Presentation on theme: "PRODUCT PURIFICATION PRECIPITATION"— Presentation transcript:
1 PRODUCT PURIFICATION PRECIPITATION ERT320 BIOSEPARATION ENGINEERINGMISS WAN KHAIRUNNISA BINTI WAN RAMLI
2 INTRODUCTIONInvolves the conversion of soluble solutes into insoluble solids, which can be separated subsequently from liquid by physical separation methodsRecovery of bulk proteinPreliminary stages of downstream processingInduced by: salt/ organic solvent addition, changing of ph value to alter the nature of the solutionADVANTAGES relatively inexpensive, simpler equipment, continuous process, formation of protein that is often stable in long-term storage
3 STRUCTURE & SIZE, CHARGE, SOLVENT PROTEIN SOLUBILITYPROTEINS PRECIPITATE AS A RESULT OF INSOLUBILITY.PROTEIN SOLUBILITY IS DETERMINED BY THE INTERACTIONS BETWEEN SURFACE REGIONS WITH SURROUNDING WATER MOLECULES.SEVERAL FACTORS AFFECTING THE PROTEIN SOLUBILITY ARE CONSIDERED IMPORTANT:STRUCTURE & SIZE, CHARGE, SOLVENT
4 FACTORS AFFECTING PROTEIN SOLUBILITY STRUCTURE & SIZEIn the native state, a protein in an aqueous environment assumes a structure thatMINIMIZES the contact of the hydrophobic amino acid residues with the water solvent moleculesMAXIMIZES the contact of the polar and charged residues with the water.CHARGEThe net charge of a protein has a direct bearing upon the protein’s solubility.The solubility of a protein increases as its net charge increases, a result of greater interaction with dipolar water molecules.SOLVENTThe solvent affects the solubility of proteins primarily through two parameters:HYDROPHOBICITY and IONIC STRENGTH
5 STRUCTURE & SIZEThe major forces to stabilize a protein in its native state hydrogen bonding, van derWaals interactions, Solvophobic interactions (driven forces of folding protein).In aqueous solution, these forces tend to push1) hydrophobic residues into the interior of the protein,2) the polar and charged residues on the protein’s surface.the surface of proteins usually contains a significant fraction of non polar atoms. The forces acting on a protein lead to the achievement of a minimum (small) Gibbs free energynative protein only marginally stable and can be destabilized by relatively small environmental changesWater molecules bind to the surface of the protein molecule because of association of charged and polar groups and immobilization by nonpolar groups.These hydration layers are thought to promote solubility of the protein by maintaining a distance between the surfaces of protein molecules (figure 1)
6 CHARGEThe solubility of protein increased as its net charge increases as a result of greater interaction with h2o molecules.A repulsive reaction between protein molecules of same charge further increases solubility.A simple way to vary the charge on a protein is by ph alterations.Isoelectric pH/ isoelectric point pH of the solution in which a protein has zero net chargeFigure 3The solubility of a protein is at its minimum, at the isoelectric point.Figure 3 The solubility (S) of insulin in 0.1 N NaCI as a function of pH. The charge Z is the average protonic charge per 12,000 g of insulin at the pH values indicated.At Charge, Z = 0, S = Smin
7 PARAMETERS OF SOLVENT AFFECTS HYDROPHOBICITYSINGLE-PHASE SOLUTIONS OF H2O & MONOHYDRIC ALCOHOLS DENATURATION AT ROOM T, CAN BE AVOIDED AT LOW TDENATURING EFFICIENCYC1-OH<C2-OH<C3-OH<C4-OHLONGER ALKYL CHAIN OF ALCOHOLS> EFFECTIVE BINDING TO POLAR GROUPS, WEAKENED INTRAPROTEIN HYDROPHOBIC INTERACTIONS DENATURATIONWhen the temperature is low, the monohydric alcohols compete for the water of hydration on the protein and cause the protein molecules to approach more closely, so that van der Waals interactions lead to aggregation.IONIC STRENGTHCAN HAVE BOTH EFFECTS SOLUBILIZING (SALTING IN) & PRECIPITATING (SALTING OUT) EFFETCSSALTING INTHE ADDITION OF SMALL QUANTITIES OF NEUTRAL SALTS TO A PROTEIN SOLUTION WHICH OFTEN INCREASES PROTEIN SOLUBILITYSALTING OUTADDITION OF SALTS ABOVE AN OPTIMAL LEVEL WHICH LEADS TO DESTABILIZATION OF PROTEINSIN SOLUTION & EVENTUALLY PROMOTES PROTEIN PRECIPITATION
8 Kirkwood’s theory for salting-in effectsthe interactions between ions & dipoles is by considering the solute size, solute shape, solute dipole moment, solvent dielectric constant, solution ionic strength, and temperature.where Sp = the solubility of the dipolar ion at ionic strength I,So = the solubility of the dipolar ion in the absence of salt,Ki = the salting-in constant, Ks = the salting-out constant.Ionic strength is defined bywhere ci, = Molar concentration of any ion in salt and zi = Ion’s charge.The salting-in and salting-out constants can be related to other variables as follows:where ε = the dielectric constant of the solvent, T = temperature,Ve = the excluded volume of the dipolar ion, u = dipole moment (C cm)E3E4E6E5
