field gave a mean value of 40,457 per mm3. This value is about 2.4 times higher than the mean number of particles per unit volume of the total thick albumen fraction (Table 1).

When a fresh thick albumen fraction gel was shaken in a closed NMR vial, the thick albumen became more fluid or sol-like and the shaking allowed this fraction to sponta­neously pass through the pores of the sepa­ration sieve. When fresh thick albumen gel was vigorously shaken in a closed NMR vial prior to microscopic slide preparation, the elongated domains were fragmented into domains of even smaller size.

The preparation of the thick albumen frac­tion for microscopic observation required application of a cover slip, thus the albumen was subjected to a capillary force prior to microscopic observation. Given this fact, it was impossible to discern if the thick albu­men might have had an even larger domain structure prior to the specimen preparation procedure. It is observed, however, that do­main structures exist in elongated band ar­rangements (Figure 3).

In summary of the structural observations it was found that the fresh unagitated thick albumen gel has a series of elongated weak­ly birefringent bands of microscopic pro­tein fibers, that fragment into domains of smaller size under the influence of mechan­ical agitation. The fragmentation of the do­main structure is associated with a loss of the gel state as judged by flow through the sieve filter. Polarization microscopy of the fresh thin albumen fractions reveals fewer birefringent particles than the thick albu­men (Table 1). The microscopic particles in the thin fractions are randomly dispersed with significantly lower concentration than in thick albumen (Table 1). No microscop­ic domain structures were observed in the thin albumen fraction. The concentration of particles in thin albumen ranged from 8900 to 13,100 per mm3 or about half the mean concentration in thick albumen and a quarter the concentration in the birefrin­gent domains of the thick albumen gel (Ta­ble 1).

Proton NMR T1 and T2 relaxation times

The results of longitudinal (T1) and trans­verse (T2) relaxation time study of thick and thin hen egg white albumen fractions are summarized in Table 2. A single T2 re­laxation time gave a good fits to a monoex­ponential decay curve. The shaking of thick albumen gel caused transition from a gel state to a sol state as shown by flow through the sieve filter. This gel to sol phase tran­sition did cause a marginally significant (P = 0.0663) shortening in the T1 relaxation time but did not cause a significant change in the T2 relaxation time.

table2

Discussion

Structural and physical state of fresh thick and thin hen albumen fractions

Brooks and Hale (1959) reported that thick hen egg white, as seen with the naked eye, “to consist of a transparent phase separated by a series of translucent bands with their centres about 1 mm apart. Microscopical­ly a band is seen to be a stratum of closely packed parallel fibres or sheets whereas few fibers can be seen between the bands.” They go on to assume that the thick albumen is held together by microscopic fibres and the translucent phase is a liquid resembling the thin albumen fraction.

When fresh thick gel was subjected to vig­orous agitation the elongated domains ob­served by polarization microscopy were no longer observed but a few smaller domain fragments were still seen. Birefringent par­ticles were more uniformly distributed in the field of view of the agitated thick albu­men but the mean concentration was not significantly different between the sol and gel states (Table 1). It was also observed that the particles in the elongated gel do­mains were immobilzed but were free to move in the sol. Fragmentation of large domains into smaller domains by vigorous agitation is associated with transformation to a sol state as determined by flow through the sieve filter.

The elastic and rigidity properties of both the translucent and transparent phase of the thick albumen have been studied (Brooks and Hale 1959). Elasticity was measured us­ing displacement of minute nickel spheres inserted into the gel when subjected to forces applied with a magnetic field. Both gel phases demonstrated elastic behavior and rigidity. Rigidity was much greater in translucent than in transparent albumen. Also the nickel spheres in the translucent gel band demonstrated greater resistance when pulled perpendicular vs. parallel to the filament long axis. No directional effect was observed in thin albumen. Thus it was concluded that the thick albumen gel is not a simple homogenous entanglement net­work.

