Section 18.3.2.3.2. Aromatic residues: tryptophan, phenylalanine, tyrosine, histidine

With the exception of generally lower σ values, tryptophan parameters remain essentially unchanged. Phenylalanine, also with generally lower σ values, is also essentially unchanged with the assumption of Gaussian distributions. However, a scatter plot of the CB—CG—CD1 versus CB—CG—CD2 angles shows an inverse correlation between these two angles, corresponding to ring rotations about an axis perpendicular to the ring face. Non-Gaussian distributions were most evident for tyrosine. In addition to the phenomenon described for phenylalanine, a clearly multimodal distribution was observed for the CE(1,2)—CZ—OH angles, with maxima at 118 and 122° (Fig. 18.3.2.2). The scatter plot of CE1—CZ—OH versus CE2—CZ—OH demonstrates that this distribution typifies individual fragments and does not arise from differing classes of fragments. This justifies an asymmetric parameterization for these angles; symmetric parameterization would require correspondingly soft force constants. The major difference between the histidine parameters listed here compared to those of EH arise from the appearance of HISD (uncharged; unprotonated at NE2) fragments in the CSD. The EH parameterization assumed values from other fragments. The total of 12 fragments is not large, but does predict some alterations in parameters involving the ring nitrogens. The fragment selection reported here did not investigate effects of noncovalent binding. For the aromatic residues, these include hydrogen-bonding effects (especially for histidine) and π-cloud interactions. Appropriate fragments exist in the database, so such dependencies are, in principle, accessible to investigation.

Figure 18.3.2.2| top | pdf | Bimodal distributions for tyrosine. (a) The Cɛ—Cζ—Oη angle distributions involving the tyrosine alcohol have maxima at 120 (2)°. (b) A scatter plot of the Cɛ2—Cζ—Oη angle against the Cɛ1—Cζ—Oη angle confirms that the Cζ—Oη bond projects asymmetrically away from the aromatic ring.