WINSIM

History

Simulation of Multiple Isotropic Spin-Trap EPR Spectra

Installing WINSIM

Public Electron Paramagnetic Resonance Software Tools

Choose the older version of WINSIM.zip and extract all...

Running a WINSIM Simulation

This is the main menu bar...

WINSIM will allow you to import an EPR data file to display along with the simulated data. In this first example, we are going to do simulations only; first select the "parameters" item under the "Simulation" tab...

Parameters

The above panel is were all simulation parameters are input. Please note the following:

1) This program can simulate up to 10 radicals in one EPR spectrum. It is very rare to have more than one radical in a spectrum, but there are cases where there is a background signal as well as a signal of interest, so...2 radicals...sometimes. To start the simulation you must select "YES"...calculate this [1] species...

2) On the right side of the parameters panel is where you enter the EPR parameters:

Set: each type/set of spins has their own parameters. For example in methane, CH₄, there is only one set of protons.

Hyperfine: this is a term which indicate the "energy" of interaction between a nuclear spin and the unparied electron.

Spin: this is the nuclear spin.

Number: how many spins are in this set

Opt: when fitting experimental EPR data an optimization routine can be run, this is where you select whether you want to optimize this parameter or not.

3) These parameters allow some fine tuning of the simulation and will be introduced later.

Simulation 1

Run a simulation for the methyl radical, CH₃^. (set 1 - check, hyperfine 15 G, Spin - 0.5, Number - 3, opt - leave as checked (but not used), click "Simulate" (bottom left)...

Sim 1 follow up...for educational purposes follow these instructions:

- change the Number to 1, and simulate...you will now see only 2 lines, displayed in a "first-derivative" signal (typical in EPR due to the detection system). The separation between these two line is 15 G (Gauss).

- Change the Number to 2, and simulate...you will now see 3 lines all separate by 15 G.

- Change the Number to 3, and simulate...you will now be back to the original EPR simulation of methyl radical; 4 lines all separate by 15 G.

Simulation 2

Run a simulation for the ethyl radical, CH₃-CH₂^....are all of these protons the same? You would need to collect the EPR signal for this radical and see if the hyperfine was the same for the protons:

If they are different, then the simulation could be setup as...

- set 1 - check, hyperfine 12 G, Spin - 0.5, Number - 2, opt - leave as checked (but not used),

- set 2 - check, hyperfine 8 G, Spin - 0.5, Number - 3, opt - leave as checked (but not used),

- click "Simulate"...

If they are all the same, then the simulation would be setup as...

(Note: for ease of setup, the simulation is still using 2 sets, but they have the same parameters, same as only 1 set with 5)

- set 1 - check, hyperfine 10 G, Spin - 0.5, Number - 2, opt - leave as checked (but not used),

- set 2 - check, hyperfine 10 G, Spin - 0.5, Number - 3, opt - leave as checked (but not used),

- click "Simulate"...

EPR Interpretation

The analysis of EPR spectra has been made considerably easier using the WINSIM (or similar) simulation software. Below are a set of EPR spectra (some simulated and some experimental) to be used to practice interpreting EPR data.

EPR Literature

Ascorbate Radical

The 1-electron oxidation of ascorbate, the conjugate base of ascorbic acid (aka vitamin C), leads to the ascorbate radical.

The ascorbate radical is one of the most studies radicals. As shown in Figure 1 (see below) of the 1993 paper by Buettner, the ascorbate radical shows a rather simple doublet (inset) indicating the unpaired electron interacts with a single proton. Interpretation of this EPR signal leads to our understanding of how the unpaired electron is localized within the molecular frame. Using the simulated data below, determine the proton hyperfine coupling constant (hfcc)...

Ascorbate radical

reference

simulated data

In addition to the hfcc, the above spectrum has been fit using the linewidth fitting parameter. In the above EPR spectrum of the ascorbate radical the linewidth is relatively broad (~0.5 G). Although linewidth is an intrinsic property of the radical species in the given sample environment, a sample of ascorbate radical can be prepared where the linewidth is significantly narrower (<0.1 G). The proton hfcc (a_H~1.8 G) is assigned to the proton at carbon-4. There are protons on carbon-5 and carobon-6, but since the unpaired electron density is highly localized on the lactone ring the hfcc for these other protons is significantly less than carbon-5 proton. Using the simulated data below, determine the proton hfcc observed in this EPR spectrum...

Ascorbate radical - small linewidth

reference

simulated data

Laroff, G. P.; Fessenden, R. W.; Schuler, R. H. The electron spin resonance spectra of radical intermediates in the oxidation of ascorbic acid and related substances. J. Am. Chem. Soc. 94:26:9062-9073; 1972

Ascorbic acid chemistry can be summaries by the following graphic that was presented by Buettner.