Glossary

24.5 Biological Amines and the Henderson–Hasselbalch Equation

3 min read•may 7, 2024

play crucial roles in our bodies, but their behavior depends on . The Henderson-Hasselbalch equation helps us understand how these molecules exist in different forms at various pH levels, especially in our cells.

Knowing the protonation state of amines is key to grasping their function. This equation lets us calculate the ratio of neutral to protonated forms, giving insight into how these molecules behave in our bodies' slightly basic environment.

Biological Amines

Protonation states using Henderson-Hasselbalch equation

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  • Relates pH, , and concentrations of and : pH=pKa+log([A−]/[HA])pH = pKa + log([A^-]/[HA])
  • For , use and ratio of base to : [pOH](https://www.fiveableKeyTerm:pOH)=pKb+log([B]/[BH+])[pOH](https://www.fiveableKeyTerm:pOH) = pKb + log([B]/[BH^+])
  • Calculate protonation state at (7.4):
    • Find pKb of amine
    • Convert pH to pOH: pOH=14−pHpOH = 14 - pH
    • Plug pOH and pKb into rearranged equation
    • Solve for [B]/[BH+][B]/[BH^+] ratio
  • [B]/[BH+][B]/[BH^+] ratio shows relative amounts of neutral and at given pH
  • This equation is crucial for understanding in biological systems

Protonated form of cellular amines

  • Cellular amines (, ) often have pKb > physiological pH (7.4)
  • When pKb > pH, amine more likely protonated
    • Amine is stronger base than OH−OH^- at physiological pH
  • Protonated form more stable under physiological conditions
  • Writing in protonated form reflects predominant cellular state
  • This concept is related to the Brønsted-Lowry theory of acids and bases

Neutral vs protonated weak bases

  • Use Henderson-Hasselbalch for weak bases to find [B]/[BH+][B]/[BH^+] at given pH
  • (CTC_T) = neutral ([B][B]) + protonated ([BH+][BH^+]):
    • CT=[B]+[BH+]C_T = [B] + [BH^+]
  • Let x=[B]x = [B], then [BH+]=CT−x[BH^+] = C_T - x
  • Substitute into equation and solve for xx
  • Neutral percentage: %B=(x/CT)×100%\%B = (x / C_T) \times 100\%
  • Protonated percentage: %BH+=((CT−x)/CT)×100%\%BH^+ = ((C_T - x) / C_T) \times 100\%

Henderson-Hasselbalch Equation

Protonation states using Henderson-Hasselbalch equation

  • Relates pH, pKa, and concentrations of weak acid and : pH=pKa+log([A−]/[HA])pH = pKa + log([A^-]/[HA])
  • For weak bases, use pKb and ratio of base to : pOH=pKb+log([B]/[BH+])pOH = pKb + log([B]/[BH^+])
  • Calculate protonation state at physiological pH (7.4):
    1. Find pKb of amine
    2. Convert pH to pOH: pOH=14−pHpOH = 14 - pH
    3. Plug pOH and pKb into rearranged equation
    4. Solve for [B]/[BH+][B]/[BH^+] ratio
  • [B]/[BH+][B]/[BH^+] ratio shows relative amounts of neutral and protonated amine at given pH

Neutral vs protonated weak bases

  • Use Henderson-Hasselbalch for weak bases to find [B]/[BH+][B]/[BH^+] at given pH
  • Total amine concentration (CTC_T) = neutral ([B][B]) + protonated ([BH+][BH^+]):
    • CT=[B]+[BH+]C_T = [B] + [BH^+]
  • Let x=[B]x = [B], then [BH+]=CT−x[BH^+] = C_T - x
  • Substitute into equation and solve for xx
  • Neutral percentage: %B=(x/CT)×100%\%B = (x / C_T) \times 100\%
  • Protonated percentage: %BH+=((CT−x)/CT)×100%\%BH^+ = ((C_T - x) / C_T) \times 100\%

Acid-Base Equilibria and Dissociation Constants

  • The Henderson-Hasselbalch equation is based on the concept of
  • (Ka) and (Kb) are key parameters in understanding acid-base behavior
  • pKa is the negative logarithm of Ka, used in the Henderson-Hasselbalch equation for acids
  • pKb is the negative logarithm of Kb, used in the Henderson-Hasselbalch equation for bases

Key Terms to Review (25)

