Figure 10.

Top panel -- views of the Trp/Trp interaction in an NPATGK-turn-flanking context. From left to right: Trp/Trp, Trp^F/Tyr^E, Tyr^F/Trp^E. The rationale for more highly shifted Hβ3 Aryl^E protons for face = Trp is apparent: for phenyl rings the Hβ3 position on the cross-strand partner is not as close and not in the middle of the shielding cone. Middle panel -- two different Trp/Trp conformers available to WIpGKW-hairpins: the standard and inverted χ₂ rotamers. The alternate conformer found flanking 2:4 turns (especially XpGX turns, or in the presence of fluoroalcohol cosolvent) is on the right. For the typical conformer, Trp^E presents its Hε3 proton to Trp^F, but in the alternate conformer Hδ1 is presented to Trp^F. Major CSD changes are observed for the turn (e.g., the Gly H_N CSD shifts from −1.0 ppm to −3.5 ppm) as its position changes relative to the Trp^E indole. Bottom – the Coulombic +XW/WX- (left) versus the hydrophobic β-cap (Ac-Ar/ArTG). In all cases, extrapolated CSD variations (Table 4) for 100% folded constructs are consistent with the aromatic ring orientations shown.