Based on the above descriptions thick albu­men is best described as a two phase gel with elastic properties. One phase consists of linear domains enriched with relatively im­mobile bi-refringent particles surrounded intervening non-domain phase with fewer particles. The present study demonstrates that the unperturbed gel (domain and non-domain) excludes dye. The thin albumen sol fraction which flows through the filter sieve, includes dye and therefore differs from both the domain and no-domain phases of the thick albumen gel (Figure 2). The thick gel has some dye which could be dissolved in a thin fraction held between the fibers but with the same concentration as in the thin fraction. This possibility seems unlikely as the thick gel fraction was 27% domain and 73% non-domain of the total volume as de­termined by morphometric measurements. If the non-domain area of thick albumen gel did not exclude dye the total thick albu­men fraction would be expected to be much more blue than was observed. Summariz­ing the physical characteristics of thick al­bumen, prior studies revealed that the thick albumen gel has elastic properties (Brooks and Hale 1959) and demonstrates thixotro­py (Cameron 2010). Cameron also demon­strated that modest shear force applied to thick gel was enough to cause loss of the dye exclusion property.

Impact of globular protein docking (polymerization) on the proton spin-lattice (T1) relaxation rate

Some background on this subject helps explain the T1 relaxation rate differences observed between thick and thin albumen and between the gel and sol state of thick albumen. Polymerization of globular pro­teins has an effect on T1 relaxation times (Fullerton et al. 1987, Fullerton and Cam­eron 1998). The effect can either cause an increase or decrease in T1. The basis for the effect is that proteins that dock with one an­other during polymerization displace a por­tion of the hydration sheath. This causes an increase in the bulk-water relative to the bound-water fraction and structured-water fractions, and will cause a decrease in the relaxation rate. The group of docked molecules is now however equivalent in mass to a much larger globular protein.This polymer rotates and translates much more slowly; it thus contributes to a more rapid T1 relaxation. These two influences are simultaneous and opposite to one an other in influence on T1. In vitro studies on actin and tubulin, two globulin proteins frequently participating in polymeriza­tion-depolymerization reactions in vivo, showed that changes in macromolecular motion are most important, the relaxation rate increases with the polymerization re­action. A study of lysozyme indicated that the displacement hydration water can also be of critical importatnce. Both changes are important in vivo and in vitro (Bray 1993, Lynch 1983, Fullerton et al. 1987, Fullerton and Cameron 1998).

This conclusion is confirmed by a study of the influence of NaCl on the spin-lattice relaxation of a 2 g lysozyme/5 ml of 0.1 M NaCl solution during a salting-out relax­ation. As shown in Figure 4 the initial addition of salt shortened the relaxation time of the solution due to increased effective molecu­lar weight caused by molecular docking. Over a period of several days, the relaxation time recovered and then exceeded the origi­nal relaxation time. This was due to con­tinued molecular docking, which formed protein crystals that further slowed the mo­tion of the lysozyme to the point where it was no longer effective in acting as a relax­ation sink for the solution. This was simul­taneous with an increase in the bulk-water fraction, due to additional dis­placed hydration water. These studies con­firm that changes in the macromolecules from monomer, to polymer, to crystal can have profound effects on the T1 relaxation time.

fig4

Figure 4: (A) The lysozyme concentration and (B) the sodium ion concentration in the supernatant in days after mixing together 2 g of lysozyme in 5 ml of water (open circles) or in 5 ml of 0.1 M NaCl (closed circles), (C) T1 measurements were made with all reactants (including crystals) in the vessel. Figure 4 is reproduced from Fullerton and Cameron (1988) with permission of Wiley VCH Publishers, Inc.