9 KIRKWOOD’S MODEL PROVES PROTEINS WITH HIGHER DIPOLE MOMENT HAVE GREATER SALTING-IN EFFECTThe salting-in term increases more than the salting-out term as the dielectric constant decreases AS A RESULT OF THE DECREASED SOLVENT POLARITYthe salting-in effect tends to predominate in relatively nonpolar solvents, while the salting-out effect is more dominant in aqueous solvents.At high ionic strength, the salting-out effect becomes predominant and can be described empirically by the Cohn equationWhere S= protein solubility,Ks’=salting-out constant characteristic of the specific protein and salt that is independent of temperature and pH above the isoelectric point.β = The hypothetical solubility of the protein at zero ionic strength, depends only on temperature and pH for a given protein and is a minimum at the isoelectric pointE7
10 which is also identical in form to the Cohn equation, with The Kirkwood equation for the solubility of dipolar ions [E3] can be arranged to give:which is also identical in form to the Cohn equation, withFigure 4From zero ionic strength, the solubility of the protein increases to a maximum as salt is added (salting in) and then continuously decreases as even more salt is added (salting out).E8E9E10Salting inSalting outFigure 4 The effect of (NH4)2SO4 and Na2SO4 on the solubility of hemoglobin: S0 is the solubility in pure water, S is solubility in salt solution
11 SALTING OUT OF A PROTEIN WITH AMMONIUM SULFATE EXAMPLE 1SALTING OUT OF A PROTEIN WITH AMMONIUM SULFATEData were obtained on the precipitation of a protein by theaddition of ammonium sulfate. The initial concentration of theprotein was 15 g/liter. At ammonium sulfate concentrations of 0.5and 1.0 M, the concentrations of the protein remaining in themother liquor at equilibrium were 13.5 and 5 g/liter, respectively.From this information, estimate the ammonium sulfateconcentration to give 95% recovery of the protein as precipitate.
12 We can use the Cohn equation [E3], to solve this problem if we can determine the constants in the equation. Since ionic strength is directlyproportional to concentration, c for a given salt [E4], we can rewritethe Cohn equation asSubstituting the experimental data into this equation givesSolving these equations for the constants yieldsFor 95% recovery, the protein solubility in the mother liquor at equilibriumis 5% of the initial protein concentration. At this solubility, from the Cohnequation
13 PRECIPITATION FORMATION PHENOMENA MECHANICAL STRENGTHLOW mechanical strength problem with excessive attrition when the dry particles are moved & considered as gel formation, which leads to major problems in filtration and centrifugationIMPORTANT CHARACTERISTICSPARTICLE SIZE DISTRIBUTIONProtein precipitates that consist largely of particle sizes with small particles (~1µm) can be difficult to filter or centrifugeDENSITYLOW particle densities filtration or centrifugation problems and can give excessive bulk volumes of the final dried precipitateINITIAL MIXINGNUCLEATIONGROWTH GOVERNED BY DIFFUSIONGROWTH GOVENRNED BY FLUID MOTIONAGINGParticle breakage during mixing limits the final size of the precipitate particlesPRECIPITATE FORMATION STEPSparticles are mixed until reaching a stable size
15 INITIAL MIXINGINITIAL MIXINGThe mixing required to achieve homogenity after the addition of a component to cause precipitationCRUCIAL to bring precipitant and product molecules into collision as soon as possibleMixing between randomly dispersed eddies is instantaneous & the mixing within eddies is diffusion limitedMean length of eddies, also known as the “Kolmogoroff length”, lewhere ρ = the liquid density, ν = the liquid kinematic viscosity,P/V = the agitator power input per unit volume of liquidE11
16 For spherical eddies of diameter le, this becomes It is necessary to mix until all molecules have diffused across all eddies. This time can be estimated from the Einstein diffusion relationshipwhere δ is the diffusion distance and D is the diffusion coefficient for the molecule being mixed.For spherical eddies of diameter le, this becomesThus, precipitation is initiated in a well-stirred tank for a period of time determined on the basis of isotropic turbulence.ISOTROPIC TURBULENCE Turbulence in which the products and squares of the velocity components and their derivatives are independent of direction or invariant with respect to rotation and reflection of the coordinate axes in a coordinate system moving with the mean motion of the fluid.E12E13
