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Comparison of heat-induced aggregation of globular proteins Roy J.B.M. Delahaije, Peter A. Wierenga, Marco L.F. Giuseppin, and Harry Gruppen J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.5b00927 • Publication Date (Web): 12 May 2015 Downloaded from http://pubs.acs.org on May 17, 2015
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Journal of Agricultural and Food Chemistry
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Comparison of heat-induced aggregation of
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globular proteins
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Roy J.B.M. Delahaije,1 Peter A. Wierenga,1 Marco L.F. Giuseppin,2 and Harry Gruppen1,*
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1
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Wageningen, The Netherlands.
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2
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*Phone: +31 317 483211, E-mail:
[email protected] Laboratory of Food Chemistry, Wageningen University, Bornse Weilanden 9, 6708 WG,
AVEBE, Prins Hendrikplein 20, 9641 GK, Veendam, The Netherlands.
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Abstract
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Typically heat-induced aggregation of proteins is studied using a single protein under various
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conditions (e.g. temperature). Since different studies use different conditions and methods, a
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mechanistic relation between molecular properties and the aggregation behavior of proteins has
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not been identified. Therefore, this study investigates the kinetics of heat-induced aggregation
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and size/density of formed aggregates for three different proteins (ovalbumin, β-lactoglobulin
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and patatin) under various conditions (pH, ionic strength, concentration, temperature). The
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aggregation rate of β-lactoglobulin was slower (>10 times) than that of ovalbumin and patatin.
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Moreover, the conditions (pH, I, C) affected the aggregation kinetics of β-lactoglobulin more
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strongly than for ovalbumin and patatin. In contrast to the kinetics, for all proteins the aggregate
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size/density increased with decreasing electrostatic repulsion. By comparing these proteins under
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these conditions, it became clear that the aggregation behavior cannot easily be correlated to the
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molecular properties (e.g. charge, exposed hydrophobicity).
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Keywords
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β-lactoglobulin, ovalbumin, patatin, pH, ionic strength, concentration
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Introduction
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Heat-induced aggregation of globular proteins is generally described to be caused by the
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exposure of the internal hydrophobic amino acids due to unfolding1, 2. After unfolding and
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exposure of the hydrophobic amino acid residues, the (partially) unfolded protein either refolds
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or irreversibly aggregates with other (partially) unfolded proteins3, 4. The likelihood of refolding
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or aggregation depends on the balance between the increased hydrophobic attraction due to
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unfolding and the mainly electrostatic barrier for aggregation. Qualitatively, aggregation has
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been investigated extensively, mainly in terms of (1) aggregation kinetics5-7, (2) aggregate size8, 9
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and (3) aggregate structure10, 11. These studies typically investigate the effect of conditions (e.g.
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ionic strength) on the aggregation of a single protein (e.g. β-lactoglobulin). A detailed
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mechanistic comparison of the aggregation of different proteins, on the other hand, is still
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missing. Exactly such information may advance the understanding of the aggregation process of
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globular proteins, especially with respect to an overall description of the effect of parameters
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influencing the aggregation process. Hence, the aim of the present paper is to obtain a
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mechanistic understanding of the aggregation process and determine the effect of the properties
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of three proteins on the aggregation behavior under various conditions.
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The aggregation kinetics (i.e. order (n) and rate (k)) are generally derived from the decrease of
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the concentration of non-aggregated protein in time. Therefore, they provide information on the
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kinetics of protein unfolding and integration of non-aggregated proteins in aggregates, whereas it
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does not contain information on the size and structure of the formed aggregates.
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Aggregation order
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To determine the order of aggregation, two approaches have been described: (A) determination
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of the concentration dependence of the initial aggregation rate5-7 and (B) fitting the decrease of
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the concentration of non-aggregated protein in time with the specific reaction equations. These
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are equations 1 and 2 for 1st and nth order kinetics, respectively12.
Ct = C0 ⋅ e−kt for n = 1 Ct = (C0
1−n
(1) 1
+ (n − 1) ⋅ k ⋅ t ) 1−n for n ≠ 1
(2)
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in which Ct and C0 is the concentration of non-aggregated protein at time t and time 0 [g/L],
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respectively, n is the aggregation order [-] and k is the aggregation rate [(g/L)(1-n)/s].
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These discrepancies between the two approaches are assumed to originate from the fact that the
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aggregation process becomes more complex at longer heating times. This is due to simultaneous
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aggregation of non-aggregated protein with other non-aggregated protein, non-aggregated with
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aggregated proteins, and aggregated proteins with other aggregated proteins.
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For ovalbumin, the initial aggregation rate was shown to be concentration independent. Hence,
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the aggregation process has been described as a first-order reaction6. The aggregation rate of
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β-lactoglobulin, on the other hand, increases with increasing concentration, indicating
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concentration dependence. This resulted in an aggregation order of 1.55,
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aggregation orders of ovalbumin and β-lactoglobulin were shown to be independent of
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temperature6, 14, 15, pH8, 14 and ionic strength6, 16.
7, 13
. In addition, the
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Aggregation rate
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For proteins, the fraction unfolded protein and the diffusion rate of the protein molecules
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typically increase with increasing the temperature17, 18. The fraction unfolded proteins depends
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on the heating temperature relative to the denaturation temperature (Td). For a more fundamental
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comparison of the aggregation properties of different proteins, the proteins are studied at a fixed
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temperature relative to Td, rather than an absolute temperature. This results in a constant fraction
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unfolded protein. An increase of the diffusion rate results in an increased likelihood of the
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proteins to meet, collide and aggregate19, 20. The aggregation rate is also affected by electrostatic
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repulsion. However, a clear link is difficult to make, since the main factors (i.e. ionic strength
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and pH) can have several effects. Increasing the electrostatic repulsion increases both the intra-
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and intermolecular repulsion. The increased intramolecular repulsion decreases the denaturation
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temperature21,
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electrostatic barrier for aggregation23. While the first effect would (when heating at given
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temperature) result in an increase of the aggregation rate, the second would decrease the
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aggregation rate. In addition, a pH change does not only affect the electrostatic interactions, but
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also changes the reactivity of the disulfide bonds. While changes in the electrostatic repulsion
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(i.e. ionic strength and pH) do not affect the aggregation rate of ovalbumin6, 24, the aggregation
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rate of β-lactoglobulin increases with increasing ionic strength (< 0.1 M)16 and pH8,
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difference indicates that the electrostatic repulsion affects both proteins differently. The
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enhancing effect of ionic strength on the aggregation rate of β-lactoglobulin showed that the
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increase of the denaturation temperature was less important than the decrease of the electrostatic
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barrier for aggregation16. At the same time, a higher denaturation temperature and a decrease of
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the reactivity of the disulfide bonds were described to reduce the aggregation rate of
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β-lactoglobulin with decreasing pH14.
