Contributions of tryptophan side chains to the far-ultraviolet circular dichroism of proteins.

It has often been assumed that the role of aromatic side chains in the far-ultraviolet region of protein circular dichroism (CD) is negligible. However, some proteins have positive CD bands in the 220-230 nm region which are almost certainly due to aromatic side chains. The contributions to the CD of interactions between tryptophan side chains and the nearest neighbor peptide groups have been studied, focusing on the indole Bb transition which occurs near 220 nm. Calculations on idealized peptide conformations show that the CD depends strongly on both backbone and side-chain conformation. Because of the low symmetry of indole, rotation about the C beta C gamma bond (dihedral angle chi 2) by 180 degrees generally leads to large changes in the CD, often causing the Bb band to reverse sign. When side-chain conformational preferences are taken into account, there is no strong bias for either positive or negative Bb rotational strengths. The observation that simple tryptophan derivatives such as N-acetyl-L-tryptophan methylamide have positive CD near 220 nm implies either that these derivatives prefer the alpha R region over the beta region, or that there is little preference for chi 2 < 180 degrees over chi 2 > 180 degrees. Nearest-neighbor-only calculations on individual tryptophans in 15 globular proteins also reveal a small bias toward positive Bb bands. Rotational strengths of the Bb transition for some conformations can be as large as approximately 1.0 Debye-Bohr magnetons in magnitude, corresponding to maximum molar ellipticities greater than 10(5) deg cm2/dmol. Although a substantial amount of cancellation occurs in most of the examples considered here, such CD contributions could be significant, especially in proteins of low helix content.