Data Analyses

NMR series

In Analysis, it is possible to display and manage series of spectra which are related to one another in terms of that some experimental condition differs within the series. For example a time delay is altered in an NMR pulse sequence so a relaxation analysis is performed. The spectra that belongs to one series are grouped into an Experiment Series and the varying parameter is set. This means to let Analysis know which spectra are grouped together and which parameter varies across the range. It is then possible to set how the parameter varies by fitting a curve or function to the data. In order to connect the results of the series’ analysis to assignments, one Peak List in the spectra has to be assigned and is then used as a reference Peak List to spread the assignments among the other peaks in the series.

In the cases where peaks do not move, e.g. for relaxation time series, but not chemical shift titrations, pseudo nD series may be of interest. A pseudo nD series is a single experiment and spectrum file, which has many planes where each plane has a different condition value. The “sampled dimension” combines many experiments, e.g. for different T1 values as different planes. A stack of effectively 2D experiments combined in this manner is typically referred to as pseudo-3D. The experiment is 3D but there are only two NMR dimensions.

Series that are constructed from multiple, separate experiments must be setup by the user, when loading stacked-plane experiments these values may come through automatically, if they are present in the spectrum header or parameter file.

Follow chemical shift changes

Peak positions (chemical shifts) may change as a response to changing conditions (e.g. temperature or ligand concentration) and this can then be used to calculate parameters that relate the condition varied to the chemical shift. For example temperature coefficients and ligand binding constants may be calculated after a best-fit curve or function has been fitted to the data. Each experiment in the series can be associated with a different chemical shift list (see Figure 1) because we expect the chemical shifts of any resonance (i.e. group of atoms) to differ under the different conditions. It would be inappropriate for the chemical shift values to be averaged over all these experiments, which is what would happen if they only use one chemical Shift List.

Figure 1. Each group of peaks is associated with at least one Resonance which is connected to a separate Shift List and a separate Experiment. The same color of Experiment and Peak is to indicate that the Peak belongs to the Experiment. The Resonance keeps track of data and directs it to the different Shift Lists.  A curve can be plotted for each peak group based on the chemical shifts versus the value of the varying parameter.

Follow intensity changes

Another reason to use NMR series can be when we want to measure the peak intensity changes after some experimental condition or parameter is changed. For example NMR delay times may be varied in a relaxation analysis to determine T1 or T2 time constants. Here we follow changes in the peak intensities for each point in the series and fit a curve or function to the data to calculate the relaxation parameters (see Figure 2). Another example is the NMR gradient strength (DOSY experiment) to measure diffusion constants.

Figure 2. A reference peak list can be used to assign peaks from different sampling points in a relaxation analysis experiment. The peak intensity data can then be used to calculate e.g. the parameters in the selected Fitting function, for example the Time constant.