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. The increase of intermolecular repulsion, on the other hand, increases the
14
. This
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Aggregate formation
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To describe the aggregate formation, the size and structure of the aggregates are typically
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determined by scattering techniques (i.e. neutron, x-ray and light scattering). The structure of the
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aggregates is described by the fractal dimension. The fractal dimension was found to be
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independent of the heating temperature15, 25. As temperature only affects the aggregation kinetics
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and not the formation of aggregates, the effect of temperature has been described as a purely
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kinetic effect15. Electrostatic repulsion, however, strongly affects the aggregate formation (size
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as well as structure). A decrease of the electrostatic repulsion, due to an increase of the ionic
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strength and/or a shift of the pH towards the iso-electric point (pI), resulted in the formation of
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larger aggregates of ovalbumin26,
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scattering. At conditions with high electrostatic repulsion (i.e. low ionic strength and pH far from
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the pI), linear aggregates with a fractal dimension of 1.7 were formed by ovalbumin30 and
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β-lactoglobulin31, 32. When the electrostatic repulsion was decreased (i.e. by increasing the ionic
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strength from 10 to 100 mM), the β-lactoglobulin and ovalbumin aggregates become more
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branched. This was indicated by a fractal dimension of 2.015,
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TEM30. When the electrostatic repulsion is minimal (i.e. close to the pI), β-lactoglobulin forms
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even more compact aggregates with a fractal dimension of 3.036. The effect of electrostatic
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repulsion on the aggregate size and structure is explained by the electrostatic barrier for
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aggregation30, 32, 35. Furthermore, the size of β-lactoglobulin and BSA aggregates increased with
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increasing concentration9,
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result of an increase of the ionic strength due to the counter ions of the protein.
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Since β-lactoglobulin has most often been used to study the aggregation process, its aggregation
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behavior is quite well understood. However, little is known on how this knowledge can be
15, 33, 37
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, β-lactoglobulin8,
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, BSA9 and patatin29, based on light
30, 33-35
, and confirmed by cryo-
. This concentration dependence has been postulated to be a
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extrapolated to describe the aggregation of other proteins. To obtain a more mechanistic
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understanding of the effect of protein molecular properties on the aggregation of globular
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proteins, this study compares the aggregation process of different proteins. Hence, the
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differences in the kinetics and factors influencing the aggregation of three proteins
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(β-lactoglobulin, ovalbumin and patatin) under various conditions (pH, ionic strength,
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temperature and protein concentration) are studied. The three proteins are selected as their
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similarities (globular structure, pI and molecular weight) and differences (surface charge,
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exposed hydrophobicity and number of disulfide bridges) (table 1) enable a comparison of the
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effect of charge, hydrophobicity and disulfide bridges on the aggregation behavior.
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Materials and methods
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Materials
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Ovalbumin (A-5503, Lot nᵒ 031M7008V; pI = 5.19; protein content 92 % (N x 6.22)38 of which
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≥ 98 % ovalbumin (based on agarose gel electrophorese)) and β-lactoglobulin (L-0130, Lot nᵒ
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SLBC2933V; pI = 4.83; protein content of 94 % (N x 6.38)38 of which 99 % β-lactoglobulin
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(based on PAGE)) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Potatoes were
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provided by AVEBE BA (Veendam, The Netherlands). Patatin was isolated from potato juice as
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described previously39, except that gel filtration was performed on a Superdex 200 column (52 x
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10 cm). The obtained patatin fraction (pI = 5.25; purity ≥ 90 % based on analytical
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size-exclusion chromatography) was dialyzed against demineralized water, freeze-dried and
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stored at -20 °C. All other chemicals were of analytical grade and purchased from either
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Sigma-Aldrich or Merck.
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Differential scanning calorimetry (DSC)
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The denaturation temperature of the proteins was determined using a VP-DSC MicroCalorimeter
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(MicroCal Inc., Northampton, MA, USA). The proteins were dissolved in 10 mM sodium
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phosphate buffer pH 7.0 at a concentration of 2 g/L. Subsequently, thermograms were recorded
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from 20 to 100 °C at a heating rate of 1 °C/min. The denaturation temperatures (Td) of
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ovalbumin, β-lactoglobulin and patatin were found to be 77.5, 75.0 and 60.0 °C, respectively
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(supporting information). This is in close agreement with the denaturation temperatures reported
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in literature6, 21, 39.
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Heat-induced aggregation
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Heat-induced aggregation was performed in a water bath. Aliquots of 1 mL were heated for
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different time intervals (0, 1, 2, 5, 10, 20, 30, 40, 50, 60, 100, 200, 300, 400, 500, 1500, 1800,
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3000, 4500 and 10080 min) in Kimax tubes. After heating, the samples were cooled on ice-
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water. Four different sets of experiments were performed:
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Effect of temperature
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Proteins were dissolved in 10 mM sodium phosphate buffer pH 7.0 at a concentration of 2 g/L.
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Subsequently, these solutions were heated at temperatures at the same distance from the
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denaturation temperature (Td): Td - 10 °C, Td - 5 °C, Td and Td + 5 °C. This was chosen to ensure
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as well as possible a similar rate of unfolding for all proteins. The reason is that at Td, for all
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proteins, the native and unfolded proteins are present in equal amounts12, 17. This equilibrium
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between native and unfolded protein shifts towards the native protein with decreasing and
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towards unfolded protein with increasing temperature. This equilibrium is, at a certain
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temperature from Td (e.g. Td - 5 °C), assumed to be equal for all proteins. For the incubations at
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the two highest temperatures, additional time intervals (i.e. 0.5, 1.5, 2.5, 3, 4, 7.5, 12.5, 15, 17.5
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and 25 min) were included.
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The aggregation rate was determined based on the decrease of the concentration of
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non-aggregated protein in time using equation 1 or 2. From the temperature dependence of the
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aggregation rate (k), the activation energy (Ea) was calculated by the Arrhenius equation.
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Effect of pH
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Proteins were dissolved in 10 mM sodium phosphate buffer pH 7.0, pH 6.0 and pH 5.0 at a
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concentration of 2 g/L. Subsequently, the solutions were heated at Td - 5 °C.
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Effect of ionic strength
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Proteins were dissolved in 10 mM sodium phosphate buffer pH 7.0 in the presence of absence of
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10, 40 and 90 mM NaCl at a concentration of 2 g/L. Subsequently, the solutions were heated at
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Td - 5 °C.
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Effect of protein concentration
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Proteins were dissolved in 10 mM sodium phosphate buffer pH 7.0 at concentrations of 1, 2, 5
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and 10 g/L. Subsequently, the solutions were heated at Td - 5 °C.
173 174
Size-exclusion chromatography (SEC)
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The concentration of non-aggregated protein (i.e. monomer concentration for ovalbumin and
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dimer concentration for β-lactoglobulin and patatin) was determined using SEC on an Äkta
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Micro equipped with a Superdex 200 PC 3.2/30 column (GE Healthcare, Uppsala, Sweden).
