What is the relationship between charge and basicity/acidity..??

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Relationship of Charge to pH

pH is the negative log of the hydrogen ion concentration in a solution. Similarly pK_A is the negative log of the acid dissociation constant for an amino acid side chain.

In solution, if pH < pK_A, then the protonated form of an amino acid side chain predominates. If pH > pK_A, then the deprotonated form of the amino acid side chain predominates. The charge on an acidic side chain can therefore vary between -1 (when pH << pK_A) and 0 ( when pH >> pK_A). Consider the example of glutamate, with a side chain pK_A = 4.4. When the pH = pK_A, the concentration of the protonated and deprotonated forms is equal and the charge is -0.5. When pH = 6.4, the glutamate is about 99% deprotonated, leading to a charge of -0.99. When pH = 2.4, the glutamate is about 99% protonated, leading to a charge of -0.01.

The charge on a basic side chain can range between 0 (when pH >> pK_A) and +1 (when pH << pK_A). The side chain on lysine normally has a pK_A value of about 9.0. When pH = pK_A, the concentration of the protonated and deprotonated forms is equal and the charge is +0.5. When pH = 11.0, the lysine is about 99% deprotonated, leading to a charge of +0.01. When pH = 7.0, the lysine is about 99% protonated, leading to a charge of +0.99.

The graph below shows the relationship between charge and pH for the side chains of glutamate, an acidic amino acid, and lysine, a basic amino acid, assuming that these amino acids are incorporated in a protein and that they havepK_A values of 4.4 for glutamate and 10.0 for lysine.

Relationship of Charge to pH

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