Site-directed circular dichroism of proteins: 1L b bands of Trp resolve position-specific features in tear lipocalin

Abstract

The absorption spectra of N-acetyl- l-tryptophanamide in various solvents were resolved into the sums of the 1L a and 1L b components. The relative intensities of the 0–0 transitions of the 1L b bands correlate linearly with the solvent polarity values ( E T N ). A novel strategy that uses a set of the experimental 1L b bands was employed to resolve the near-UV circular dichroism (CD) spectra of tryptophanyl residues. Resolved spectral parameters from the single-tryptophan mutants of tear lipocalin (TL), F99W and Y87W, corroborate the fluorescence and structural data of TL. Analysis of the 1L b bands of the Trp CD spectra in proteins is a valuable tool to obtain the local features. The dimethyl sulfoxide (DMSO)-like 1L b band of Trp CD spectra may be used as a “fingerprint” to identify the tryptophanyl side chains in situations where the benzene rings of Trp have van der Waals interactions with the side chains of its nearest neighbor. In addition, the signs and intensities of the components hold information about the side chain conformations and dynamics in proteins. Combined with Trp mutagenesis, this method, which we call site-directed circular dichroism, is broadly applicable to various proteins to obtain the position-specific data.