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Prior to analysis, the samples were centrifuged (16,100g, 10 min, 20 °C) to remove insoluble
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aggregates. Subsequently, samples (20 µL) were injected and eluted with 10 mM sodium
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phosphate buffer pH 7.0 at a flow rate of 0.06 mL/min. The elution was monitored at 214 nm.
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The calibration was performed with globular proteins with a mass range of 13.7 - 67 kDa (GE
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Healthcare). The typical experimental error for the determination of the amount of non-
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aggregated protein was < 10 % as determined from duplicate time incubations of ovalbumin and
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patatin, 2 g/L, Td - 5 °C, 10 mM sodium phosphate buffer pH 7.0.
185 186
Determination of the aggregation kinetics
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The aggregation kinetics are determined in several ways: (1) concentration dependence of the
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time required to aggregate half of the protein (i.e. th as indication of the aggregation rate), (2)
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fitting the decrease of the concentration of non-aggregated protein in time using equations 1 and
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2 and (3) fitting the natural logarithm of concentration (n = 1.0), reciprocal of the square root of
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concentration (n = 1.5) and the reciprocal of the concentration (n= 2.0) against time with a linear
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function, fixing the intercept with the y-axis corresponding to the initial concentration (C0). The
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first approach only includes the data points Ct ≥ 0.5C0, whereas the other two approaches are
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applied to the regime Ct ≥ 0.05C0. For the second approach the data is fitted by minimizing the
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sum of squares error using the Microsoft Excel solver.
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Static light scattering (SLS)
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The scattered light intensity was determined in time using a Malvern Zetasizer Nano ZS
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(Malvern Instruments, Worcestershire, UK). The protein solutions (as described in the section
200
heat-induced aggregation) were filtered over a 0.1 µm PTFE filter (Puradisc 13; Whatman, Kent,
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UK). Subsequently, the filtered solutions were heated at the temperatures described in the section
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on heat-induced aggregation. The scattered light intensity was measured every 30 seconds for
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7200 seconds. The intensity of the scattered light is generally described by equation 313, 40.
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I (q ) ∝ K ⋅ C ⋅ M w ⋅ P (q) ⋅ S (q )
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(3)
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in which I(q) is the intensity of the scattered light at angle q (i.e. 173°), K is an optical constant,
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c is the concentration [g/L], Mw is the molar mass of the particles [g/mol], P(q) is the particle
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form factor which describes the shape and size of the particles and S(q) is the structure factor
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that describes the spatial arrangement of the particles.
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As the concentration (C) and the optical constant (K) are constant during and between the
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measurements equation 3 can be simplified to:
I ( q) ∝ M w ⋅ P( q ) ⋅ S ( q)
(4)
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This shows that an increase of the scattered light intensity relates to an increase of the aggregate
211
size and/or a more dense aggregate structure.
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Results and discussion
214
Aggregation order
215
Generally, the aggregation kinetics are described by the aggregation order and aggregation rate.
216
The first parameter used to compare the aggregation behavior of different proteins is the
217
aggregation order (n). To exclude the effect of aggregation rate and thereby only obtain
218
information on the influence of conditions on the aggregation order, the decrease of the
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concentration of non-aggregated protein is plotted against the time divided by the time required
220
to aggregate half of the proteins (th) (figures 1A-C). All curves of one protein superimpose onto
221
one master curve, indicating that the aggregation order is not affected by temperature,
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concentration, pH and ionic strength. In other words, the general aggregation process of each
223
protein can be described by a single aggregation order under the different conditions studied.
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The small variations for ovalbumin and patatin at Td and Td + 5°C were ascribed to small
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inaccuracies as a result of the fast aggregation rate (i.e. indicated by a th ≤ 2 min; table 3). The
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observed discrepancies for patatin under conditions with a slower aggregation rates (i.e.
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indicated by a th ≥ 50 min; table 3) are not completely understood, but could be caused by a
228
stronger tendency to form larger aggregates in the initial stage of the aggregation process.
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The aggregation order for each of the three proteins was determined using three approaches. The
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first approach derives the aggregation order from the concentration dependence of th (figure 2
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and table 2). The th is concentration independent for ovalbumin and patatin, indicating a first
232
order reaction. For ovalbumin, this is in line with previous data6, whereas for patatin no previous
233
data is present. For β-lactoglobulin, on the other hand, th depends on the concentration. The slope
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of the concentration dependence of th is -0.5. This shows that, within the experimental error, th
235
scales with √C, implying an order of 1.57. This is in line with literature5, 7. The second approach
236
is based on fitting the decrease of the concentration of non-aggregated protein in time using
237
equations 1 and 2. For all proteins, the order of the aggregation process is higher than second
238
order kinetics (i.e. n = 2.5, 2.1 and 2.9 for β-lactoglobulin, ovalbumin and patatin, respectively)
239
(table 2). This indicates a complex aggregation process, in which non-aggregated proteins, for
240
example, aggregate with other non-aggregated proteins as well as aggregated proteins. The third
241
approach is based on fitting the decrease of ln Ct, 1/√Ct or 1/Ct in time to a linear function. This
242
resulted in the best fit (i.e. R2) of an aggregation order of 2.0 for ovalbumin and patatin (R2 =
243
0.95 ± 0.04 and 0.97 ± 0.01 for ovalbumin and patatin, respectively) (table 2). Moreover, it
244
showed that the first order kinetics for ovalbumin and patatin, as determined by the first
245
approach, the low R2 (i.e. R2 = 0.28) disqualifies the first order kinetics. This discrepancy
246
between the concentration and time fit has been described previously41. Since the time fit
247
approaches include all the data for fitting, it is concluded that the overall aggregation can be well
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described based on second order kinetics. The best for β-lactoglobulin was an aggregation order
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of 1.5 (R2 = 0.96 ± 0.04). The fit of the data of β-lactoglobulin for a second order reaction is,
250
however, not significantly different from that of an order of 1.5. Although the aggregation
251
process of β-lactoglobulin can overall be described by an aggregation order of 1.5 or 2.0, the
252
aggregation process at pH 5.0 shows a better fit with first order kinetics (i.e. R2 = 0.99 and 0.84
253
for n = 1.0 and 1.5, respectively).
254
Summarizing, the aggregation process of all tested proteins, β-lactoglobulin, ovalbumin and
255
patatin, is described by an order of 2.0 (R2 > 0.90 for all proteins). The aggregation process of
256
β-lactoglobulin at pH 5.0 is a first order process.
257 258
Aggregation rate
259
In the previous section, the reaction order was determined from rescaled data (i.e. t/th). The
260
aggregation rate (k) is determined by fitting the decrease of the concentration of non-aggregated
261
protein versus real time (t) with equation 2. To allow quantitative comparison of the aggregation
262
rates, the data of all proteins is fitted with an aggregation order of 2.0 (R2 > 0.90 for all proteins)
263
(table 2).