17 NUCLEATION NUCLEATION Particles Generation of ultramicroscopic size For the particles of a given solute to form, the solution must be SUPERSATURATED of solute component, with the intention that the concentration of the solute in solution > the normal equilibrium solubility of the soluteDEGREE OF SUPERSATURATIONThe difference between the actual concentration in solution and the equilibrium solubilityNEGATIVE EFFECTS of high supersaturation The precipitate tends to be in the form of a colloid, a gel, or a highly solvated precipitateTo obtain precipitate particles having desirable characteristics, the supersaturation should be kept relatively low
18 Figure 5: Nucleation rate as a function of degree of supersaturation Figure 5: Nucleation rate as a function of degree of supersaturation. The normal equilibrium solubility is at A and the supersaturation limit is at BFIGURE 5 RATE OF NUCLEATION increases exponentially up to the maximum level of supersaturation/ supersaturation limit.The rate of nucleation increases to a very high value at the supersaturation limit.
19 GROWTH GOVERNED BY DIFFUSION After NUCLEATION, precipitate growth is limited by diffusion until the particles grow to a limiting particle size defined by the fluid motion.When dissolved solute diffuses to the particles, the initial rate of decrease of particle number concentration (N) can be described by a second-order rate equation (Smoluchowski):For convenience, N0 is taken as the initial number concentration of dissolved solute moleculesThe constant KA is determined by diffusivity D and diameter Lmol, of the molecules that are adding to the particles as follows:E16E14E15
20 EQ. (16) can be rewritten as with M as the MW of particles at time t and M0 as the MW of the soluteso thatThis equation has been verified experimentally bymeasuring the MW of precipitating α–casein (FIGURE 6)The Stokes-Einstein equation Estimate the diameter of globular proteins, as spheres:where k is the Boltzman constant, T is the absolute temperature and μ is the liquid viscosityE18E19E20E17
21 Calculation of Concentration of Nuclei in a Protein Precipitation EXAMPLE 2Calculation of Concentration of Nuclei in a Protein PrecipitationWe wish to precipitate the protein α2 –macroglobulin contained in 100 liters of aqueous solution at 20°C in a tank at a concentration of 0.2 g/liter. α2 –Macroglobulin is a globular protein with a molecular weight of 820,000 and a diffusion coefficient of 2.41 x 10-7 cm2/s at 20°C. The precipitate particles have a density of 1.3 g/cm3. The solution is stirred with a 75 W (0.1 hp) motor. Calculate the concentration of nuclei at the end of the “initial mixing” period.
22 During the initial stirring, diffusion-limited molecular collisions occur, and molecules must travel by diffusion across an eddy in order to meet.The time required for this is determined by combining the Einstein diffusion eq. for spherical eddies [E13] and the eq. for the Kolmogoroff eddy length le or the average diameter of an unstirred zone [E11].Assume the properties of water since the solution is dilute, so thatρ = l g/cm3 and v = kinematic viscosity = 0.01 cm2/s.First, convert power per unit volume to appropriate units:Therefore,Since diffusion is limiting during this 6s period, the number concentration of nuclei N can be determined from the integrated form of the second-order rate equation for the growth of particles limited by diffusion [E16]:
23 where KA =8πDLmol.Since globular proteins are approximately spherical, we can estimate the diameter Lmol of the molecule from the Stokes-Einstein equation [E17]:This value of Lmol allows us to calculate KA:N0 is the number concentration of protein molecules that are dissolved before starting precipitation, so that
24 Substituting into E16 for N, we obtain after the initial 6 s mixing period In this calculation we note that the term involving N0 is very small, and thus the initial number of molecules is not important to the calculation.
25 GROWTH GOVERNED BY FLUID MOTION The growth of particles is governed by fluid motion after the particles have reached a critical size, typically 1μm in diameterIn this growth regime, particles tend to grow by colliding and then sticking together FLOCCULATION processFlocculation is enhanced when electrostatic repulsion between particles is reduced in comparison to the attractive van der Waals ForceCan be accomplished by raising the ionic strength and lowering the temperature, to reduce the thickness of the electrical double layer or Debye length, around particles.
