water chemistry 2012

8/2/2019 Water Chemistry 2012

1/27

WaterChemistry

Dr.AtifHassanKhirelsied

DepartmentofBiochemistry

InternationalUniversityofAfrica,Khartoum,Sudan


2/27

Biomoleculesassumespecificshapesinwater.

Biomoleculesundergochemicalreactionsinan

a ueous environment.

a er sa eyreac an nmanyreac ons,usua y n

theformofH+andOH ions.


3/27

Waterisoxidizedinphotosynthesistoproduce

molecularoxygen,

O2.

Ex enditure of ener under aerobic conditions

leadsto

the

reduction

of

O2 backtowater.


4/27

Thestructureofwaterapproximatesatetrahedron

withits

two

hydrogen

atoms

and

the

two

lone

pairs

ofelectronsofitsoxygenatomoccupyingthe

verticesofthetetrahedron.

Thehi helectrone ativit ofox enrelativeto

hydrogenresultsintheestablishmentofa

ermanentdi ole in

water

molecules.


5/27

The

3dimensional

structure

of

awater

molecule,


6/27

Thepolar natureofwaterresultsinelectrostatic

interaction(hydrogenbond) betweenneighboringwatermolecules.

Hydrogenbond


7/27

H dro enbonds

Waterisstronglyhydrogenbonded

Each

water

molecule

participates

in

four

hydrogen

bondswithitsneighbors.


8/27

H dro enbonds

Hydrogenbondscommonlyformbetweenwatermolecules

andthepolarfunctionalgroupsofbiomolecules,or

betweenthepolarfunctionalgroupsthemselves.


9/27

Thestronglyhydrogenbondedcharacterofwateris

responsiblefor

many

of

its

characteristic

properties,

mostnotably:

(a)Ahighheatoffusion,whichallowswatertoact

asaheatsink suchthat reaterheatlossis

necessaryforthefreezingofwatercomparedto

othersubstances

of

similar

molecular

mass.


10/27

moreheatmustbeinputtovaporizewatercompared

c

n

a y

o

sso vemos

po ar

compoun s.

(d)Anopenstructuremakesicelessdensethanliquidwater,therebymakingicefloat,insulatingthewaterbeneathit,andinhibitingtotalfreezingoflarge

bodiesofwater.


11/27

Avarietyofweakelectrostaticinteractionsarecritical

tothe

structure

and

reactivity

of

biological

molecules.

These interactions include in order of increasin

strength,

,

dipoledipoleinteractions,

,

ionicinteractions


12/27

Waterisanexcellentsolventofpolarandionicsu s ances ue o sproper yo surroun ngpo ar

moleculesandionswithorientedshellsofwater

therebyattenuatingtheelectrostaticforcesbetween

thesemoleculesandions.


13/27

Thetendencyofwatertominimizeitscontactwith

nonpolar

(hydrophobic)

molecules

is

called

the

hydrophobiceffect. This

effect

is

largely

driven

by

the

increase

in

entropy

causedb thenecessit forwatertoorderitself

aroundnonpolarmolecules.


14/27

Thehydrophobiceffectcausesthenonpolarmolecules

to

aggregate,

thereby

reducing

the

surface

areathatwatermustorderitselfabout.

Consequently,nonpolar substances

are

poorly

solubleina ueoussolution.


15/27

Manybiologicalmoleculeshavebothpolar(or

charged)and

non

polar

functional

groups

and

are

thereforesimultaneouslyhydrophilicand

hydrophobic.

Thesemoleculesaresaidtobeam hi hilic or

amphipathic.


16/27

Osmosisisthemovementofsolventacrossasem permea e mem rane romareg ono ower

concentrationofsolutetoare ionofhi her

concentrationofsolute.


17/27


18/27

Diffusion istherandommovementofmoleculesin

so u on or n egasp ase .

Itisresponsibleforthemovementofsolutesfroma

regionofhighconcentrationtoaregionoflow

concentration.


19/27

Waterisaneutral,polarmoleculethathasaslight

tendencyto

ionize

into

H

+

andOH

.

The roton is never free and binds to a water

moleculeto

form

H3O+

(hydroniumion).


20/27

Theionizationofwaterisdescribedasanequilibrium

betweenthe

unionized

water

(reactant)

and

its

ionizedspecies(products)

Inwhich


21/27

Theconcentrationofwaterisessentiallyconstant

(55.5M),

the

concentration

of

H2O

is

incorporated

intothevalueofK,whichisreferredtoasKw,theionizationconstantofwater.


22/27

ThevaluesofbothH+ andKareinconvenientlysmall;

hence,their

values

are

more

conveniently

expressed

asnegativelogarithms,sothat


23/27

BrnstedLowrydefinition,anacidisasubstancethat

can

donate

aproton,

and

abase

is

asubstance

thatcanacceptaproton.

Thestrength

of

aweak

acid

is

proportional

to

its

dissociationconstant whichisex ressedas


24/27

T epHo aso utiono awea aci is etermine y

the

relative

concentrations

of

the

acid

and

its

conjugate ase.

Theequilibrium

expression

for

the

dissociation

of

a

weakacidcanberearran edto


25/27

H n r nH l l h i n

Whentheconcentrationofaweakacidise ualto

theconcentrationofitsconjugatebase,pH=pK.

Hence,the

stron er

the

acid,

the

lower

its

K.


26/27

SolutionsofaweakacidatpHsnearitspK resistlargechangesinpHasOH orH+ isadded.

Added rotons react with the weak acids con u ate

baseto

reform

the

weak

acid;

whereas

added

OH

combineswiththeacidtoformitscon u atebase

andwater.


27/27

Asolution

of

aweak

acid

and

its

conjugate

base

(in

theformofasalt)isreferredtoasabuffer. Buffers

are

effective

within

1pH

unit

of

the

pK ofthe

.

water chemistry 2012

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