264
Effect of temperature (2 g/L in 10 mM buffer pH 7.0)
265
Quantitatively, the effect of temperature on the aggregation rate varies significantly between the
266
proteins (figures 1D-F and table 3). Whereas the decrease of the concentration of non-aggregated
267
protein for ovalbumin and patatin at Td - 5°C (pH = 7.0, I = 10 mM and C = 2 g/L) is in the same
268
order of magnitude (i.e. th ~ 10 min), the decrease of non-aggregated β-lactoglobulin is
269
significantly slower (i.e. th ~ 220 min) (table 3). In addition, the temperature dependence of the
270
aggregation rate varies significantly between the proteins. This temperature dependence is
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reflected in the activation energy (Ea), calculated using the Arrhenius equation, which was found
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to be 343, 293 and 220 kJ/mol for patatin, ovalbumin and β-lactoglobulin, respectively
273
(supporting information). These activation energies are lower than reported in literature (i.e. ~
274
260-300 kJ/mol for β-lactoglobulin42,
275
conditions (i.e. pH 6.5-7.0).
276
Summarizing, the aggregation rate decreases from ovalbumin ≥ patatin >> β-lactoglobulin.
277
Based on the relative exposed hydrophobicity, the order was expected to be β-lactoglobulin >
278
patatin > ovalbumin. Based on the net surface charge density no large differences were expected.
279
Considering the possibility that the free sulfhydryl groups can interchange with disulfide
280
bonds44, the expected order is ovalbumin > β-lactoglobulin > patatin. From the previous it is
281
concluded that there is no clear unambiguous relation between one of these protein properties
282
and the determined rate of aggregation. One parameter that has not often been considered is the
283
extent of protein unfolding. The extent of unfolding may be limited by the presence of disulfide
284
bonds. Therefore, patatin and ovalbumin are expected unfold more completely than β-
285
lactoglobulin, due to the presence of 2 disulfide bonds in β-lactoglobulin. Consequently, less
286
hydrophobic amino acids become exposed for β-lactoglobulin than for the other two proteins,
287
leading to less non-covalent interactions.
288
Effect of pH (2 g/L in 10 mM buffer heated at Td - 5 °C)
289
The aggregation rate of ovalbumin is not affected by the pH in the range from pH 5 to pH 7
290
(figure 1D and table 3). The aggregation rates of β-lactoglobulin and patatin, on the other hand,
291
are slower at a pH closer to the iso-electric point (pI), with th of 223 and 10.8 min at pH 7.0,
292
compared to th of 1052 and 73 min at pH 5.0 for β-lactoglobulin and patatin, respectively (figures
293
1E and F and table 3).
43
and 338 kJ/mol for ovalbumin24) under similar
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For patatin, the effect of pH seems to be due to a difference in the denaturation temperature.
295
When the aggregation rates of patatin at pH 5 and pH 6 are compared to pH 7 at different
296
temperatures, they are similar to pH 7 at a temperature between Td - 5 °C and Td - 10 °C (table
297
3). This corresponds with a theoretical increase of Td of ~ 4 °C, which is in close agreement with
298
literature, i.e. 3 °C increase of Td from pH 7 to pH 645. For β-lactoglobulin, the aggregation rate
299
at pH 5 and pH 6 is slower than at pH 7 and Td - 10 °C (table 3). The observed decrease of the
300
aggregation rate by shifting the pH towards the pI is in line with literature5,
301
based on the temperature dependence of the aggregation rate (table 3), the Td should increase by
302
7-10 °C to explain the decrease in aggregation rate. Experimentally, the Td of β-lactoglobulin
303
was only reported to increase by 2 °C when the pH was decreased from pH 7 to pH 516.
304
Consequently, another factor also has to be influenced by the pH change. The association state of
305
β-lactoglobulin, as measured by size-exclusion chromatography, was not affected by the pH
306
changes (data not shown). The thiol groups, on the other hand, become less reactive with
307
decreasing pH8. Therefore, the decreased reactivity of the thiol groups can explain the observed
308
differences.
309
Effect of ionic strength (2 g/L in buffer pH 7.0 heated at Td - 5 °C) and concentration (10 mM
310
buffer pH 7.0 heated at Td - 5 °C)
311
For β-lactoglobulin, the aggregation rate increases with increasing ionic strength (i.e. th decreases
312
from 223 min at 10 mM to 87 min at 100 mM) and protein concentration (i.e. th decreases from
313
255 min at 1 g/L to 84 min at 10 g/L) (figure 1E and table 3). The effect of ionic strength is
314
explained by a decrease of the electrostatic repulsion within and between the protein molecules.
315
This was described to result in an increase of the denaturation temperature (i.e. 2 and 5 °C from 0
316
to 100 mM for β-lactoglobulin and BSA, respectively)21,
22
7, 8, 14
. However,
. In addition, it also results in a
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decrease of the electrostatic barrier for aggregation23. As in this case the aggregation rate
318
increases rather than decreases, the enhancing effect of electrostatics was more important than
319
the observed increase of the denaturation temperature. The effect of concentration on the
320
aggregation rate is line with previous data7. The effect is postulated to be caused by the fact that
321
at higher protein concentrations the absolute number of unfolded protein increases, resulting in
322
an increased likelihood of proteins to meet, collide and aggregate. In contrast to the observation
323
for β-lactoglobulin, the aggregation rates of ovalbumin and patatin are neither affected by the
324
ionic strength nor by the protein concentration (figures 1D and F). These results show that it is
325
important to use different proteins, as the behavior of proteins cannot be extrapolated from one
326
protein to another.
327 328
Formation of aggregates
329
Whereas some protein solutions remained transparent after heating, others became translucent or
330
opaque. These clear visual differences between the heated protein solutions are also reflected in
331
the ionic strength dependence of the aggregated protein in time determined by size-exclusion
332
chromatography (SEC) (figure 3).
333
At all ionic strengths, the UV peak area corresponding to the aggregates initially increases and
334
subsequently decreases. This decrease shifted to shorter heating times with increasing ionic
335
strength. This decrease at increased ionic strength and longer times is due to the formation of
336
larger aggregates, which are removed during the centrifugation step prior to SEC analysis.
337
To obtain insights in the effect of the conditions (e.g. pH) on the formation of aggregates, the
338
scattered light intensity is monitored in time (figures 4A-C). According to equation 4, an increase
339
in the scattered light intensity is caused by an increase of the aggregate size and/or alterations of
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the aggregate shape (at a given concentration). Based on the aggregation kinetics (table 3), the
341
time is recalculated into the fraction aggregated protein (figures 4D-F). This provides insights in
342
the aggregation mechanism (e.g. preference towards the formation of small or large aggregates).
