studying interactions between protein, small molecules and...
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Studying Interactions between Protein, Small Molecules and Water by DSC
Recent DSC work – interaction of small molecules with proteins
Differential scanning calorimetry scans of lysozyme with varying
sodium phytate concentrations in 10mM NaCl, pH 7.0.
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The Effect of the Anions Phosphate, Sulphate and Chloride on 1 mg/ml Lysozyme
pH 7
Binding of anions to proteins effect on stability does not follow the
Hofmeister series
(mM)
At higher anion concentrations there are two distinct phases of stabilisation
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Sodium Chloride
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At higher anion concentrations there are two distinct phases of stabilisation
Disodium Sulphate
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At higher anion concentrations there are two distinct phases of stabilisation
Disodium Hydrogen Phosphate
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Sodium Iodide
Sodium Perchlorate
Low charge density ions behave like detergents
Current theory of stabilisation
+ve
Anionic
molecule -
High charge density ions bind to the
protein’s basic side chains by
electrostatic interaction. Alteration of
the charge results in a change in force
applied to the peptide backbone.
Mechanism 1. Strong direct interaction of anions with opposite
charged side chains
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Current theory of stabilisation
Anionic
molecule -
Anions interact with the protein’s
backbone by electrostatic
interaction – has a weak effect on
stability
Mechanism 2. Weak direct interaction of anions with partially
charged peptide backbone
9
10
Current theory of stabilisation
Mechanism 3. Competition for water
between the solvation layers of the
protein and the ions
Mechanism 4. Wetting agent or detergent-like mechanism
Low charge density ions reduce the
free energy required to wet the
newly exposed hydrophobic interior
of the protein.
ΔΔGsolv layer
+ve
Chaotropic anions or
charged organic molecule
11
Current theory of stabilisation
Sodium Iodide
Sodium Perchlorate
Reality – multiple mechanisms may occur concurrently
• Direct strong interaction
• Competition for water
+ve
Phosphate
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Current theory of stabilisation
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Arginine – a widely used excipient in
bio-pharmaceutical formulations
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Glycine
Guanidinium HCl
Arginine – a widely used excipient in
bio-pharmaceutical formulations
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Arginine – a widely used excipient in
bio-pharmaceutical formulations
Arginine
16
Arginine – a widely used excipient in
bio-pharmaceutical formulations
Glycine
17
Arginine – a widely used excipient in
bio-pharmaceutical formulations
Guanidinium HCl
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Arginine – a widely used excipient in
bio-pharmaceutical formulations
Glycine
Guanidinium HCl
Arginine
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Glycine
Guanidinium HCl
Arginine – an excipient that could have
been reverse engineered
Mechanism 2 & 3
Mechanism 4
Goals
• Better understand the role of small molecules in protein stability
• Challenge existing theories on the interaction between water and
cosolutes (small inorganic, organic and macromolecules)
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