BCHM-3050 Lecture Notes - Lecture 1: Amphoterism, Buffer Solution, Acid Dissociation Constant
19 May 2018
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Acids v Bases
Remember: acids and bases are OPPOSITES
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Acids
Bronsted-Lowry Acids: compounds that donate protons (H+)
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Arrhenius acids: compounds that form H3O+ if dissolved in water
Involves water
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Lewis Acids: compounds that accept electrons
Why do they accept electrons?
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pH scale: based on auto-ionization of water
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Water: Amphoteric and Amphiprotic molecule
Amphoteric: water is able to react as acid or base
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Amphiprotic: can donate or accept proton
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Terms are not the same
Ex. Al2O3
Amphoteric but isn't amphiprotic (doesn’t have proton)
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Biologically relevant weak acids and their conjugate bases
𝑝𝐾𝑎 = − log 𝑘𝑎
Dihydrogen phosphate ion (H2PO4-)
Monohydrogen phosphate ion (HPO42-)
Bicarbonate ion (HCO-3)
Note: Conjugate acid/base pairs differ by one proton
Lower pKa indicates stronger acid
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pH Scale
Most bio rxns take place b/w pH 6.5 and 8.0
Physiological ph range
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Human blood: pH 7.35 -7.45
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Physiological pH: 7.4
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Molecular charge depends on pH
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Buffer solutions and Cells
Buffer: resist changes in pH following addition of acid or base that have +
1 pH units difference from pKa of solution
+3 pH units: the group is deprotonated or fully protonated and the
buffer effect ceases
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pH = pKa of solution: group is 50% protonated/deprotonated
50% ionized
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Things are neutralized a bit
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Henderson-Hassel Bach equation: can be used to calculate average
charge on ionizable group at any pH
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[𝑏𝑎𝑠𝑒]
[𝑎𝑐𝑖𝑑]
[] = concentration
Acid-Base titration curves
Henderson Hassel Bach full equation
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𝑝𝐻 =𝑝𝐾𝑎 +log([𝐴 −]/[𝐻𝐴])
HA: acid
A- : base
pH < pKa: concentration of acid is greater than concentration of base
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pH> pKa: concentration of acid is less than concentration of base
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Buffering Example
Acetic acid buffer (HOAc pKa = 4.76) containing 1M HOAc and 1M Na+ Oac-
𝑝𝐻 =𝑝𝐾𝑎 +log([𝐴 −]
[𝐻𝐴] )
𝑝𝐻 = 4.76 +log([1]
[1])
𝑝𝐻 = 4.76
NEXT! Add 0.1 mol of HCL to 1.0 L of Buffer
[HOAc] = 1 M + 0.1 M = 1.1 M
[OaC-] = 1M -0.1 M = 0.9 M
This is what happens to the pH of the buffer with the addition of acid.
The acid drives the concentration of total acids up while bringing
down the concentration of base causing the pH to go down.
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𝑝𝐻 = 4.76 +log([0.9]
[1.1])
𝑝𝐻 = 4.673@𝑜𝑟@4.7
Titration of Glycine
Isoelectric point (pI): pH at which average charge on molecule is neutral
(0)
Average of two pKa values surrounding isoelectric species
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𝑝𝐼 =𝑝𝐾𝑎@𝐻𝐴 + 𝑝𝐾𝑎@𝐴 −
2
Ionic properties of amino acid side chains
Surface properties that effect protein ligand binding
Ex. DNA-binding proteins
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Example of electrostatic interactions b/w macro-ions
DNA carries negative charge at neutral pH
Phosphate ions for backbone
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DNA molecules will repel each other BUT DNA with Protein will attract
each other
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Dependence of protein solubility on pH
Buffer is required to fully solubilize a given protein
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Chapter 2: Acids and Bases
Tuesday, May 15, 2018
9:06 PM
Document Summary
Arrhenius acids: compounds that form h3o+ if dissolved in water. Why do they accept electrons? ph scale: based on auto-ionization of water. Amphoteric: water is able to react as acid or base. Biologically relevant weak acids and their conjugate bases. Note: conjugate acid/base pairs differ by one proton. Most bio rxns take place b/w ph 6. 5 and 8. 0. Buffer: resist changes in ph following addition of acid or base that have + 1 ph units difference from pka of solution. +3 ph units: the group is deprotonated or fully protonated and the buffer effect ceases ph = pka of solution: group is 50% protonated/deprotonated. Henderson-hassel bach equation: can be used to calculate average charge on ionizable group at any ph. A- : base ph < pka: concentration of acid is greater than concentration of base ph> pka: concentration of acid is less than concentration of base. Acetic acid buffer (hoac pka = 4. 76) containing 1m hoac and 1m na+ oac-