343
For ovalbumin and β-lactoglobulin, the scattered light intensity does not significantly increase
344
when the proteins are heated at Td - 5 °C in a concentration of 2 g/L. For patatin, on the other
345
hand, the intensity of the scattered light increases significantly (i.e. I/I0 = 8 after 7200 seconds).
346
This indicates that the patatin aggregates are larger and/or denser than the ovalbumin and
347
β-lactoglobulin aggregates. These differences cannot be explained based on the aggregation
348
kinetics (i.e. th of ovalbumin and patatin is similar). This shows that for patatin the formation of
349
less, but larger aggregates is favored over more, smaller aggregates as is the case for ovalbumin
350
and β-lactoglobulin under these conditions.
351
Effect of pH and ionic strength
352
For β-lactoglobulin, a lag phase precedes the intensity increase (figure 4B). Moreover, the
353
intensity of the scattered light in time increases for all three proteins (i.e. ovalbumin,
354
β-lactoglobulin and patatin) with increasing ionic strength or a shift of the pH towards the pI
355
(figures 4A-C). The intensity, however, increases more strongly for patatin (i.e. I/I0 = 47 and 12
356
after 200 seconds for pH 5 and 100 mM, respectively) compared to β-lactoglobulin (i.e. I/I0 = 10
357
and 22 after 7200 seconds for pH 5 and 100 mM, respectively) and ovalbumin (i.e. I/I0 = 12 after
358
900 seconds and 8 after 7200 seconds for pH 5 and 100 mM, respectively). The observed lag
359
phase for β-lactoglobulin has also been reported in literature where it is ascribed to the
360
monomer-dimer equilibrium46 or the formation of small aggregates16. Furthermore, it was
361
concluded that a decrease of the electrostatic repulsion results in the formation of larger and/or
362
denser aggregates. This is explained by a lower electrostatic repulsion between the aggregates.
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Consequently, aggregates can continue to grow to larger sizes and/or the formed aggregates are
364
denser. In literature, the molecular mass and size of β-lactoglobulin aggregates were also
365
described to increase when the pH shifts towards the pI and/or the ionic strength increases8, 47, 48.
366
However, as shown, this effect is not the same for all proteins. The difference among the proteins
367
indicates that the magnitude by which the aggregate size/structure in time is affected by
368
electrostatics depends on the protein.
369
Recalculating time into the fraction aggregated protein provides information on the mechanism
370
of aggregation (figures 4D-F). Close to the pI (i.e. pH 5), for patatin and β-lactoglobulin, the
371
intensity increases with the incorporation of even a minor fraction of the protein into the
372
aggregates (i.e. ~ 5 %) (figures 4E and F). For ovalbumin, the intensity increases after ~ 25 % of
373
the protein aggregated (figure 4D). At a pH away from the pI or at lower ionic strength, a larger
374
fraction aggregated protein is required for the intensity to increase. These results show that for
375
patatin and β-lactoglobulin at low electrostatic repulsion (i.e. pH 5) even in the initial stages (i.e.
376
< 10 % aggregated protein) large and/or dense aggregates are formed. For ovalbumin, on the
377
other hand, the lag phase indicates that initially (i.e. < 25 % aggregated protein) smaller and/or
378
more open aggregates are formed. Subsequently, at a higher fraction of aggregated protein, large
379
and/or denser aggregates are formed. Similarly, also at higher electrostatic repulsion initially
380
smaller and/or more open aggregates are formed, followed by the formation of larger and/or
381
denser aggregates. The differences between the proteins show that patatin has the highest
382
tendency to form larger and/or denser aggregates.
383
Effect of concentration and temperature
384
In general, changes in temperature or concentration only resulted in minor changes of the light
385
scattering intensity as function of heating time (figures 4A-C). This shows that these two
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386
parameters do not affect the aggregate formation (i.e. size and structure) as strongly as ionic
387
strength or pH. For patatin, the intensity increase upon heating is concentration independent,
388
whereas it decreases with decreasing temperature (i.e. I/I0 = 4 after 7200 seconds) (figure 4C).
389
For β-lactoglobulin and ovalbumin, the intensity in time increases slightly with increasing
390
concentration (i.e. I/I0 = 4 and 3 at 10 g/L after 7200 seconds for β-lactoglobulin and ovalbumin,
391
respectively) (figures 4A and B). This increase is in line with literature, as the mass and size of
392
β-lactoglobulin aggregates have been described to increase with concentration (at higher
393
concentrations until 110 g/L)7, 28, 49. The effect of temperature on the aggregation of patatin is
394
expected to be purely determined by kinetics, as a consequence the aggregate formation in time
395
is reduced.
396
From the intensity as function of the fraction aggregated protein it can be observed that, even at
397
large fractions of aggregated protein, the intensity is not affected by concentration and
398
temperature. This confirms that, also after correcting for the aggregation kinetics, the aggregate
399
size and structure are, within the tested temperature and concentration range, independent of
400
temperature and concentration. Hence, at high electrostatic repulsion (i.e. pH 7.0 and 10 mM),
401
relatively small and/or open aggregates are formed.
402
In summary, aggregate formation is limited by the electrostatic repulsion between proteins or
403
protein aggregates due to their charge. In case of charge screening (i.e. increased ionic strength)
404
or a decreased surface charge (i.e. pH closer to pI), this electrostatic barrier decreases and larger
405
and/or denser aggregates are formed. Concentration and temperature, on the other hand, did not
406
strongly affect the aggregate size and/or structure. Although for all proteins aggregate formation
407
was qualitatively similar, quantitatively the behavior was significantly different. This shows that
408
the information on one protein cannot directly be related to another protein. In general,
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ovalbumin showed to have the least tendency to form larger and/or denser aggregates, followed
410
by β-lactoglobulin. Patatin, on the other hand, directly forms large and/or dense aggregates.
411
In conclusion, the comparison of different proteins shows that the sensitivity towards system
412
conditions (i.e. pH, ionic strength and concentration) varies significantly between the proteins.
413
While it was expected that the aggregation behavior relates to the molecular properties of a
414
protein, there is no clear link between the observed aggregation behavior and known protein
415
molecular properties such as surface charge and exposed hydrophobicity. This was for instance
416
illustrated by the lower tendency of ovalbumin to form larger, denser aggregates than patatin and
417
β-lactoglobulin, while the surface charge of these proteins is similar. Therefore, the generic
418
relations commonly assumed in literature need to be considered with care.
419
In addition, although the classical reaction rate kinetics are often used to describe the aggregation
420
process of proteins it was shown to contain little information about the actual size/density of the
421
formed aggregates. Consequently, for a more complete description of the aggregation process,
422
not only the loss of monomers, but also a detailed analysis of the aggregates is required.
423
For a fundamental understanding of the influence of molecular properties on the aggregation
424
process and to exclude factors relating to differences in unfolding behavior, it is important to
425
compare the aggregation behavior at a constant distance from Td. This is in contrast to the fact
426
that for practical situations a comparison at a constant temperature is more valuable (i.e.
