| |
|
The ultraviolet CD spectrum of proteins can predict important characteristics of their secondary structure. CD spectra can be readily used to estimate the fraction of a molecule that is in the alpha-helix conformation, the beta-sheet conformation, the beta-turn conformation, or some other (random) conformation. These fractional assignments place important constraints on the possible secondary conformations that the protein can be in. CD can not, in general, however say where the alpha helices that are detected are located within the molecule or even completely predict how many there are. Despite this, CD is a valuable tool. It can, for instance, be used to study how the secondary structure of a molecule changes as a function of temperature or of the concentration of denaturing agents. In this way it can reveal important thermodynamic information about the molecule that can not otherwise be easily obtained. Anyone attempting to study a protein will find CD a valuable tool for verifying that the protein is in its native conformation before undertaking extensive and/or expensive experiments with it. Also, there are a number of other uses for CD spectroscopy in protein chemistry not related to alpha-helix fraction estimation.
It may be of interest to note that the protein CD spectra used in secondary structure estimation are related to the pi-star orbital absortions of the amide bonds linking the amino acids. These absorbtion bands lie partly in the so-called vacuum ultraviolet (wavelengths less than about 200 nm). This wavelength region is actually inaccessible in air because of the strong absorbtion of light by oxygen at these wavelengths. In practice these spectra a measured not in vacuum but in an oxygen-free instrument (filled with pure nitrogen gas).
At the quantum mechanical level, the information content of circular dichroism and optical rotation are identical.
See also: