FIGURE 6.

Mg²⁺ association with P4. (A) Selected regions of 2D ¹³C/¹⁵N/¹H HSQC spectra of P4 in the presence (red) and absence (black) of 15 mM MgCl₂. Significant chemical shifts (>0.08 ppm) are highlighted. Mg²⁺-induced chemical shift perturbations in P4 (>0.08 ppm) observed for C6H6/C8H8 (circles), C5H5 (squares), C1′H1′ (stars), and N1H1/N3H3 (“x”) with the highest shifts across all sites shown in red. (B) Representative titration curves as a function of total Mg²⁺ concentration with K_d values highlighted on the P4 secondary structure (terminal residues not shown): red <1.0 mM, orange 1–2 mM, green 2–4 mM, and blue >4 mM. Chemical shift mapping data were available in certain cases without corresponding K_d values because titration spectra were recorded with reduced resolution in the indirect dimension. (C) Electrostatic surface potential of P4 computed using the B-type X-ray structure (Glemarec et al. 1996; Kazantsev et al. 2005) with negative and positive potentials indicated in red and blue, respectively.