427
realistic).
428 429
Notes
430
The authors declare no competing financial interest.
431
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Supporting information
433
Supporting information available:
434
Thermograms of the heat-induced unfolding of ovalbumin, β-lactoglobulin and patatin (2 g/L, 10
435
mM sodium phosphate buffer pH 7.0).
436
Arrhenius plots of ovalbumin, β-lactoglobulin and patatin (2 g/L, 10 mM sodium phosphate
437
buffer pH 7.0).
438 439
References
440
1.
441
176.
442
2.
443
USA, 2002.
444
3.
Lumry, R.; Eyring, H., Conformation changes of proteins. J. Phys. Chem. 1954, 58, 110-120.
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4.
Mahler, H.-C.; Müller, R.; Frieβ, W.; Delille, A.; Matheus, S., Induction and analysis of aggregates in a
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liquid IgG1-antibody formulation. Eur. J. Pharm. Biopharm. 2005, 59, 407-417.
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Biochem. 1994, 226, 883-889.
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aggregation of ovalbumin at neutral pH described by irreversible first-order kinetics. Protein Sci. 2003, 12, 2693-
451
2703.
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induced denaturation. Macromolecules 1999, 32, 6120-6127.
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of bovine serum albumin studied by asymmetrical flow field-flow fractionation. Anal. Chim. Acta 2010, 675, 191-
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198.
Pauling, L., Protein interactions: aggregation of globular proteins. Discuss. Faraday Soc. 1953, 13, 170-
Creighton, T. E., Proteins: structure and molecular properties. 2 ed.; W.H. Freeman: New York, NY,
Roefs, S. P. F. M.; de Kruif, K. G., A model for the denaturation and aggregation of β-lactoglobulin. Eur. J.
Weijers, M.; Barneveld, P. A.; Cohen Stuart, M. A.; Visschers, R. W., Heat-induced denaturation and
Le Bon, C.; Nicolai, T.; Durand, D., Kinetics of aggregation and gelation of globular proteins after heat-
Hoffmann, M. A. M.; van Mil, P. J. J. M., Heat-induced aggregation of β-lactoglobulin as a function of pH.
Yohannes, G.; Wiedmer, S. K.; Elomaa, M.; Jussila, M.; Aseyev, V.; Riekkola, M.-L., Thermal aggregation
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globular protein gels at pH 7. Ovalbumin. Macromolecules 2004, 37, 8709-8714.
461
11.
462
aggregation? Biophys. J. 2007, 92, 1336-1342.
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van Boekel, M. A. J. S., Kinetic modeling of reactions in foods. CRC Press: Boca Raton, FL, USA, 2009.
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Hoffmann, M. A. M.; Roefs, S. P. F. M.; Verheul, M.; van Mil, P. J. J. M.; de Kruif, K. G., Aggregation of
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β-lactoglobulin studied by in situ light scattering. J. Dairy Res. 1996, 63, 423-440.
466
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pH, heating temperature, and protein composition. J. Agric. Food Chem. 1998, 46, 4909-4916.
468
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Int. J. Food Sci. Technol. 1999, 34, 451-465.
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Agric. Food Chem. 1998, 46, 896-903.
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analysis. J. Pharm. Sci. 2009, 98, 2909-2934.
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18.
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2010, 390, 89-99.
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solution: mechanism and driving forces in nonnative protein aggregation. Pharm. Res. 2003, 20, 1325-1336.
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Biotechnol. Bioeng. 1997, 54, 333-343.
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transitions during heat denaturation as studied by differential scanning calorimetry. Food Hydrocolloids 2009, 23,
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2287-2293.
483
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denaturation. Effects of ionic strength and SDS concentration. Int. J. Biol. Macromol. 1997, 20, 193-204.
485
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486
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Weijers, M.; Visschers, R. W.; Nicolai, T., Influence of the ionic strength on the structure of heat-set
Krebs, M. R. H.; Devlin, G. L.; Donald, A. M., Protein particulates: another generic form of protein
Verheul, M.; Roefs, S. P. F. M., Structure of particulate whey protein gels: effect of NaCl, concentration,
Le Bon, C.; Nicolai, T.; Durand, D., Growth and structure of aggregates of heat-denatured β-lactoglobulin.
Verheul, M.; Roefs, S. P. F. M.; de Kruif, K. G., Kinetics of heat-induced aggregation of β-lactoglobulin. J.
Mahler, H.-C.; Friess, W.; Grauschopf, U.; Kiese, S., Protein aggregation: pathways, induction factors and
Wang, W.; Nema, S.; Teagarden, D., Protein aggregation - pathways and influencing factors. Int. J. Pharm.
Chi, E. Y.; Krishnan, S.; Randolph, T. W.; Carpenter, J. F., Physical stability of proteins in aqueous
Speed, M. A.; King, J.; Wang, D. I. C., Polymerization mechanism of polypeptide chain aggregation.
Haug, I. J.; Skar, H. M.; Vegarud, G. E.; Langsrud, T.; Draget, K. I., Electrostatic effects on β-lactoglobulin
Giancola, C.; De Sena, C.; Fessas, D.; Graziano, G.; Barone, G., DSC studies on bovine serum albumin
De Young, L. R.; Fink, A. L.; Dill, K. A., Aggregation of globular proteins. Acc. Chem. Res. 1993, 26, 614-
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generation of aggregation-prone conformers. Biomacromolecules 2007, 8, 1648-1656.
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25.
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shear modulus of heat-set globular protein gels. Soft Matter 2008, 4, 893-900.
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26.
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property. J. Agric. Food Chem. 1996, 44, 2086-2090.
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27.
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denatured ovalbumin investigated with a multiangle laser light scattering technique. J. Food Sci. 2008, 73, C41-C49.
495
28.
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globular proteins. Phys. A 2002, 304, 253-265.
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patatin studied in situ. J. Agric. Food Chem. 1999, 47, 4600-4605.
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of ovalbumin. Macromolecules 2002, 35, 4753-4762.
501
31.
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Int. J. Biol. Macromol. 2008, 43, 129-135.
503
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of aggregates and gels formed by heat-denatured whey protein isolate and β-lactoglobulin at neutral pH. J. Agric.
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Food Chem. 2007, 55, 3104-3111.
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33.
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induced aggregation of the globular protein β-lactoglobulin at pH 7. Int. J. Biol. Macromol. 2004, 34, 21-28.
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34.
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denaturation of globular proteins. Macromolecules 1994, 27, 583-589.
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35.
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protein gels at pH 7. β-lactoglobulin. Macromolecules 2004, 37, 8703-8708.
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36.
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disaggregation of native β-lactoglobulin in low salt. Langmuir 2013, 29, 4584-4593.
