Fig. 1

Evidence for β-turn exchange in MPD-ub (black) and cubic-PEG-ub (red) crystals. a Structures of ubiquitin in the two crystal forms. Top: superposition of the main chains in the two crystals, shown as C^α backbone traces; for cubic-PEG-ub only one of the two non-equivalent chains is shown. For clarity, the semi-transparent cartoon representation is shown only for MPD-ub. Bottom: zoom into the β-turn region. The peptide bond undergoing a flip is highlighted in yellow. b–d BMCRD profiles of ¹⁵N sites in the two ubiquitin crystal forms, obtained at a sample temperature of 300 K. Data shown here for D58 and T55 were obtained at B ₀ field strengths corresponding to ¹H Larmor frequencies of 600 MHz, and those of I23 at 950 MHz, respectively. Error bars of R _1ρ values were obtained by standard Monte Carlo analyses, described in the Methods section. Solid lines correspond to a global two-site exchange model fit of data from residues I23, V26, K27, T55, D58 within each of the crystal forms (solid lines). The fits were performed separately for each of the two crystal forms using in each case the same cluster of the aforementioned five residues. Note that the resonances of E24 and N25 are not visible, presumably as a consequence of the conformational exchange. The comprehensive set of BMCRD data and fitting results can be found in Supplementary Figs. 2, 3, and 5. e ϕ _ex values, \({{{{\phi }}_{{\rm{ex}}}} = {p_{\rm{A}}}{p_{\rm{B}}}{{\left( {{\rm{2}}\pi {\rm{\Delta }}\delta } \right)}^{\rm{2}}}}\) in cubic-PEG-ub (red) and MPD-ub (black), obtained from the joint fit of residues I23, V26, K27, T55, and D58, exemplified by curves in b–d. Here p _A and p _B are the populations of the two conformers, while Δδ is the chemical shift difference expressed in the units of ppm. Fitted exchange time constants are reported in Table 1