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You mean "why is a protein least soluble when the pH of aqueous media matches the value of its isoelectric point?".
The answer is that at its isoelectric point the protein surface carries no net charge. As protein solubility is based upon favourable electrostatic interactions between the charges (negative or positive) on the protein surface and the delta-negative or delta-positive dipoles on water molecules, when there is LEAST charge on the protein surface you can also expect their to be the least favourable interactions between water molecules and the protein. As the protein-water interaction is in competition with protein-protein interactions, being at the pI most favours protein-protein interactions, which leads to precipitation, compared to protein-water interaction which lead to solvation.
Isoelectric. "equal electric [charge]"
The previous answer is incorrect in multiple ways and correct in some. It states "protein solubility is based upon electrostatic interactions between the charges (negative or positive) on the protein surface and the ...dipoles of water". Firstly, there cannot exist an electrostatic interaction (coulombic) with one charged molecule and an uncharged water molecule/induced dipole. An electrostatic force of a charged particle must have an electrolyte in solution for interaction.
What the explanation aimed to describe is the decreased ion-induced dipole forces from a charged amino acid side chain to molecules of water at the pI of the protein. The singular force of which is stronger than H-bonding (and much stronger than van der waals, london/dispersion forces). Collectively H-bonding of the exterior amino acids is a much stronger force than total of the ion-induced dipoles.
...However, if we were talking about "solvation" of the molecule, then his answer would be completely correct where at the isoelectric point the least solvation would occur, largely due to the absence of charged outer residues...
Why is the solubility not dependent on ion-induced dipole? We only have 5 natural occurring amino acid R groups that can be charged around neutral pH: aspartate, glutamate - whose side chains behave as any other organic acid in the physiological range, reacting with other amine side chains, going through esterification with alcohols, chelation of divalent metal ions, etc.
The basic residues - lysine and arginine, lysine will undergo a number of reactions with or without a protonated nitrogen. Arginine however is likely going to carry a net charge at any protein pI and is the least reactive A.A. (however its' frequency in proteins is 5.7%, of this percent a fraction of which will go toward ion-dipole surface interaction with water. The same goes for histidine but it is even less frequent in proteins (2.2%).
The pI of most any protein will be nearer to the physiological range than to either extreme, so we can rule out mass protonation/deprotonation of side chains at its pI, which is the only way ion-induced dipole forces would participate in such solubility changes anyhow.
So what does make a protein soluble/insoluble in water?
A protein's 'solubility' comes first from the specific volume of water being much smaller than the specific volume of the protein. Meaning although our protein may be more dense than water, it has a tendency to be suspended in solution because of the much larger volume occupied (this of course is not true for many types of proteins, seen more in globular proteins but the theory is the same).
The major factor of protein solubility according to pI is hydrophilic residues (vs hydrophobic) on the exterior. As the pH of the solution nears an equilibrium of oxidation/reduction of the hydrophobic exterior residues, less water molecules are forming h-bonds, dipoles, van der waals, etc on the protein's exterior. You could imagine the H2O 'dispersing' away from the membrane, decreasing the volume occupied by H2O molecules near the protein exterior. This action allows the protein to 'fall' from solution and form a precipitate at the pI of the solution. The physical chemistry behind this action is much more involved, but this should give some idea of the biochemistry of the event.
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RossA protein is least soluble at its isoelectric point because at this pH, the protein has no net charge and is less likely to interact with water molecules. This lack of charge reduces the protein's solubility, causing it to precipitate out of solution.
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What is the relationship between pH and pI in terms of their impact on protein charge?
The relationship between pH and pI is that the pH of a solution can affect the charge of a protein, while the pI (isoelectric point) is the pH at which a protein has no net charge. At a pH below the pI, the protein will have a net positive charge, and at a pH above the pI, the protein will have a net negative charge.
The isoelectric point of an amino acid is?
