Download Ionization of Functional Groups in Water: pH Effects and Buffer Role - Prof. Jerry J. Bran and more Study notes Biology in PDF only on Docsity!
BIO 311CSpring 2009
Graded exams will be returned during discussion periods today and tomorrow. A Key to Exam 1 will be placed on the course web site on Wednesday. If you believe your Exam 1 has been scored inaccurately, take your exam toJingjie and explain your concern to her on or before this Friday, Feb. 20. If youstill have a concern, then please see me. We will not consider any changes afternoon on Monday, Feb. 23. The average for Exam 1 was 72%. The high grade was 94%. Approximately 1/3 of the class received a grade higher than 80%. Lecture 11 – Monday 16 Feb.
Reversible Ionization of Water
+^ HO+ OH 3
HO + H^2
IonizationO 2 De-ionization
Definition: pH = - log
+[HO]^ molar 10 3
In pure water, at any instant of time: [H
+-O] = [OH] = 1 X 10 3
-7^ M
Thus, in pure water: pH = - log
-7(1 X 10) = 7.0 10
Note: reversible arrows for the reaction show that the reaction goes inboth directions.^ A longer arrow pointing toward H
O shows that at equilibrium there 2 is more HO than H^2
+^ - Oor OH. 3
Review & Summary
The name of an ionizable functional group is slightly different,depending on its ionization state. The ionized and non-ionized formsof functional groups have very different chemical properties.
O R C O^ H
O R C- O
carboxylic acid
carboxylate
H R N^ H
H +H R N^ H
amine^
ammonium ion
phosphoric acid
phosphate (monobasic)
phosphate^ (dibasic)^
Although the pH of pure
water is 7.0, some substances change the pH
Def. When one or more substances is dissolved in water, the result is anaqueous solution.^ A teaspoon of sugar dissolved in a glass of water produces an aqueous soln.^ The cytoplasmic matrix of cells is an aqueous soln.when they are dissolved in water.^ Example: When a molecule withcarboxylic acid functional group isdissolved in water, the resulting aqueoussolution has an increased concentration+of HO. This can be expressed as a^3 decrease in the pH of the solution. Def: An aqueous solution with a pH of (or very near to) 7 is said to be neutral
An aqueous solution with a pH lower than 7 is said to be acidic
An aqueous solution with a pH higher than 7 is said to be alkaline
or basic.^ *
R-COOH
+R-NH (^3) R O -O O P^ OH
R^ O
O - O PO -
acidic pH Carboxylic acid Ammonium ion Phosphate ion
neutral pH and basic pH
Carboxylate ion
acidic pH and neutral pH
basic pH Amine
Phosphate^ ion
acidic pH^
basic pH
neutral^ pH (7)
acidic^
basic
pH 2^
pH 12
Not all compartments in a living cell are at the same pH value.
Textbook Fig. 6.9, p. 100^ The cytoplasmic matrix andmost compartments are nearneutral pH (~ 7.4).
The Mitochondrial matrix is at a alkaline pH (~8) Lysosomesare at anacidic pH (~2).
acidic or neutral compartment of acell such as the cytoplasmic matrix
basic compartment of a cell such atthe matrix of a mitochondrion
Effect of pH of a Cellular Compartment on the Ionization
State of an Amine Functional Group
H +H R N^ H
H R N^ H
ammonium ion^
amine
~ ½^ ~ ½
Effect of pH of a Cellular Compartment on the IonizationState of a phosphate Functional Group
neutral compartment^ of a cell acidic compartment^ of a cell
basic compartment^ of a cell
monobasic^ phosphate Note: in a very acidic environment (more acidic than shownhere) phosphate would not carry any net negative chargeand should be called phosphoric acid.
dibasic phosphate
If only one or a very few molecules with an ionizable functionalgroup are placed into a compartment of a cell, then the pH (therelative amounts of H
- (^) Oand OH) of the compartment will 3 determine the ionization state of the functional group.^ If a large number of molecules with an ionizable functional groupare placed into a compartment of a cell (large enough to make itsconcentration of the functional group as high or higher than theconcentration of H
- (^) Oor OH), then the pH of the compartment 3 will be affected by the ionization state of the functional group.
Effects of Adding Molecules with an Ionizable Functional Group^ to a Compartment of a Cell
pH Buffer:^
A chemical buffer that resists a change in theconcentration of H
+^ Oin an aqueous solution. 3
Definitions: Buffer: an agent that prevents or retards a change in thestate of a chemical or physical system. Chemical Buffer: a chemical substance agent resists a
change in concentration of someother chemical substance in asolution.
note: In molecular biology "buffer" always means "pH buffer” unless stated otherwise. note: "The concentration of H
+^ O" is sometimes written in more abbreviated form as 3 "the concentration of H
+^ +", or as “[ H
]”.
note: in order to act as an effective pH buffer, the concentration of the functionalgroup must be as high or higher than the concentration of H
+^ - Oor OH. 3
A population of ionizable functional groups can serve as a buffer at pH valuesnear where half are in one ionization state and half in another ionization state.
Example: Typical carboxylic acid^ O^ R^ C ½^ O^ H^ O^ R^ C ½-^ O
solution at apH of 3.
Definition: Consider a population of identical organic molecules with an ionizablefunctional group at a specific location on the molecule. The pK value forthat functional group is the pH at which, at any instant of time, thefunctional group on half of the molecules is in one ionization state and inthe other half of the molecules is in the other ionization state. The carboxylic acid functional group shown here has a pK value of 3.0. Itcould serve as an effective buffer at pH values ranging from ~2 to ~4.
compartment 1^ pH 2.5 compartment 2^ pH 5. The exact pK value of a specific functional group dependents on thenature of the R-group to which it is attached.
This carboxylic acid, with a pK value of2.5, would serve as a good buffer in anycompartment with a pH somewherebetween approximately 1.5 - 3.5.^ This carboxylic acid, with a pK value of5.5, would serve as a good buffer in anycompartment with a pH somewherebetween approximately 4.5 - 6.5.
The effectiveness of a functional group as a pH buffer depends on: (1) the pK value of the functional group, and (2) the pH of compartment within which it resides.
negatively^ charged^ surface
Electrovalent (Ionic) Bonding of an Organic Ammonium
Ion to a Negatively Charged Surface
electrovalent bond
Electronic Bonding of a Carboxylate Ion^ to an Organic Ammonium Ion
R^2
electrovalent bond
