The chiroptical properties of proteins. II. Near‐ultraviolet circular dichroism of lysozyme

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Abstract
A study of the near‐uv CD spectrum of lysozyme was carried out in the presence and absence of the inhibitor tri‐N‐acetylglucosamine, and theoretical chiroptical calculations based on the tetragonal crystal structure of the enzyme and the enzyme‐inhibitor complex were performed. The results of these calculations indicate that the near‐uv CD spectrum of lysozyme can be adequately explained in terms of negative rotatory strengths arising from the tryptophan 1La (293–300 nm) and the disulfide n‐σ* bands (250 rm), and positive rotatory strength contributions from the tryptophan 1Lb bands (291 nm) and the tyrosine 1Lb bands (275 nm). Contributions to the rotatory strength of each band were approximated in terms of specific interactions between chromophores. It was found that the rotatory strength of most of the near‐uv transitions arises primarily from coupling interactions involving other side‐chain chromophores and amide groups which are in close proximity. Changes which are observed in the lysozyme CD spectrum on binding of tri‐N‐acetylglucosamine may be explained in terms of changes in the rotatory strength which result from interactions of the 1La transitions of the active‐site tryptophans with the acetamide groups of the inhibitor. The reasonable agreement which is found between the experimental and calculated rotatory strengths implies that the crystal conformation of lysozyme must resemble the solution conformation.