To further investigate this hypothesis, relaxation data were obtained for selected samples with varying Cu II concentrations (Figures and, Table ). Increasing the concentration up to 2.5 mM (20 molar equivalents), the echo decay became faster, as would be expected for increasing close-range interactions between Cu II -sites. However, data clearly revealed a very marked step upon further increasing the ratio from 20 to 50 molar equivalents of Cu II , but keeping the Cu II concentration constant. The resulting echo decay displayed an additional slow component. This slow component completely dominated the remaining signal at even higher equivalents.