The above description of T1 relaxation time changes due to docking of globular proteins during agregation is consistent with ob­served measure of proton T1 relaxation time difference between thick and thin egg white albumen fractions and between the gel and sol state of thick albumen. The T1 relaxation time of fresh unperturbed thick albumen has significantly longer T1 relaxation time than thin albumen which has a significantly higher water content 9.6 g water/g DM (dry material) vs. 7.3 g/g DM for thick albumen (Cameron 2010). This difference in T1 relax­ation between thick and thin albumen is not therefore attributed to the water content but is attributed to extreme slowing of pro­teins in the domain structure of the thick albumen gel. Apparently the rotational and translocation motion of the proteins and particles in the gel is slowed to the extent that the proteins and particles form a rela­tively immobile domain structure which is no longer effective in acting as a relaxation sink for protons in solution. The fragmen­tation of the large domains of the gel state of thick albumen by agitation (shaking) might be expected to have a shorter T1 relaxation time due to increased mobility of proteins released by dissolution (depolymerization) of the large, relatively immobile domains. Data in Table 2 indicates that gel domain fragmentation does have a shorter mean T1 relaxation time as predicted.

Summary and Conclusion

We conclude that hen egg white structure is a useful model to study factors impacting heterogeneous distributions of particles, proteins, metabolites, salts and other small molecules in cellular structures of biology due the following observations:

1. The thick albumen gel, as visualized by the polarized light microscopy, is composed of elongated weakly birefringent domains enriched in strongly birefringent particles separated by areas with fewer particles causing heterogeneous distributions of these particles.

2. Thick gel agitation fragments the large domain structure of the gel and is associ­ated with a more homogenous distribution and increased mobility of these birefringent particles.

3. Flow of thick albumen gel through the sieve following mechanical agitation (tran­sition to sol) shows depolymerization of protein filaments to create smaller domains due to domain fragmentation that corre­lates with redistribution of proteins mol­ecules and particles.

4. Shorter proton NMR T1 relaxation times following mechanical agitation of the thick albumen gel imply de-polymerization of protein structures induced by mechanical forces implying very weak binding mecha­nisms.

5. Exclusion of blue dye by thick albumen gel and loss of dye exclusion following me­chanical agitation confirms that cosolute concentrations are correlated with protein aggregation caused by polymerization.

6. In a prior report the gel to sol transfor­mation of thick albumen was demonstrated to be reversible following a long (4 day) pe­riod of rest (Cameron 2010) which demon­strates self association similar to the pro­cess of tendon formation by spontaneous association of collagen molecules.

7. The gel to sol and back to gel phase trans­formation of hen thick albumen appears to be similar to the contraction induced non-Newtonian cytoplasmic flow characteristics observed in amoeba and Physarum (Tay­lor and Condeelis 1979, Kamiya 1981, Bray 1993).

8. The vital dye excluding property of the thick albumen hydrogel resembles that of the cytoplasmic “antidomain” structures of a giant algal cell (Shepherd 2006).

9. We conclude that the formation of hy­drogels by the spontaneous interaction of cellular protein causes heterogeneous distributions of particles, proteins and co­solutes and is likely a alternative mecha­nism to active membrane “pump” trans­port causing heterogeneous distributions of ions, cosolutes and other biologically active molecules in biology; these sponta­neous, cytoplasmic pump mechanisms can be studied using the hen egg white model demonstrated in this study.

Acknowledgements

Dr. Yongsook Lee’s help with NMR measure­ments is greatfully acknowledged as is the sec­retarial help of Cathy A. Bunnell.

References

Bray, D. Cell Movement. Garland, NY. 1993.

Brooks, J. and Hale, H.P. The mechanical properties of the thick white of the hen’s egg. Biochem. Biophys. Acta. 1959. 32: 237-250.

Brooks, J. and Hale, H.P. The mechanical properties of the thick white of the hen’s egg II: the relationship between rigidity and composition. Biochem. Biophys. Acta. 1961. 46: 289-301.

Cameron, I.L. Dye exclusion and other physical properties of hen egg white. Water. 2010. Volume 2. pp. 83-96.

Fels, J., Orlov, S.N., Grygorczyk, R. The hydrogel nature of mammalian cytoplasm contributes to osmosensing and extracellular pH sensing. Biophys. J. 2009. 96: 4276-4285.

Fullerton, G.D. and Cameron, I.L. Relaxation of biological tissues. Biomedical Magnetic Resonance Imaging. 1988. pp. 115-155. Eds. F.W. Webili, D. Shaw, J.B. Kneeland. Vch Publishers, Weinheim.

