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SN1 Reaction: Mechanism

S_N1 Reaction: Mechanism

Kinetic studies of ionization of a tertiary halide in a protic solvent suggest that only the substrate participates in the rate-determining step (slow step). The nucleophile is involved only after the slowest step. The SN1 reaction takes place in a multiple-step mechanism.
Firstly, the haloalkane ionizes to generate a carbocation intermediate and a halide ion. This heterolytic cleavage is highly endothermic with large activation energy. The ionization of the substrate, facilitated by a polar...

SN2 Reaction: Kinetics

S_N2 Reaction: Kinetics

Kinetic Studies and Significance
In a chemical reaction, a relationship exists between the concentration of reactants and the rate at which the reaction proceeds. The study to measure this relationship is known as the kinetics of a chemical reaction. Kinetic studies are used to deduce the rate law of a chemical reaction, which provides information about the species involved during the transition state of the rate-determining step. Thus, kinetic studies help to derive the mechanism of a reaction.

Rate-Determining Steps

Rate-Determining Steps

Relating Reaction Mechanisms
In a multistep reaction mechanism, one of the elementary steps progresses significantly slower than the others. This slowest step is called the rate-limiting step (or rate-determining step). A reaction cannot proceed faster than its slowest step, and hence, the rate-determining step limits the overall reaction rate.
The concept of rate-determining step can be understood from the analogy of a 4-lane freeway with a short-stretch of traffic-bottleneck caused due to...

Cationic Chain-Growth Polymerization: Mechanism

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The cationic polymerization mechanism consists of three steps: initiation, propagation, and termination. In the initiation step of the polymerization process, the π bond of a monomer gets protonated by the Lewis acid catalyst, which is formed from boron trifluoride and water. The protonation of the π bond generates a carbocation stabilized by the electron‐donating group. In the propagation step, the π bond of the second monomer acts as a nucleophile and attacks the generated carbocation,...

Nucleophilic Addition to the Carbonyl Group: General Mechanism

Nucleophilic Addition to the Carbonyl Group: General Mechanism

The carbonyl carbon in an aldehyde or ketone is the site of a nucleophilic attack due to its electron-deficient nature. Depending on the strength of the incoming nucleophile, the reaction occurs via different mechanistic pathways.
A stronger nucleophile can directly attack the electrophilic center, the carbonyl carbon. The HOMO orbital of the nucleophile interacts with the LUMO (π* antibonding) orbital present on the carbonyl carbon. This interaction breaks the π bond and shifts the π bonding...

Crystal Growth: Principles of Crystallization

Crystal Growth: Principles of Crystallization

Crystallization is a phase transformation process in which crystals are precipitated from a supersaturated solution or formed from other sources. During crystallization, atoms or molecules arrange themselves into a well-defined, rigid crystal lattice to minimize energy.
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Updated: Jul 5, 2026

Computation of Atmospheric Concentrations of Molecular Clusters from ab initio Thermochemistry

Computation of Atmospheric Concentrations of Molecular Clusters from ab initio Thermochemistry

Published on: April 8, 2020

How general is the nucleation-condensation mechanism?

Bengt Nölting¹, David A Agard

¹Department of Biochemistry and Biophysics, University of California, San Francisco, California 94158-2517, USA.

|May 24, 2008

Summary

Small globular proteins predominantly fold via a nucleation-condensation mechanism. This process involves concurrent secondary and tertiary structure formation, with a single folding nucleus driving consolidation.

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Computation of Atmospheric Concentrations of Molecular Clusters from ab initio Thermochemistry

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Area of Science:

Protein folding dynamics
Biophysics
Structural biology

Background:

Understanding protein folding mechanisms is crucial for deciphering biological functions.
Previous models like hydrophobic collapse and zipper mechanisms offer partial explanations.
The nucleation-condensation mechanism proposes a specific pathway for protein folding.

Purpose of the Study:

To investigate the folding transition states of nine proteins.
To correlate published Phi-values with inter-residue contact maps.
To synthesize findings into a unified nucleation-condensation model.

Main Methods:

Analysis of published Phi-values.
Correlation with inter-residue contact maps.
Comparative analysis across 15 small globular proteins.

Main Results:

At least 10 of 15 proteins studied fold via nucleation-condensation.
Folding consolidation is nonuniform, with a single dominant nucleus.
Helix- and sheet-forming residues exhibit higher Phi-values, indicating their role in nuclei.

Conclusions:

A unified nucleation-condensation mechanism explains protein folding.
Secondary structure formation is a key driving force in nucleation.
This model integrates aspects of framework, hydrophobic collapse, zipper, and funnel models.