Broersen, K.; Weijers, M.; de Groot, J.; Hamer, R. J.; de Jongh, H. H. J., Effect of protein charge on the
Mehalebi, S.; Nicolai, T.; Durand, D., The influence of electrostatic interaction on the structure and the
Mine, Y., Laser light scattering study on the heat-induced ovalbumin aggregates related to its gelling
Choi, S. J.; Moon, T. W., Influence of sodium chloride and glucose on the aggregation behavior of heat-
Durand, D.; Gimel, J. C.; Nicolai, T., Aggregation, gelation and phase separation of heat denatured
Pots, A. M.; ten Grotenhuis, E.; Gruppen, H.; Voragen, A. G. J.; de Kruif, K. G., Thermal aggregation of
Weijers, M.; Visschers, R. W.; Nicolai, T., Light scattering study of heat-induced aggregation and gelation
Mehalebi, S.; Nicolai, T.; Durand, D., Light scattering study of heat-denatured globular protein aggregates.
Mahmoudi, N.; Mehalebi, S.; Nicolai, T.; Durand, D.; Riaublanc, A., Light-scattering study of the structure
Baussay, K.; Le Bon, C.; Nicolai, T.; Durand, D.; Busnel, J. P., Influence of the ionic strength on the heat-
Gimel, J. C.; Durand, D.; Nicolai, T., Structure and distribution of aggregates formed after heat-induced
Pouzot, M.; Durand, D.; Nicolai, T., Influence of the ionic strength on the structure of heat-set globular
Yan, Y.; Seeman, D.; Zheng, B.; Kizilay, E.; Xu, Y.; Dubin, P. L., pH-dependent aggregation and
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37.
515
β-lactoglobulin aggregates. J. Agric. Food Chem. 1997, 45, 2949-2957.
516
38.
517
patatin gels compared with β-lactoglobulin, ovalbumin, and glycinin. J. Sci. Food Agric. 2011, 91, 253-261.
518
39.
519
P.; Voragen, A. G. J., Effects of pH and heat treatments on the structure and solubility of potato proteins in different
520
preparations. J. Agric. Food Chem. 2001, 49, 4889-4897.
521
40.
522
fractionation. John Wiley: Hoboken, NJ, USA, 2011.
523
41.
524
1996, 61, 477-486.
525
42.
526
reconstituted whole milk. J. Agric. Food Chem. 1996, 44, 422-428.
527
43.
528
1988, 53, 258-263.
529
44.
530
reactions on the heat-induced aggregation kinetics of β-lactoglobulin. Int. Dairy J. 2007, 17, 1034-1042.
531
45.
532
structural stability of patatin. J. Agric. Food Chem. 1998, 46, 2546-2553.
533
46.
534
dynamic light scattering. Int. Dairy J. 1996, 6, 343-357.
535
47.
536
lactoglobulin heat aggregation at high pH. Biophys. J. 2000, 79, 1030-1038.
537
48.
538
characterization of thermally-induced β-lactoglobulin aggregates. J. Food Sci. 2010, 75, E261-E268.
539
49.
540
kinetics. Biomacromolecules 2010, 11, 864-871.
Hoffmann, M. A. M.; Sala, G.; Olieman, C.; de Kruif, K. G., Molecular mass distributions of heat-induced
Creusot, N.; Wierenga, P. A.; Laus, M. C.; Giuseppin, M. L. F.; Gruppen, H., Rheological properties of
van Koningsveld, G. A.; Gruppen, H.; de Jongh, H. H. J.; Wijngaards, G.; van Boekel, M. A. J. S.; Walstra,
Podzimek, S., Light scattering, size exclusion chromatography and asymmetric flow field flow
Van Boekel, M. A. J. S., Statistical aspects of kinetic modeling for food science problems. J. Food Sci.
Anema, S. G.; McKenna, A. B., Reaction kinetics of thermal denaturation of whey proteins in heated
Dannenberg, F.; Kessler, H.-G., Reaction kinetics of the denaturation of whey proteins in milk. J. Food Sci.
Mounsey, J. S.; O’Kennedy, B. T., Conditions limiting the influence of thiol–disulphide interchange
Pots, A. M.; de Jongh, H. H. J.; Gruppen, H.; Hessing, M.; Voragen, A. G. J., The pH dependence of the
Elofsson, U. M.; Dejmek, P.; Paulsson, M. A., Heat-induced aggregation of β-lactoglobulin studied by
Bauer, R.; Carrotta, R.; Rischel, C.; Øgendal, L., Characterization and isolation of intermediates in β-
Zúñiga, R.; Tolkach, A.; Kulozik, U.; Aguilera, J., Kinetics of formation and physicochemical
Ako, K.; Nicolai, T.; Durand, D., Salt-induced gelation of globular protein aggregates: Structure and
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541
50.
542
Protein concentration and protein-exposed hydrophobicity as dominant parameters determining flocculation of
543
protein-stabilized oil-in-water emulsions. Langmuir 2013, 29, 11567-11574.
Delahaije, R. J. B. M.; Wierenga, P. A.; van Nieuwenhuijzen, N. H.; Giuseppin, M. L. F.; Gruppen, H.,
544
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Table 1. Molecular protein properties of β-lactoglobulin, ovalbumin and patatin as obtained from
546
the Swiss-Prot database (http://www.expasy.org).
547
Protein
Expasy code
Mw [kDa]
pI
#COOH/#NH2b
S-S bridges/free SH groups
σw [mC/m2]c
Rel. QH [-]d
β-lactoglobulin
P02754
18.4/36.8a
4.83
26/18
2/1
-21.9
1.00
ovalbumin
P01012
44.5
5.19
47/35
1/4
-24.2
0.19
patatin
P07745
40.0
5.25
43/32
0/1
-17.9
0.73
548
a
549
b
550
acid) and positively charged (arginine and lysine) groups in the primary sequence, respectively.
551
c
552
d
Molecular weight of a β-lactoglobulin dimer (native state under the selected conditions). #COOH and #NH2 are the maximum numbers of negatively charged (aspartic and glutamic
σw is the theoretical net surface charge density at pH 7. Rel. QH is the relative exposed hydrophobicity of the proteins as determined previously50.
553 554
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Table 2. Aggregation order of ovalbumin, β-lactoglobulin and patatin determined by different
556
methods. The variation reported for method 3.1-3.3 represent the standard deviation between R2
557
values obtained from fitting the individual conditions separately.
