When the regularization parameter X is small, G(P) is dominated by the quality of the fit
with experimental data, and as the regularization parameter is increased, G(P) is driven by a
larger contribution from smoothness of the fit, as illustrated in Figure 2-23. A quality L-curve
contains an "elbow," where the slope of the line changes dramatically, and this point corresponds
to the optimal regularization parameter. By selecting this value of X, the optimal distance
distribution profile can be determined.
Zero-time selection
A B
Zero time = 4 ns
S0.00.5
I \ II I I I I
0.0 0.5 1.0 1.5 2.0 2.5 3.0 20 30 40 50
,C (s) Distance (A)
Figure 2-24. Example of zero-time selection for dipolar modulated echo data; A) selection of
zero-time, and B) results of incorrect zero-time selection.
In order to obtain the most accurate distance profile, it is imperative that the correct zero-
time be determined. DEER data is collected with a small amount of negative time, as shown in
Figure 2-24A. If the incorrect zero-time is selected, the distance distribution will be shifted
slightly towards either smaller or greater distances, as demonstrated in Figure 2-24B.
In order to do this, truncated dipolar modulated echo curve in the region of -300 300 ns is
plotted in Origin8.0 and fit to a GaussAmp function of the form shown in Equation 2-15, where
yo is the y-offset, w is the width, A is the amplitude, and x, is the center of the Gaussian-shaped
function, and thus designated as the zero time.