Figure 6.

(a) Plot of Asn χ₁ vs. χ₂ values for the 166 left-handed examples (red triangles and contours) and for the 310 strictly α-helical examples (×s and black contours). Contours representing the Gaussian smoothed distributions are shown in regions of clustering and stray points elsewhere to indicate the remaining scatter in the data. Note that χ₁p is forbidden for left-handed as well as helical Asn. The two large α peaks for χ₁m represent two mutually exclusive ways in which Asn interacts with residue i−4 on an α-helix. (b) Asn-99 of 1UDC with χ₂ = −81°, for which Nδ forms H bonds to the i−4 backbone CO (shown by the overlapping dots). (c) Asn-12 of 1AB1 with χ₂ = −21°, in which the amide makes broad van der Waals contact with the i−4 CO.