26 The initial rate of decrease of particle number concentration (N) due to collisions can be described by a second-order rate equation:α = the collision effectiveness factor (fraction of collisions that result in permanent aggregates)L = the diameter of the particles and γ = the shear rate (velocity gradient)Assuming that the volume fraction of the particles (ф = πL3N/6) is constant during particle growth governed by fluid motion, EQ (21) becomes;E21E22
27 Integrating EQ. (22) yields Where N0 = The particle number concentration at the time [t=0] in at which particle growth starts to be governed by fluid motionFor turbulent flow, the average shear rate can be estimated by the following equation developed by Camp and Stein:where P/V = Power dissipated per unit volume and ρ and ν are the density and kinematic viscosity of the liquid, respectivelyE23E24
28 PRECIPITATE BREAKAGEWhen precipitate particles grow large enough by colliding and sticking together, they become susceptible to breakage during collisionsThe rate of precipitate breakage depend on the shear rate and particle concentrationA model that has successfully described the breakup of protein precipitates: The displacement model, which depicts the rate of aggregate size change as a function of displacement from an equilibrium aggregate diameter, Le:where the rate constant k would be expected to depend on the volume fraction of particles ф and the shear rate γE25
29 The equilibrium diameter Le depend on the shear rate For soy protein precipitate in laminar Couette shear2000s-1≤ γ ≤ 80,000s-1And for casein precipitated by salting out in continuous stirred tank reactor12s-1≤ ≤ 154s-1= the equilibrium particle size at the volume mean of theparticle size distributionE26E27
30 PRECIPITATE AGINGFIGURE 7Protein precipitate particles reach a stable size after a certain length of time in a shear field AGING time
31 Camp number The strength of protein particles correlated with the product of the mean shear rate and aging time.FIGURE 8 soy protein particles: the mean particle size becomes approximately constant after reaching a Camp number of 105Aging of precipitates helps the particles withstand processing in pumps and centrifuge feed zones without further size reduction
32 METHODS OF PRECIPITATION ADDITION OF ORGANIC SOLVENTSISOELECTRIC PRECIPITATIONMETHODS OF PROTEIN PRECIPITATIONADDITION OF SALTADDITION OF NONIONIC POLYMERS
33 ISOELECTRIC PRECIPITATION BASIS OF SEPARATION the solubility of a given protein is generally at a minimum at the isoelectric point (pI) of the protein.A convenient method when fractionating a protein mixture.The pH should be adjusted above the highest pI or below the lowest pI of all the proteins present.The pH is then changed to the nearest pI where precipitate is allowed to form and is then removed.ADVANTAGES when acids are added to cause precipitation: mineral acids are CHEAP, and several acids (e.g., phosphoric, hydrochloric, sulfuric) are acceptable in protein food products.NOT for all proteins Gelatin, which is a very hydrophilic protein, does not precipitate at its isoelectric point in solvents with low ionic strength
34 ADDITION OF ORGANIC SOLVENT Several organic solvents have been used to precipitate proteins, including alcohols, acetone, and ether.Alcohols - the most widely used in industry.Cohn Process One of the most important processes utilizing alcohol to precipitate protein to purify therapeutic proteins from human plasma.Uses ethanol at temperatures below 0°C to minimize denaturation by the organic solvent.Manipulated variables: pH, ionic strength, and ethanol concentration.Ionic strength is kept low, which leads to a salting-in effectThe salting-in effect is enhanced when ethanol is added.
35 Figure 9 Cohn’s method (1946) for blood protein fractionation: Г/2 = ionic strength. Sem. II, 05/06
36 ADDITION OF SALTSALTING OUT Salt is dissolved in the solution containing the proteins. The protein solubility decreases as the salt ionic strength rises (Cohn equation)IMPORTANT CONSIDERATION in salting out The type of salt that is used.Salts with multiply charged anions Sulfate, phosphate, and citrateCation monovalent ionsFollowing the Hofmeister or lyotropic series, the salting-out ability of the common multiply charged anionscitrate2- > phosphate3- > sulfate2- ;for the common monovalent cations the order isNH4+ > K+ > Na+DESIRABLE SALT for precipitating proteins ammonium sulfate.
