1D Exchange Analysis

The Exchange1D module in NMRAnalysis.jl provides tools for analysing chemical exchange processes using 1D NMR experiments. It performs joint fitting of CEST (Chemical Exchange Saturation Transfer), on- and off-resonance R1ρ relaxation dispersion, and R1 relaxation data using Bloch-McConnell equations to extract exchange kinetics, populations, and chemical shift differences.

This page covers how to launch an analysis and what to expect from the interactive workflow. For the exchange models themselves — what each one represents physically and which parameters it exposes — see Exchange Models. For the underlying Bloch-McConnell and simulation equations, see Theory and Calculation Methods.

Launching Exchange Analysis

Via automatic dispatch

If your experiments have appropriate annotations, the analyse() function will detect a CEST or off-resonance R1ρ experiment (any accompanying R1 or on-resonance R1ρ experiments are included automatically) and offer exchange analysis:

using NMRAnalysis
analyse(["data/101", "data/102", "data/103"])

See Automatic Analysis for details on how dispatch works.

Direct launch with a directory

Call exchange1d() with a folder path to select experiments interactively:

exchange1d("data/exchange_expts")

Direct launch with specific files

Provide experiment paths directly:

exchange1d(["data/101", "data/102", "data/103", "data/104"])

Direct launch with a file dialog

Call with no arguments to open a file selection dialog:

exchange1d()

Supported Experiment Types

Exchange1D currently supports four experiment types, which can be combined in a joint fit:

When using automatic dispatch via analyse(), at least one CEST or off-resonance R1ρ experiment must be present to offer exchange analysis; on-resonance R1ρ and R1 experiments can be included in the joint fit but do not by themselves trigger it. Calling exchange1d() directly accepts any combination of the four experiment types above; R1 experiments are always optional and help constrain the longitudinal relaxation rate during fitting.

Analysis Workflow

The interactive text-based workflow proceeds through the following steps:

1. Model selection

A menu is presented listing all available exchange models. Select the model appropriate for your system — see Exchange Models for what each one represents and when to use it.

2. Experiment loading

Each input file is loaded and classified automatically based on its annotations. The loaded experiments are listed with their type and magnetic field strength.

3. Molecule mapping (binding models only)

For models involving multiple molecules (e.g. two-state binding), you are prompted to assign molecule roles (observed species, titrant) to the sample components found in the experiment metadata.

4. Peak integration

You are prompted for three values:

• Peak position (ppm): Centre of the signal of interest
• Integration width (ppm): Width of the integration region (default: 0.1 ppm)
• Noise position (ppm): Centre of a signal-free region for noise estimation

All experiments are integrated at these positions. Noise is estimated from the standard deviation of the noise region across the variable dimension (e.g. saturation offsets for CEST, delays for R1).

5. Parameter review and fitting

Default parameters are assembled from the model and experiments. An interactive editor lets you review and modify initial values before fitting. After fitting, results are displayed as a table and diagnostic plots.

You can then choose to:

• Save the results to a folder
• Adjust parameters and refit
• Quit without saving

6. Saving results

If you choose to save, you are prompted for an output folder. The following files are written:

• exchange1d_fit.pdf — Combined plot of all experiments
• exchange1d_expt_N.pdf — Individual experiment plots
• exchange1d_params.txt — Fitted parameters with uncertainties

Parameter Structure

Parameters are organised into three sections using a ComponentArray:

Field-dependent parameters are tagged with the magnetic field strength (e.g. R2_14p1T for 14.1 T). When experiments at multiple fields are combined, separate parameters are created for each field.

Chemical shifts (delta) and transverse relaxation rates (R2) are vectors with one value per state. Longitudinal relaxation (R1) is shared across states (length 1).

Automatic dispatch

When exchange analysis is available (at least one CEST or off-resonance R1ρ experiment is present), it appears in the analyse() menu as "Exchange analysis (CEST / R1rho)". See Automatic Analysis for how dispatch works in general, and Analysis Rules for how to register custom analysis routines.