FIGURE 10.

¹³C NMR chemical-shift index of proinsulin and predicted protein dynamics. A, ¹³C NMR chemical shift index (CSI) values of B-domain (blue), C-domain (black), and A-domain (red): upper and lower panels pertain to ¹³C_α and ¹³C_β secondary chemical shifts. Data pertain to an engineered proinsulin monomer at neutral pH as described (26). Helical segments are indicated by spirals at the bottom; red arrow indicates the segmental attenuation of CSI values in the A1–A8 segment, presumed to reflect motions at time scales longer than nanoseconds. Horizontal arrow indicates the B24–B27 β-strand. B and C, CSI values observed in proinsulin may be interpreted in relation to a random-coil index and predicted model-free order parameters (75). B, random-coil index values were calculated based on main chain secondary chemical shifts. C, predicted main chain order parameters as inferred by random-coil index values by the method of Berjanskii and Wishart (75). These parameters pertain to putative subnanosecond fluctuations. Whereas the A domain exhibits near-uniform high order parameters, the B domain N- and C-terminal segments exhibit increasing disorder (“ramps”) at its N terminus (residues B1–B8) and BC junction (residues B25–B30).