Fullerton, G.D., Finnie, M.F., Hunter, K.E. et al. The influence of macromolecular polymerization on proton NMR T1 relaxtion of water solutions. J. Magn. Reson. Imag. 1987. 77: 426-445.

Hiidenhovi, J. Chapter 9 ovomucin. Bioactive Egg Compounds. 2007. pp. 61-68.

Hiidenhovi, J., Makinen, I., Huopalahti, R., and Ryhanen, E.L. Comparison of different egg albumen fractions as sources of ovomucin. J. Agric. Food Chem. 2002. 50: 2840-2845.

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Laghi, L., Cremonine, M.A., Plaucci, G., Sykora, S., Wright, K., and Hills, B. A proton NMR relaxation study of hen egg quality. Magn Reson. Imag. 2005. 23: 501-510.

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Discussion with Reviewers

Mae Wan Ho1: Would you comment on hydrogel acting as an alternative to pump transport?

Ivan Cameron and Garry Fullerton: Clearly the hen thick albumen hydrogel excludes vital dye like methylene blue. Such vital dye exclusion, in cells, is usually attributed to a membrane. The thick albumen hydrogel excludes the same vital dye without a membrane pump. The hydrogel is proposed to exclude the vital dye due to the presence of non-dye-solvent water. Agitation of the albumen hydrogel converts the gel to sol state which causes loss of its dye excluding property but with time for rest the sol state can return to a dye excluding gel state. Thus the thick albumen hydrogel is an alternate to a cell membrane pump transport system.

Ho: Why do you suppose that the thick gel albumen consist of globular aggregates rather than extended protein aggregated, as suggested by Gilbert Ling?

Cameron and Fullerton: We have not made this supposition but will comment that Gil­bert Ling’s association induction process re­quires binding of a cardinal adsorbent, like ATP, to induce a transformation of a folded (introverted) globular protein state to the unfolded (extroverted) state. This trans­formation requires a cardinal adsorbent , like ATP, that allows multilayer of polarized water with solute excluding properties to form over the exposed hydrophilic surface. Loss of ATP, causes to transformation back to the folded non-solute excluding globular protein state. As far as is known, the solute (dye) excluding property of the thick albu­men gel does not depend on the presence of a “cardinal adsorbent” like ATP. All that may be needed is the presence a hydrophilic “water loving” gel surface to forms a solute excluding zone over the hydrophilic surface of a globular protein monomer or aggre­gate.

Ho: Is it possible that you may have more than one NMR relaxation time in the gel phase?

Cameron and Fullerton: Yes. For example water protons in a protein crystal can be­come so immobile that their water proton NMR relaxation rate is too fast to be detect­ed. This happens with water in the form of ice. However addition of cold liquid water to an ice cube gives only the water proton NMR relaxation rate/time of the cold liq­uid water. Both relaxation times occur but only one is measureable. The hen egg white gel is also proposed to have more than one NMR relaxation rate fraction but one frac­tion may relax too fast to be measured.

Ho: Are your results on dye exclusion relat­ed in a way to Pollack et al.’s exclusion zone next to hydrophilic gels?

Cameron and Fullerton: Yes as just alluded to in my response to the previous question. Pollack’s gel exclusion zone extends over the hydrophilic gel surface and grows with time, can be destroyed by physical pertur­bations, and can then re-grow with time. These are the same properties observed in the thick hen egg white gel.

Ho: Do you think transition between heavy and light water could be involved in liquid and gel phases of albumen?

Cameron and Fullerton: We assume by heavy and light water you refer to water den­sity. We have not made measures of water density in the hen albumen fractions how­ever we assume that water hydrogen bond­ed to a hydrophic surface is more densely packed than bulk water. Thus changes in protein folding and aggregation is expected to change the proportion of dense to less dense water between the gel and the sol phase of the thick albumen.

1 Visiting Professor of Biophysics, Catania Univer­sity, Sicily  BlueSquare

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