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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 higher...

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 Aromatic Amines

Basicity of Aromatic Amines

The basicity of aromatic amines is much weaker than that of aliphatic amines due to the involvement of the lone pair of electrons over the N atom in resonance with the aryl rings. Generally, the electron-donating ability of any substituents on the aryl ring of aromatic amines increases the basicity of the amine by increasing electron density, and hence the availability of lone pair on the nitrogen. On the other hand, electron-withdrawing functional groups on the aryl ring of amines decrease the...

Amino acids

Amino acids

Amino acids are the monomers that comprise proteins. Each amino acid has the same fundamental structure, which consists of a central carbon atom, or the alpha (α) carbon, bonded to an amino group (NH2), a carboxyl group (COOH), and to a hydrogen atom. Every amino acid also has another atom or group of atoms bonded to the central atom known as the R group. There are 20 common amino acids present in proteins, each with a different R group. Variation in the amino acid sequence is responsible for...

Physical Properties of Amines

Physical Properties of Amines

Amines with low molecular weight are usually gaseous at room temperature, while those with high molecular weight are liquid or solids in nature. Usually, low molecular weight amines have a rotten fish-like smell. Diamines typically have a pungent smell. For instance, cadaverine and putrescine, depicted in Figure 1, are two molecules responsible for decaying tissue.

Overview of Functional Groups

Overview of Functional Groups

Functional groups are a group of atoms with characteristic properties, which when linked to the carbon skeleton of a molecule, alter the properties of that molecule. For example, certain functional groups will make a molecule hydrophilic, whereas others will make them hydrophobic. These functional groups are an indispensable part of organic chemistry and important components of biological molecules, such as carbohydrates, proteins, lipids, and nucleic acids. Each functional group is a unique...

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Related Experiment Video

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Determination of the Gas-phase Acidities of Oligopeptides

Determination of the Gas-phase Acidities of Oligopeptides

Published on: June 24, 2013

Gas phase basicities of polyfunctional molecules. Part 3: Amino acids.

¹Département de Chimie, Laboratoire des Mécanismes Réactionnels, Ecole Polytechnique, 91120 Palaiseau, France. bouchoux@dcmr.polytechnique.fr

Mass Spectrometry Reviews

|May 22, 2012

Summary

This review compiles experimental and theoretical data on the gas-phase protonation thermochemistry of 20 proteinogenic amino acids. It presents a unified dataset and discusses comparisons between experimental and computational findings.

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Area of Science:

Physical Chemistry
Computational Chemistry
Biochemistry

Background:

Gas-phase protonation thermochemistry is crucial for understanding molecular interactions.
Amino acids, as fundamental biological building blocks, exhibit complex protonation behaviors.
Previous reviews have established a foundation for studying polyfunctional molecules.

Purpose of the Study:

To provide a comprehensive overview of gas-phase protonation thermochemistry for 20 proteinogenic amino acids.
To present a harmonized dataset of experimental values and theoretical calculations.
To compare experimental and theoretical results and propose evaluated thermochemical parameters.

Main Methods:

Compilation and re-assignment of experimental data (equilibrium, thermokinetic, kinetic methods) using the Hunter & Lias 1998 gas-phase basicity scale.
Review of theoretical investigations, including conformational analysis of neutral and protonated amino acids.
High-level theoretical computations (e.g., Gn procedures) for proton affinities and thermochemical parameters using Boltzmann distribution at 298K.

Main Results:

An extensive collection of experimental gas-phase protonation data for amino acids, re-evaluated for consistency.
Theoretical proton affinities derived from the most stable conformers and Boltzmann-averaged calculations.
Detailed comparison between experimental and theoretical protonation thermochemistry.

Conclusions:

A unified and evaluated dataset of proton affinities, gas-phase basicities, and protonation entropies for amino acids is proposed.
Theoretical methods, when applied to stable conformers, provide reliable predictions of protonation thermochemistry.
This work serves as a valuable resource for researchers in physical chemistry, biochemistry, and mass spectrometry.