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Basicity of Heterocyclic Aromatic Amines

Basicity of Heterocyclic Aromatic Amines

Heterocyclic amines, where the N atom is a part of an alicyclic system, are similar in basicity to alkylamines. Interestingly, the heterocyclic amine having a nitrogen atom as part of an aromatic ring has much less basicity than its corresponding alicyclic counterpart. For this reason, as presented in Figure 1, piperidine (pKb = 2.8) is significantly more basic than pyridine (pKb = 8.8).

Basicity of Aliphatic Amines

Basicity of Aliphatic Amines

Amines can behave as Brønsted–Lowry bases by accepting a proton from the acid to form corresponding conjugate acids. Due to a lone pair of nonbonding electrons, aliphatic amines can also act as Lewis bases by forming a covalent bond with an electrophile.
To measure the basicity of amines, two conventions are generally used. The first defines Kb as the basicity constant for the deprotonation reaction of water by the amine, as presented in Figure 1. Conventionally, lower Kb indicates...

Regioselectivity and Stereochemistry of Acid-Catalyzed Hydration

Regioselectivity and Stereochemistry of Acid-Catalyzed Hydration

The rate of acid-catalyzed hydration of alkenes depends on the alkene's structure, as the presence of alkyl substituents at the double bond can significantly influence the rate.

Polyprotic Acids

Polyprotic Acids

Acids are classified by the number of protons per molecule that they can give up in a reaction. Acids such as HCl, HNO3, and HCN that contain one ionizable hydrogen atom in each molecule are called monoprotic acids. Their reactions with water are:

¹H NMR of Labile Protons: Deuterium (²H) Substitution

¹H NMR of Labile Protons: Deuterium (²H) Substitution

This lesson illustrates the role of deuterium substitution in simplifying the NMR spectrum of compounds comprising labile protons. One method employed is the use of deuterium. Amongst the three isotopes of hydrogen, deuterium (2H) has a nucleus composed of one proton and one neutron. When the D2O solvent is added to a pure dry ethanol solution, its labile proton is substituted with deuterium.

Proton (¹H) NMR: Chemical Shift

Proton (¹H) NMR: Chemical Shift

Organic molecules primarily contain carbon and hydrogen atoms. While all the hydrogen isotopes are NMR-active, protium or hydrogen-1 is the most abundant. It has a significant energy separation between its nuclear spin states due to its large gyromagnetic ratio. As per Boltzmann's distribution, an increase in the energy separation implies a greater excess population of nuclei available for excitation, resulting in a strong NMR absorption signal.
Absorption signals of all the protium nuclei...

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Related Experiment Video

Updated: Jun 19, 2025

Sedimentation Equilibrium of a Small Oligomer-forming Membrane Protein: Effect of Histidine Protonation on Pentameric Stability

Sedimentation Equilibrium of a Small Oligomer-forming Membrane Protein: Effect of Histidine Protonation on Pentameric Stability

Published on: April 2, 2015

Protonation of histidine rings using quantum-mechanical methods.

Nigel W Moriarty¹, Jonathan Moussa², Paul D Adams¹

¹Molecular Biophysics and Integrated Bioimaging, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.

Acta Crystallographica. Section D, Structural Biology

|July 25, 2024

Summary

Understanding histidine protonation is key for its biological roles. This study introduces a quantum-mechanical method to accurately determine histidine’s most likely protonation state in situ, improving predictions of its function.

Keywords:

histidine macromolecular refinement quantum-mechanical predictions

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Sedimentation Equilibrium of a Small Oligomer-forming Membrane Protein: Effect of Histidine Protonation on Pentameric Stability

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Published on: April 2, 2015

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Neutron Crystallography Data Collection and Processing for Modelling Hydrogen Atoms in Protein Structures

Neutron Crystallography Data Collection and Processing for Modelling Hydrogen Atoms in Protein Structures

Published on: December 1, 2020

Area of Science:

Biochemistry
Computational Chemistry
Structural Biology

Background:

Histidine's imidazole ring has two nitrogen atoms, leading to multiple protonation states.
The stereochemical environment influences which protonation state is adopted.
Accurate determination of histidine protonation is crucial for understanding protein function, enzyme catalysis, and metal binding.

Purpose of the Study:

To present a novel quantum-mechanical method for determining histidine protonation states.
To accurately predict the most likely in situ protonation state of histidine.
To enhance the understanding of histidine's role in biological systems through precise structural analysis.

Main Methods:

Utilizing quantum-mechanical calculations to determine minimum geometry and energy.
Performing in situ analysis of histidine's stereochemical environment.
Comparing calculated protonation states to identify the most stable and likely form.

Main Results:

The quantum-mechanical method provides a more accurate identification of histidine protonation states compared to atomic modeling.
Calculations allow for precise prediction of histidine's in situ protonation based on its local environment.
The study validates the efficacy of computational approaches in resolving stereochemical ambiguities.

Conclusions:

Quantum-mechanical calculations offer a direct and accurate method for determining histidine protonation states.
This approach improves the prediction of histidine's role in hydrogen bonding and metal coordination.
The findings contribute to a more precise understanding of histidine's function in complex biological structures.