posted on 2015-12-17, 00:22authored byOktay K. Gasymov, Adil R. Abduragimov, Ben J. Glasgow
Trp is the most spectroscopically
informative aromatic amino acid
of proteins. However, the near-UV circular dichroism (CD) spectrum
of Trp is complicated because the intensity and sign of 1La and 1Lb bands vary independently.
To resolve vibronic structure and gain site-specific information from
complex spectra, deconvolution was combined with cooling and site-directed
tryptophan substitution. Low temperature near-UV CD was used to probe
the local tertiary structure of a loop and α-helix in tear lipocalin.
Upon cooling, the enhancement of the intensities of the near-UV CD
was not uniform, but depends on the position of Trp in the protein
structure. The most enhanced 1Lb band was observed
for Trp at position 124 in the α-helix segment matching the
known increased conformational mobility during ligand binding. Some
aspects of the CD spectra of W28 and W130 were successfully linked
to specific rotamers of Trp previously obtained from fluorescence
lifetime measurements. The discussion was based on a framework that
the magnitude of the energy differences in local conformations governs
the changes in the CD intensities at low temperature. The Trp CD spectral
classification of Strickland was modified to facilitate the recognition
of pseudo peaks. Near-UV CD spectra harbor abundant information about
the conformation of proteins that site directed Trp CD can report.