Acid Dissociation Constant: The acid dissociation constant, denoted as Ka, is a quantitative measure of the strength of an acid in a solution. It represents the equilibrium constant for the dissociation of an acid in water, providing insight into the extent of ionization and the relative acidity of different acids.
Amino Acids: Amino acids are the fundamental building blocks of proteins, which are essential macromolecules that perform a vast array of functions in living organisms. These organic compounds contain an amino group (-NH2), a carboxyl group (-COOH), and a side chain (R-group) that varies among the different types of amino acids. The unique combination and sequence of amino acids give rise to the diverse structures and functions of proteins.
Base Dissociation Constant: The base dissociation constant, denoted as Kb, is a measure of the strength of a base in an aqueous solution. It quantifies the extent to which a base dissociates and releases hydroxide ions (OH-) when dissolved in water. This term is particularly relevant in the context of biological amines and the Henderson-Hasselbalch equation, as these topics involve the acid-base behavior of compounds containing amine functional groups.
Biogenic Amines: Biogenic amines are organic, nitrogenous compounds that are produced naturally in living organisms through the decarboxylation of amino acids. These compounds play crucial roles in various biological processes, including neurotransmission, hormone regulation, and cellular signaling.
Biological Amines: Biological amines are organic compounds containing a basic nitrogen atom. They are found naturally in living organisms and play crucial roles in various physiological processes, including neurotransmission, hormone regulation, and cellular signaling.
Buffer Solutions: Buffer solutions are aqueous solutions that resist changes in pH upon the addition of small amounts of an acid or base. They maintain a relatively stable pH and are essential in various chemical and biological applications, including organic chemistry and biochemistry.
Conjugate acid: A conjugate acid is formed when a base gains a proton (H+ ion) during a chemical reaction. It is the species that remains after a base has accepted a proton in the context of the Brønsted–Lowry acid-base theory.
Conjugate Acid: A conjugate acid is the species formed when a base accepts a proton (H+) in a Brønsted-Lowry acid-base reaction. It is the acid that results when a base is protonated, and it is a weaker acid than the original acid. Conjugate acids play a crucial role in understanding acid-base chemistry, the strength of acids and bases, and their behavior in various reactions, including SN1 reactions, amine basicity, and the Henderson-Hasselbalch equation for biological amines and amino acids.
Conjugate base: A conjugate base is the species that remains after an acid has donated a proton (H+ ion) during a chemical reaction. It is capable of gaining a proton in the reverse reaction, forming the original acid.
Conjugate Base: A conjugate base is the species formed when an acid loses a proton (H+) in an acid-base reaction. It is the base that is left behind when an acid donates a proton to another substance, becoming the conjugate acid-base pair. This term is central to understanding acid-base chemistry, as well as its applications in organic reactions and biological systems.
Henderson–Hasselbalch equation: The Henderson–Hasselbalch equation is a mathematical expression that relates the pH of a solution to the equilibrium concentrations of the acid and conjugate base forms of a weak acid. It is a fundamental concept in understanding the pH of biological systems, particularly the pH of blood and other bodily fluids.
Ionic Equilibrium: Ionic equilibrium refers to the dynamic balance between the concentrations of ions in a solution, where the rate of forward and reverse reactions involving the ions are equal. This concept is crucial in understanding the behavior of biological amines and the application of the Henderson-Hasselbalch equation.
Neutral Amine: A neutral amine is a type of organic compound containing a nitrogen atom with a lone pair of electrons, where the overall charge of the molecule is neutral. These amines play a crucial role in biological processes and the Henderson-Hasselbalch equation, which describes the relationship between pH and the acid-base equilibrium of a solution.
PH: pH, or the potential of hydrogen, is a measure of the acidity or basicity of a solution. It is a scale that ranges from 0 to 14, with 7 being neutral, values less than 7 being acidic, and values greater than 7 being basic or alkaline. The pH of a solution is directly related to the concentration of hydrogen ions (H+) present, and it is a critical factor in many chemical and biological processes.
Photon: A photon is a quantum of electromagnetic energy, essentially a particle of light that carries energy but has no mass. In the context of spectroscopy, photons interact with molecules to cause transitions between energy levels, which is fundamental to understanding molecular structure through techniques like infrared spectroscopy.
Physiological pH: Physiological pH refers to the normal range of pH values found in the body's fluids and tissues that are essential for maintaining optimal biological functions. It is a crucial factor in understanding the behavior and interactions of biological acids and bases as well as the role of biological amines in the body.
PKa: pKa, or the acid dissociation constant, is a measure of the strength of an acid in a solution. It represents the pH at which a particular acid is 50% dissociated into its conjugate base. This value is crucial in understanding the behavior and properties of acids, bases, and their reactions in organic chemistry.
PKb: pKb, or the base dissociation constant, is a measure of the strength of a base in an aqueous solution. It is the negative logarithm of the equilibrium constant (Kb) for the dissociation of a base, and it provides information about the extent to which a base will dissociate in water to form hydroxide ions and the conjugate acid.
POH: pOH is the negative logarithm of the hydroxide ion (OH-) concentration in a solution, and it is a measure of the acidity or basicity of a solution. It is an important concept in the context of biological amines and the Henderson-Hasselbalch equation, which are used to understand the pH and acid-base balance in living systems.
Protonated Amine: A protonated amine is a nitrogen-containing organic compound where the nitrogen atom has gained a positive charge by accepting a proton (H+) from the surrounding environment. This phenomenon is particularly relevant in the context of biological amines and the Henderson-Hasselbalch equation, which describe the acid-base equilibria of these compounds.
Protonation States: Protonation states refer to the different forms that a molecule can take on based on the presence or absence of protons (H+ ions) in its structure. This concept is particularly important in understanding the behavior and reactivity of biological amines and the application of the Henderson-Hasselbalch equation.
Total Amine Concentration: Total amine concentration refers to the combined concentration of all amine groups present in a given system or solution. Amines are organic compounds containing a nitrogen atom with one or more attached hydrogen atoms, and they play a crucial role in various biological processes as well as in the context of the Henderson-Hasselbalch equation.
Weak Acid: A weak acid is a type of acid that only partially dissociates into hydrogen ions (H+) and the conjugate base of the acid when dissolved in water. This partial dissociation results in a solution with a pH that is higher than that of a strong acid at the same concentration.
Weak Bases: Weak bases are chemical compounds that have the ability to accept protons (H+) in an aqueous solution, forming a conjugate acid-base pair. They are considered weak because they only partially dissociate in water, resulting in a pH that is less than 7 but greater than 0. In the context of 24.5 Biological Amines and the Henderson–Hasselbalch Equation, weak bases play a crucial role in regulating pH and understanding the behavior of amine-containing compounds in biological systems.
α-Amino acids: α-Amino acids are the building blocks of proteins, consisting of a central carbon atom (the α-carbon) to which an amino group, a carboxyl group, a hydrogen atom, and a distinctive side chain (R group) are attached. They play a crucial role in biochemistry as they polymerize to form peptides and proteins.
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