Figure 12.31

Figure 12.31

Figure 12.33 summarizes all of the secondary structure evidence for Heregulin-a EGF domain. Exchange protection is indicated by black circles (very slow H/D exchange) and gray circles (slow H/D exchange). 3 J coupling between Ha and HN is indicated by an arrow pointing up (J > 8 Hz, ^-strand O angle) or pointing down (J < 6 Hz, a-helical O angle). Sequential and medium-range NOEs are indicated by bars connecting the two residues horizontally (short bar: weak NOE, tall bar: strong NOE, gray bar: NOE ambiguous due to overlap). The chemical-shift index (for Ha protons) is shown at the bottom along with the secondary structure elements found in the final calculated structure. Strong aN sequential connectivity is found in the ^-sheet regions, with NN connections limited to the helix and various turns. The rare i ^ i + 2 NOEs are indicative of turns, which occur between elements of regular secondary structure. Only two i ^ i + 3 NOEs are found (K5-E8 and E8-K11), both within the a-helix. These medium-range (i to i + 3 and i to i + 4) NOEs

Figure 12.33

result from the proximity of one rung of the helix to the next rung in space (Fig. 12.12) because the helix repeat is about 3.6 residues per turn.

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