Water & pH

Water

• The predominant chemical component of living organisms

• The ability to solvate a wide range of organic and inorganic molecules– Dipolar structure

• a high dielectric constant– Reflecting the number of dipoles

– Capacity for forming hydrogen bonds

Water

• Influences the structure of biomolecule • Water is a reactant or product in many

metabolic reactions

Water• Water has a slight propensity to dissociate

into hydroxide ions and protons• The acidity of aqueous solutions is generally

reported using the logarithmic pH scale• Buffers normally maintain the pH of

extracellular fluid between 7.35 and 7.45.• Suspected disturbances of acid-base balance

are verified by measuring the pH of arterial blood and the CO2 content of venous blood

WATER IS AN IDEAL BIOLOGIC SOLVENT

• Water Molecules Form Dipoles

WATER IS AN IDEAL BIOLOGIC SOLVENT

• the strength of interaction F between oppositely charged particles is inversely proportionate to the dielectric constant ε of the surrounding medium.

• Water therefore greatly decreases the force of attraction between charged and polar species relative to water-free environments with lower dielectric constants.

Water Molecules Form Hydrogen Bonds

Biomolecules,have functional groups

Water as solvent

• Hydrophilic

• Hydrophobic – The forces that hold the nonpolar regions of the

molecules together are called hydrophobic interactions.

• Most biomolecules are amphipathic;– Proteins – Phospholipid bilayer– Biomolecules fold to position polar & charged

groups on their surfaces• It minimizes energetically unfavorable contact between

water and hydrophobic groups

• WATER IS AN EXCELLENT NUCLEOPHILE– Nucleophilic attack by water generally results in

the cleavage of the amide, glycoside, or ester bonds

• Hydrolysis

Water as a product

• when monomer units are joinedtogether to form biopolymers

– such as proteins or glycogen

• The effect of solutes on osmolarity depends on the number of dissolved particles, not their mass

• The high concentration of albumin and other proteins in blood plasma contributes to its osmolarity.

• Cells also actively pump out ions such as Na into the interstitial fluid to stay in osmotic balance with their surroundings.

Regulation of water balance

• Hypothalamic mechanisms that control thirst• Antidiuretic hormone (ADH)• Retention or excretion of water

– Kidneys, and on evaporative loss

• Nephrogenic diabetes– Inability to concentrate urine– Unresponsiveness of renal tubular osmoreceptors

to ADH.

Acidity of aqueous solutions (pH)• pH = −log [H+ ]• Low pH values

– Correspond to high concentrations of H+

• High pH values• Correspond to low concentrations of H+

• Acids– are proton donors

• Bases– are proton acceptors

• K w=[H+][OH−]=10−14

• pH+pOH=14• pH = −log [H+ ]

pH• Strong acids

– Have larger dissociation constants

• Weak acids dissociate only partially• Strong bases (eg, KOH or NaOH)

– the strong base KOH is completely dissociated in solution and that the concentration of OH ions is thus equal to that of the KOH

• Weak bases (eg, Ca[OH]2)

• Many biochemicals are weak acids

• Many biochemicals possess functional groups that are weak acids or bases

• We express the relative strengths of weak acids and bases in terms of their dissociation constants (Ka)– expressing the extent of ionization of water in

quantitative terms.

• The equilibrium constant is fixed and characteristic for any given chemical reaction at a specified temperature.

The tendency of any acid (HA) to lose a proton and form its conjugate base (A-) is defined by the equilibrium constant

• pKa is used to express the relative strengths of both acids and bases.

• polyproteic compounds– Containing more than one dissociable proton, a

numerical subscript is assigned to each in order of relative acidity

• the pKa is the pH at which the concentration of the acid (R-NH3+) equals that of the base (R-NH2).

• pKa of an acid group is the pH at which the protonated and unprotonated species are present at equal concentrations

• The pKa Values Depend on– Molecular Structure

• The presence of adjacent negative charge– Decreases with distance

– Properties of the Medium

• Buffer(s)– Solutions of weak acids or bases and their

conjugates exhibit buffering– Maximum buffering capacity

• Most effectively in the pH range pKa ± 1.0 pH unit.

– Physiologic buffers

– The value of pKa relative to the desired pH is the major determinant of which buffer is selected.

• The pH of an aqueous solution can be approximately measured using– indicator dyes including

• Litmus, phenolphthalein, and phenol red, which undergo color changes as a proton dissociates from the dye molecule

• a glass electrode that is selectively sensitive to H+

concentration

The pH of some aqueous fluids

• Disturbances of acid-base balance– Measuring the pH of arterial blood– The CO2 content of venous blood

• Acidosis (blood pH < 7.35)– Causes

• include diabetic ketosis• Lactic acidosis

• Alkalosis (pH > 7.45)– Vomiting of acidic gastric contents

Weak Interactions Are Crucial to Macromolecular Structure and Function

• Macromolecules such as proteins, DNA, and RNA contain so many sites of potential hydrogen bonding or ionic, van der Waals, or hydrophobic interactions

• the cumulative effect of the many small binding forces can be enormous.

• For macromolecules, the most stable (that is, the native) structure is usually that in which weak-bonding possibilities are maximized

• The folding of a single polypeptide or polynucleotide chain into its three-dimensional shape

• The binding of an antigen to a specific antibody• Formation of an enzyme-substrate complex• The binding of a hormone or a

neurotransmitter to its cellular receptor protein

Titration curve for an acid of the typeHA. The heavy dot in the center of the curve indicatesthe pKa 5.0.

H+(H2O)n

Water Is a Weak Electrolyte

Protons that dissociate interact with oxygens of other water molecules to form clusters of water molecules.

• The dissociation of an acid increases with increasing temperatures.

• Keq will be a – Small number if the degree of dissociation of a

substance is small.– Large if the degree of dissociation is large

• An extremely small number of water molecules actually dissociate– At 25°C the value of Keq for dissociation of water is

about 1.8 x 10-16

• The relationship between pK' and Keq is an inverse one– The smaller the Keq the larger the pK’

• The Relationship between pH and Concentrations of Conjugate Acid and Base

• If [base]/[acid] is 1: 1, pH equals the pK’

• Must be taken into account for a buffer – (best)Buffering range

– Between 1 pH unit below and 1 pH unit above pK‘.

– Buffering capacity• Depends on concentrations of conjugate acid and base

• To monitor the acid-base parameters of a patient's blood.– Values of interest to a clinician

• pH,HC03 - and CO2 concentrations.

• Normal values

pH = 7.40, [HC03 -] =24.0 mM, [C02] =1.20 mM.