2 - pH & Buffers

⬇pH = __ Proton concentration

⬇pH = ⬆️ Proton concentration

what’s the formula regarding pH and H⁺?

pH = -log[H⁺] / [H⁺] = 10^-pH

mM = 10 _

μM = 10 _ 

nM = 10 _

mM = 10^-3

μM = 10^-6

nM = 10^-9

at lower pH values, more __ are present to __ different functional groups

at lower (⬇) pH values, more H+ are present to protonate different functional groups

what does pKa values measure?

what is the formula for pKa?

• measures the strength of an acid (HA), tells the pH at which a functional group loses/gains H⁺

• pKa = -log Ka, where Ka = [H⁺][A^-] / [HA]

  • Ka = acid dissociation constant

  • [H⁺] = concentration of H⁺

  • [A^-] = concentration of conjugate base (deprotonated form of acid)

  • [HA] = concentration of undissociated acid

if pH < pKa:

if pH > pKa:

if pH = pKa:

if pH < pKa: mostly protonated

if pH > pKa: mostly deprotonated

if pH = pKa: equal probability of protonated/deprotonated species

strong acids have _ Ka and _ pKa

strong acids have ⬆ Ka (high dissociation) and ⬇ pKa → donate protons readily, it will be deprotonated @ physiological pH

which functional groups are neutral when protonated at pH = 1.0?

• pH = 1.0 is highly acidic, most groups with pKa values > 1 will be protonated

• e.g. -COO^- → COO, -NH² → NH³⁺

• one can also be protonated multiple times & have multiple pKa values

What is Isoelectric Point, what does it measure?
What’s the formula?

• Isoelectric point (pl) is the pH at which a molecule carries no net electrical charge → a zwitterion is present. (+ & - charges on the molecule are balanced)

• pl = (pKa1 + pKa2) / 2

  • (pKa1 + pKa2) = the average of 2 pKas around the neutral species

    • pKa1 = pKa for 1st proton lost

    • pKa2 = pKa for 2nd proton lost… so on

What’s a Zwitterion?

a neutral molecule that has separate + & - charged functional groups

pH < pl:

pH > pl:

pH < pl: molecule tends to carry net positive charge (more groups protonated) - cation

pH > pl: molecule tends to carry net negative charge (more groups deprotonatd) - anion

calculate the pl

What’s the Henderson-Hasselbalch Equation? What does it measure?

• pH = pKa + log ([A^-]/[HA])

  • x = log → 10^x

• determines the pH of a solution depending on the amount of ionization of a weak acid (protonated [HA] vs. deprotonated [A^-])

• can be used to determine the pH of a mixture of weak acid & conjugate base

What’s a Buffer? When are they most effective?

• mixture of weak acids & conjugate bases that can resist pH changes, maintaining a protein’s structure/function

• by neutralizing small addition/loss of H⁺, keeping pH stable

• most effective at pKa (within range of ±1 pH unit around pKa)

• H₂O is a poor buffer - lacks significant amounts of a weak acid and its conjugate base

What’s the equation for Total buffer concentration?

C_total = [HA] + [A^-]

what’s the main buffering species in Blood?

Bicarbonate (HCO₃^-)

what’s the Bohr Effect?

• pH can affect oxygen-carrying ability of Hb (hemoglobin)

• ⬇ blood pH / ⬆ blood CO₂ = ⬇ Hb affinity for O₂, therefore ⬆ Hb release O₂