<p class="first" id="P1">Electron paramagnetic resonance (EPR) inversion recovery
curves for vanadium catecholates
and iron-sulfur clusters were analyzed with three models: the sum of two exponentials,
a stretched exponential, and a model-free distribution of exponentials (UPEN). For
all data sets studied fits with a stretched exponential were statistically indistinguishable
from the sum of two exponentials, and were significantly better than for single exponentials.
UPEN provides insights into the structures of the distributions. For a vanadium(IV)
tris catecholate the distribution of relaxation rates calculated with UPEN shows the
contribution from spectral diffusion at low temperatures. The energy of the local
mode for this complex, found from the temperature dependence of the spin lattice relaxation,
is consistent with values expected for a metal-ligand vibration. For the [2Fe-2S]
<sup>+</sup> cluster in pyruvate formate lyase activating enzyme (PFL-AE) the small
exponential β values (0.3) at low temperature and the distributions calculated with
UPEN reflect the contribution from a second rapidly relaxing species that could be
difficult to detect by continuous wave EPR. The distributions in 1/
<sub>1</sub> for the [4Fe-4S]
<sup>+</sup> clusters in MftC were about a factor of four wider than for the three
studied. The very broad distribution of relaxation rates may be due to protein mobility
and distributions in electronic energies and local environments for the clusters.
UPEN provides insight into several situations that can result in low values of stretch
parameter β including contributions from spectral diffusion, overlapping signals from
distinguishable clusters, or very wide distributions.
<div class="figure-container so-text-align-c">
<img alt="" class="figure" src="/document_file/7117e82d-755a-4a02-b722-2e4a6bc145f0/PubMedCentral/image/nihms-1539746-f0012.jpg"/>