An Analysis of the Circular Dichroism of the Lysozyme-α-Lactalbumin Group of Proteins

Circular dichroism data are presented for eight lysozymes and α-lactalbumins. All have multiple Cotton effects in the near-ultraviolet region, associated with the absorption bands of the aromatic amino acids. All but the avian lysozomes have predominantly negative optical activity in this region. Hen and duck egg lysozymes are dominated by positive rotational strength, which is interpreted as arising from tryptophan residues. Calculations of on- and off-res-onance interactions between the two adjacent active-site tryptophan residues show that these can give rise to sufficient rotational strength to explain the observed difference between these proteins and their homologs, which lack the two adjacent tryptophans. The apparently unrelated goose egg lysozyme, which has a low content of aromatic amino acids, shows only very small optical activity in the region of the aromatic absorption band, and marked differences in the peptide region. Differences between the egg lysozymes and the other proteins in the series in this region can be rationalized in terms of aromatic contributions. Similarities in the high-field proton magnetic resonance spectra of lysozymes, due to ring-current shifted aliphatic proton resonances, and similar effects in the same region of the inhibitor, 7V-acetyl-glucosamine, are described.