17O NMR studies of Dy- and Yb-M8-SPy

The hydration state, q, of Dy- and Yb-M8-SPy was determined by measuring the lanthanide induced chemical shift of the free water ¹⁷O signal of the two samples, using H217O as a reference (Djanashvili and Peters 2007), and using the equation q = Δ/(<S_z> •F), where Δ = δ_obs/P_m (δ_obs is the difference in ¹⁷O chemical shifts of the water resonances and P_m is the mole fraction [Ln(III)]/[H₂O] = 0.00114). The <S_z> is the reduced value for the average spin polarization [28.591 and 2.591 for Dy and Yb at 333 K, respectively, calculated as in (Pinkerton et al. 1985)], and F ≈ (−2.40 9 10⁴)/T, where F is given in ppm, and T is the absolute temperature. The experiments were conducted in water at 333 K with 19 % H₂ ¹⁷O (Cambridge Isotope Laboratories, Tewkesbury, MA) and 7 % D₂O, using 128 scans on a Bruker Avance 600 MHz spectrometer equipped with a room-temperature probe. Chemical shifts were also measured as a function of temperature for both compounds, using 8 scans for each data point with the deuterium providing the lock signal.