The isoelectric point of an amino acid is the pH at which the amino acid carries no net charge. It is the pH at which the amino acid exists in its zwitterionic form, with equal numbers of positive and negative charges.
Why agarose gel electrophoresis is not used for protein analysis?
Agarose gel electrophoresis is primarily used for separating and analyzing nucleic acids based on their size, as it provides good resolution for DNA and RNA molecules. However, proteins have different properties (charge, size, and shape) compared to nucleic acids, making agarose gel less suitable for protein analysis. For protein analysis, techniques like SDS-PAGE and isoelectric focusing are commonly used, as they are designed specifically for separating proteins based on their size, charge, and isoelectric point.
The pI of a lysozyme is 11. What is the pH range in which the overall charge of the lysozyme is positive?
The overall charge of a protein is positive when the pH is below the pI (isoelectric point). For lysozyme with a pI of 11, the pH range in which its overall charge is positive would be below pH 11.
What are the differences between the N-terminal and C-terminal regions of a protein?
The N-terminal region of a protein is the starting point of the protein chain, while the C-terminal region is the end point. These regions can have different functions and structures, influencing how the protein functions in the body.
What is the significance of the isoelectric point in the context of protein chemistry?
The isoelectric point (pI) of a protein is the pH at which the protein carries no net electrical charge. This is significant in protein chemistry because at the isoelectric point, the protein is least soluble and is least likely to interact with other molecules. This property is important for protein purification and separation techniques.
What is the isoelectric point of cysteine and how does it affect its chemical properties?
The isoelectric point of cysteine is around pH 5.0. At this pH, cysteine carries no net charge and is least soluble in water. This affects its chemical properties as it can form disulfide bonds with other cysteine molecules, impacting protein structure and function.
What is the significance of isoionic point in the context of protein structure and function?
The isoionic point, also known as the isoelectric point, is the pH at which a protein has no net charge. This is significant in protein structure and function because it affects the protein's solubility, stability, and interactions with other molecules. At the isoionic point, a protein is least soluble and may undergo conformational changes that impact its function.
What does the isoelectric point graph reveal about the behavior of a molecule in different pH environments?
The isoelectric point graph shows how a molecule's charge changes in different pH environments. At the isoelectric point, the molecule has no net charge and is least soluble. Above the isoelectric point, the molecule is negatively charged, and below it, the molecule is positively charged. This information helps understand how the molecule interacts with its environment at different pH levels.
What is the pI of a protein?
The pI (isoelectric point) of a protein is the pH at which the protein carries no net charge. It is the pH at which the protein will not migrate in an electric field.
What is the isoelectric point of lysine and how does it affect its chemical properties?
The isoelectric point of lysine is around pH 9.74. At this pH, lysine carries no net charge and is least soluble in water. This affects its chemical properties by influencing its solubility, reactivity, and ability to interact with other molecules.
What is isoelectric pH?
Isoelectric pH, often referred to as the pI (isoelectric point), is the pH at which a molecule or substance carries no net electrical charge. It is the pH at which the molecule is neutral or balanced between positive and negative charges. At the isoelectric pH, the molecule tends to be least soluble in water due to its minimum ionization state.
What is the isoelectric point of lysine?
The isoelectric point of lysine is approximately 9.74.
What is the isoelectric point of tyrosine?
The isoelectric point of tyrosine is approximately 5.66.
What is meant by isoelectric point of protein?
It is the pH at which a particular molecule or surface carries no net electrical charge
What is meant by isoelectronic point of protein?
The isoelectric point of a protein is the pH at which the protein has zero net charge. At this pH, the number of positively charged amino acids equals the number of negatively charged amino acids in the protein, resulting in a neutral overall charge.
What is the isoelectric point of tyrosine and how does it affect its chemical properties?
The isoelectric point of tyrosine is around pH 5.66. At this pH, tyrosine carries no net electrical charge. This affects its chemical properties by making it less soluble in water and more likely to interact with other molecules through hydrogen bonding.