37 ADVANTAGES OF AMMONIUM SULFATE Its solubility is very high (approximately 4 M in pure water) and varies very little in the range of 00 to 300C.The density of its saturated solution is gcm-3 , enough below the density of protein aggregates (approximately 1.29 gcm-3 ) to allow centrifugation.Protein precipitates are often very stable for years in 2 to 3 M saltProteolysis and bacterial action are prevented in concentrated ammonium sulfate solutions.The only DISADVANTAGES: Cannot be used above pH 8 because of the buffering action of ammonia.Sodium citrate is very soluble and is a good alternative to ammonium sulfate when the precipitation must be performed above pH 8
38 ADDITION OF NONIONIC POLYMERS Several nonionic polymers have been used to precipitate proteins, including dextran, poly(vinyl pyrrolidone) (PVP), poly(propylene glycol), and poly(ethylene glycol) (PEG)Of these polymers, by far the most extensively studied is PEG.POLYETHELYNE GLYCOL [PGE]:Solutions of PEG up to 20% w/v can be used without viscosity becoming a problem.PEG’s with molecular weights above found to be the most effectiveProtein destabilization in PEG solutions does not occur until the temperature is significantly higher than room temperature (>40 0C)
39 DESIGN OF PRECIPITATION SYSTEM The safest procedure is to base on the design on a lab or pilot plant system that has given acceptable resultsImportant consideration in obtaining the best possible plant design:The type of precipitation reactor,Processing conditions (flow rates, concentration etc.)Assumption used to scale up to the plant scaleBATCHPRECIPITATION REACTORCONTINUOUS-STIRRED TANKTUBULAR
40 BATCH REACTORThe simplest of the three types – tried first at small scaleSlowly adding the precipitating agent to a protein solution that is being mixedAddition of the precipitating agent continues the desired level of supersaturation is reached with respect to the protein being precipitatedNucleation begins, and precipitation proceeds through the steps of particle growth and aggregationMixing continues until the precipitation is complete - generally turbulentProtein particles precipitated tend to be more compact and regular in shape than those precipitated in a tubular reactor, apparently because of the different shear profiles existing in the two reactors and the length of time the particles are exposed to this shearThe shear field in a tubular reactor – essentially homogeneous; by contrast in the batch reactor the precipitate particles are exposed to a very wide range of shears and to much longer times of exposure than in the tubular reactor, resulting in improved precipitate mechanical stability
41 TUBULAR REACTORPrecipitation takes place in volume elements that approach plug flow as they move through the tubeThe distance-particle size distribution history of the particles in a volume element moving through a tubular reactor is comparable to the time-particle size distribution history of a stationary volume element in a batch reactorThe feed protein solution and the precipitating agent are contacted in a zone of efficient mixing at the reactor inletThe flow pattern in the reactor can be turbulent, a property that can be promoted by wire meshes at intervals along the reactorAdvantages: short fluid residence times, an absence of moving mechanical parts, uniformity of flow conditions throughout the reactor, a simple and inexpensive design and a relatively small holdup of fluidFor particles that grow relatively slow, the length of the tubular reactor can be excessive
42 CSTRFresh protein feed is contacted with a mixed slurry containing precipitate aggregatesThe mixing conditions in a CSTR are similar to those in a batch rectorUpon entering the CSTR, fresh protein feed nucleates, the nucleate particles grow by diffusion and the submicrometer-sized “primary” particles collide with and adhere to growing aggregatesThe degree of supersaturation can be more easily controlled than in the batch or tubular reactor which means tha not prone to form precipitates with undesirable properties
43 PROCESSING CONDITIONTURBULENT FLOW flow must be high enough to avoid inadequate mixing and high supersaturation but low enough to avoid excessive particle breakage leading to particles that are smaller than desirableFor both the batch and tubular reactors, the flow regime can be changed from turbulent to laminar during the particle growth phase to avoid excessive particle breakageThe rate of addition of precipitant should be kept low enough to avoid high supersaturations that lead to colloidal, highly solvated precipitatesThe added concentrations of the precipitant with lower concentrations leading to lower supersaturationKEY PARAMETER for scaleup of precipitation is MIXING: Recommended approach Consider using geometric similarity and constant power per unit volume (P/V).
44 Volume Scaleup Factor (Tip speed)large/ (Tip speed)small 10 100 1000 If the precipitate is susceptible to shear breakage - the assumption of constant P/V for scale up may not be satisfactoryThe impeller tip speed, which determines the max. shear rate, rises when P/V is held constant upon scale up of the reactor volume, as seen in table 1:These results assume turbulent flow, where the power number is constant, so thatVolume Scaleup Factor(Tip speed)large/(Tip speed)small10100100010,0001.31.72.22.8Table 1: Scale-up of Turbulent Agitation, assuming constant P/VE28