Determination of the aggregation order
Ovalbumin
β-Lactoglobulin
Patatin
1.0
1.5
1.0
2.1
2.5
2.9
(3.1) Linear fit of ln(Ct) in time (n = 1.0) (Ct ≥ 0.05C0)
R2 = 0.28 ± 0.32
R2 = 0.85 ± 0.13
R2 = 0.28 ± 0.30
(3.2) Linear fit of 1/√C in time (n = 1.5) (Ct ≥ 0.05C0)
R2 = 0.80 ± 0.13
R2 = 0.96 ± 0.04
R2 = 0.83 ± 0.09
(3.3) Linear fit of 1/C in time (n = 2.0) (Ct ≥ 0.05C0)
R2 = 0.95 ± 0.04
R2 = 0.93 ± 0.07
R2 = 0.97 ± 0.01
(1)
Concentration dependence of the th (Ct ≥ 0.5C0)
(2) Fit concentration in time using equations 1 and 2 (Ct ≥ 0.05C0)
558 559
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A - ova
0.9
0.8
0.8
Fraction C0 [-]
Fraction C0 [-]
1 0.9 0.7 0.6 0.5 0.4 0.3
0.7 0.6 0.5 0.4 0.3
0.2
0.2
0.1
0.1
0 0.0001
D - ova
0 0.01
1
100
10000
0.1
1
t/th [-] 0.9
10000
0.8
0.8
0.7 0.6 0.5 0.4 0.3
E - β-lg
0.7 0.6 0.5 0.4 0.3
0.2
0.2
0.1
0.1 0 0.01
1
100
10000
0.1
1
t/th [-]
10
100
1000
10000
Time [min]
1
1
C - pat
0.9
0.9
0.8
0.8
Fraction C0 [-]
Fraction C0 [-]
1000
1
B - β-lg
Fraction C0 [-]
Fraction C0 [-]
1
0.7 0.6 0.5 0.4 0.3
F - pat
0.7 0.6 0.5 0.4 0.3
0.2
0.2
0.1
0.1
0 0.0001
100
Time [min]
0.9
0 0.0001
10
0 0.01
1
100
10000
0.1
1
10
100
1000
10000
560 561
Figure 1. Fraction of non-aggregated protein (C0) as function of the time normalized by th (A-C)
562
and as function of the time (D-F) for ovalbumin (A and D), β-lactoglobulin (B and E) and patatin
563
(C and F). The markers represent the different conditions: Td - 10 °C (), Td (), Td + 5 °C
564
(), pH 5.0 (), pH 6.0 (), 5 g/L, 10 g/L, 40 mM NaCl and 90 mM NaCl () and the average
565
of all remaining conditions (Td - 5 °C, 2 g/L, 10 mM NaCl and all not indicated by the other
566
markers), with error bars indicating the standard deviation (). Solid lines in D-F are guides to
567
the eye. The solid lines in A-C represent the best fits (n = 2.1 for ovalbumin, n = 2.5 for β-
568
lactoglobulin, and n = 2.9 for patatin).
t/th [-]
Time [min]
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th [min]
100
10
1 1
569
10
Concentration [g/L]
570
Figure 2. Concentration dependence of the time required to aggregate half of the proteins (th) for
571
ovalbumin (), β-lactoglobulin () and patatin () heated at Td - 5 °C. The solid line has a
572
slope of -0.5. The dashed line is a guide to the eye.
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Table 3. Half times (th) and aggregation rate (k) of the ovalbumin, β-lactoglobulin and patatin at
575
different heating conditions. k [L/g s] (n =2)a
th [min] Variable
C [g/L]
pH
I [mM]
T [°C]
Ova
β-Lg
Pat
Ovalbumin
β-Lactoglobulin
Patatin
Ref
2
7
10
Td - 5
8.7
223
10.8
9.64E-04
3.74E-05
7.71E-04
2
7
10
Td - 10
81
330
144
1.02E-04
2.52E-05
5.78E-05
2
7
10
Td
2.0
29.2
1.5
4.21E-03
2.85E-04
5.66E-03
2
7
10
Td + 5
1.1
16.0
0.5
7.87E-03
5.20E-04
1.60E-02
2
7
20
Td - 5
7.8
188
10.7
1.07E-03
4.42E-05
7.76E-04
2
7
50
Td - 5
8.7
93
9.9
9.63E-04
8.94E-05
8.38E-04
2
7
100
Td - 5
6.8
87
7.2
1.22E-03
9.63E-05
1.16E-03
1
7
10
Td - 5
9.9
255
11.1
1.69E-03
6.53E-05
1.50E-03
5
7
10
Td - 5
8.9
128
10.8
3.75E-04
2.59E-05
3.07E-04
10
7
10
Td - 5
8.0
84
13.3
2.08E-04
1.99E-05
1.26E-04
2
6
10
Td - 5
7.1
621
49.1
1.17E-03
1.34E-05
1.70E-04
2
5
10
Td - 5
4.8
1052
73
1.72E-03
7.92E-06
1.14E-04
T
I
C
pH
576 577
a
aggregation rate determined based on n = 2.
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A280 [mAU mL]
Journal of Agricultural and Food Chemistry
0.1
579
1
10
100
1000
10000
Time [min]
580
Figure 3. UV peak area of aggregated patatin (10 mM sodium phosphate buffer pH 7.0, 2 g/L,
581
Td - 5 °C) in time determined by size-exclusion chromatography. The markers represent samples
582
heated at different NaCl concentrations: 0 mM NaCl (), 10 mM NaCl () and 90 mM NaCl
583
().
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14
14
A - ova 5
12
5
12
10
10
8
8 6
I/I0 [-]
I/I0 [-]
D - ova
7 4 3,6 2 1,9-11
4 2
8
8 6
7 4 2 3,6
4 2
0 0
1200
2400
3600
4800
6000
7200
0
0.2
Time [s] 25
0.4
0.6
25
B - β-lg
E - β-lg
15
15
5
10 5
I/I0 [-]
20
I/I0 [-]
20
0 1200
2400
3600
4,8
4800
6000
2,7
5
1,3,6,9-11
0 0
7200
0.2
Time [s] 50
50
0.6
0.8
1
F - pat
45 40
35
35
30
30
5
I/I0 [-]
40
I/I0 [-]
0.4
Fraction aggregated protein [-]
C - pat 5
45
1
5
10
2 7 1,3,4,6,9-11 0
0.8
Fraction aggregated protein [-] 8
25
25
20
20
7
15
8
4,6 1-3,10,11 9
10 5 0
7
15
8
6 1-3 10 11
10
4
5
9
0 0
585
1,9-11
0
1200
2400
3600
4800
6000
Time [s]
7200
0
0.2
0.4
0.6
0.8
1
Fraction aggregated protein [-]
586
Figure 4. Normalized intensity (I/I0) of heated ovalbumin (A and D), β-lactoglobulin (B and E)
587
and patatin (C and F) in time (A-C) and as a function of the fraction aggregated protein (D-F).
588
The numbers represent the different conditions: (1) Td - 5 °C and 2 g/L, (2) 10 g/L, (3) 5 g/L, (4)
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pH 6.0, (5) pH 5.0, (6) 10 mM NaCl, (7) 40 mM NaCl, (8) 90 mM NaCl, (9) Td - 10°C, (10) Td
590
and (11) Td + 5°C. The error bars indicate the standard deviation between the different
591
conditions with similar behavior.
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Table of Contents Graphic
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