Rotational correlation times are influenced by molecular size and shape and by solvent viscosity, although the last of these can be ignored in the present work, because the same solvent composition was used for all measurements. In mononuclear Cu(II) complexes, the major factors affecting the correlation times

are, therefore, the size and number of ligand molecules that are coordinated to the copper. The rotational correlation times increase in the order Complex I < Complex II < Complex III for each of the polyphenols, and are consistent with a progressive increase in molecular mass, as proposed from analysis of the spectral AC220 cost parameters in the previous paragraph. The values for the Cu/EGCG system are also appreciably greater than the corresponding values for Cu/GA, as expected for the larger size of the EGCG ligand. Although the trend is the same for X- and S-band results – the rotational correlation times are higher with Complex III than with Complex II – the absolute values differ between the two spectrometer frequencies (Table 2). This result is puzzling, but it may Decitabine be the consequence of the difficulty in precisely analysing the spectra when the solutions contain a mixture of species. With both polyphenols, there is a mixture of complexes at most alkaline pH values,

and with EGCG there is the further complication of two resorcinol groups in the polyphenol. Finally, there is the potential problem that the axial symmetry model may not be precisely correct for all of such components. Thus it was not considered appropriate Sinomenine to attempt to further refine the values

reported in Table 2. Since the effect of molecular rotational correlation time on the shape of an EPR spectrum is dependent on the spectrometer operating frequency, measurements at lower frequency (S-band) [17] and [18] provided better resolution of fluid solution spectra than those at X-band frequencies. Thus the isotropic spectral parameters for Complexes II and III were able to be determined directly from the S-band spectra, and these results confirmed that the anisotropic hyperfine coupling constants have the same sign, and thus provide agreement with the restricted motion analysis of the X-band spectra. With each complex, there are small differences between the parameters from the simulations of the frozen and fluid solution spectra, the biggest deviation being observed for Complex III. There are a number of possible explanations for these discrepancies. Firstly, the axial symmetry model assumed for the low temperature simulations may not be strictly correct, and the g- and A-matrices may not be co-axial; in addition there could also be a quadrupolar interaction as a result of the appreciable electric field gradient that can exist at the Cu atom in tetragonal symmetry.