biochem 1 lm03slides_s16

7/24/2019 Biochem 1 LM03slides_S16

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Lecture Notes for

Chapter 2Aqueous Chemistry

Essential BiochemistryThird Edition

Charlotte W. Pratt | Kathleen Cornely

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+

!Hydrogen ions or protons combine with H2O to

form hydronium ions (H3O+)

! Probably delocalized

!Proton jumping

! H+relayed through network of water molecules

! Mobility greater than simple diffusion

!

Like crowd surfing

(Free) Protons Dont Exist2


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+

!

Ionization of water! H2O"#H

++ OH-

![H2O] (55.5 M) >> [H+] or [OH-]

! DefineKW, ionization constant of water

The pH Scale3

K=

H+!" #$ OH

%!" #$

H2O[ ]

KW = K H

2O[ ] = H+!" #$ OH

%!" #$


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+

If [H+] is > 10-7M, then [OH-] must be


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+

!Concentrations of H+and OH-on very small scale

!Lets convert to positive whole integers

!

pH = power of hydrogen(etymology uncertain)

pH Scale5

pH = ! log H+"# $%


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+

!What happens if [H+] increases

above 10-7M?

!Will the number in the exponent get

larger or smaller?!So, as solution gets more acidic, pH

decreases

pH Scale6


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+Acid-Base Chemistry

!Using Brnsted-Lowry definition of acids &

bases

!Acid = proton donating

!HCl + H2O#H3O++ Cl-

!Base = proton accepting

!NaOH + H3O+#Na++ 2H2O

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!

Most biological acids/bases are not strong!CH3COOH + H2O"#CH3COO

-+ H3O+

!Ka= acid dissociation constant

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Much larger

K=

CH3COO

!"# $% H3O+"# $%

CH3COOH[ ] H2O[ ]

Ka = K H

2O[ ] =

CH3COO!"# $% H

+"# $%

CH3COOH[ ]


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!Kastill a small numberlets do the whole integer

thing again

!The more acidic the compound, the larger the value

ofKa

!The larger the value ofKa, the smaller the value of pK

!As pK$, acid strength%

! Greater tendency to donate H+

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pK = pKa= ! logKa


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+Henderson-Hasselbalch

!How are pH and pKrelated?

!Consider the dissociation of a weak acid:

!HA = acid; A-= conjugate base

!Expression forKa:

10

HA! A"

+ H+

Ka =

A!"# $% H

+"# $%

HA[ ]


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+ Henderson-Hasselbalch

!Rearrange:

!Take the (-) log of both sides

!Different form:

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!log H+"# $%= ! logKa! logHA[ ]A!"# $%

pH=pK+ logA!"# $%

HA[ ]

H+!" #$= Ka

HA[ ]A

%!" #$

Ka =

A!"# $% H

+"# $%

HA[ ]


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!

What happens when [A-] = [HA]?

!pH = pK

!So the pKis the pH at which!of the groups

are deprotonated (ionized)

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Conjugate base

Acid

Different forms

of same grouppH=pK+ logA!"# $%

HA[ ]


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!Many groups on biological molecules candonate or accept H+

!Act as acids and bases

! E.g. NH2/NH3+ and COO-/COOH on amino

acids

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pH=pK+ logA!"# $%

HA[ ]


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+Polyprotic acids ionize more than

once and have multiple pK

s!H3PO4= phosphoric acid pK1= 2.15

!H2PO4-= monobasic pK2= 6.82

!HPO42- = dibasic pK3= 12.38

!PO43-= tribasic

H3PO4 H2PO4- HPO4

2- PO43-

+

H+

+

H++

H+

pK1 pK2 pK3

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+Determining Protonation State

!When pH = pK, [HA] = [A-]; group is!protonated

!When pH < pK:

!So if pH < pK, the group is more protonated

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pH - pK = logA!"# $%

HA[ ]

If pH < pK, then pH - pK= (-) = logA!"# $%

HA[ ]

A!"# $%

HA[ ] A

!"# $%


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+Determining Protonation State

!When pH = pK, [HA] = [A-]

!When pH > pK:

!So if pH > pK, the group is more deprotonated

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pH - pK = logA!"# $%

HA[ ]

If pH > pK, then pH - pK= (+) = logA!"# $%

HA[ ]

A!"# $%

HA[ ]>1 and A

!"# $% > [HA]


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+Aqueous Buffers

!When you add a strong acid to water, all of the acid

contributes to pH$

! HCl#H++ Cl-

!

When you mix a strong acid with a weak acid(conjugate base), the pH change is less

! HCl + A-#HA + Cl-

! The conjugate base of the weak acid can accept some

of the H+

!Acid neutralized

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+Aqueous Buffers

!Same thing goes for strong base + weak acid

! NaOH + HA#Na++ A-+ H2O

! Weak acid donates H+to some of the OH-

!

Base neutralized

!Buffers resist pH changes

! Buffers: acid (HA) + conjugate base (A-)

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+Buffering Capacity

!

Titration curve for acetic acid

!Rapid change in pH at the start and end points

!More gradual change in-between

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!Best buffering capacity in

flat area of curve

!

~1 pH unit on either side ofmidpoint (pK)


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+Convenient Rule of Thumb

pH (range) = pK 1

! This means [A] = 10 [HA]

! This means [A] = 1/10 [HA] or [HA] = 10 [A-]

10

0.1

Buffers tend to resist changes in pH when [HA] and [A]

differ by no more than a factor of 10.

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+Maintaining pH in Blood

!

Normal internal pH is 6.9-7.4

!Metabolic reactions generate acid

! How can the pH be adjusted?

!

CO2(g) in blood forms carbonic acid (H2CO3):! CO2+ H2O"#H2CO3"#H

++ HCO3-

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pK~3.6


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+Maintaining pH in Blood

!

CO2(g) in blood forms carbonic acid (H2CO3):! CO2+ H2O"#H2CO3"#H

++ HCO3-

! Occurs spontaneously, but carbonic anhydrase helps

! Lungs expel CO2(g), pulls equilibrium away from low pH

!

Law of mass action

! Changes in lung function adjust blood pH in min or hours

!What about a more long term way of adjusting pH?

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+Maintaining pH Homeostasis

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!Kidneys play major role in buffering metabolic acids! Filter HCO3

- from bloodstream into kidney filtrate

! Need way to reclaim HCO3- for buffering acids inside the

kidney cell

1) Antiporter moves H+

out of cell and Na+

in (ionexchanger)

From

bloodstream

2) H+combines with HCO3-to

form CO2! CO2diffuses across membrane


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3) Converted back to HCO3- to

neutralize acids in the cell

! (H+pumped out)

Frombloodstream

Metabolites are in

constant motion


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!How do you replenish bicarbonate in the bloodstream?

!Metabolic processes generate CO2

!

Use carbonic anhydrase to regenerate H++ HCO3-

!Pump H+out into filtrate (urine) and HCO3-into blood

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biochem 1 lm03slides_s16

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