Near UV protein CD

Definition: Protein circular dichroism (CD) spectroscopy is usually separated into (1) far UV or backbone CD with data collected from ~190 to 250 nm and (2) near UV or aromatic CD with data collected from 250 to 300 nm. The aromatic chromophores of protein side chains (tryptophans, tyrosines, and phenyl alanines) are planar and so have no intrinsic CD. However, their location as part of an amino acid results in an induced CD signal in their transitions. This is further enhanced when they are located in the chiral environment of a peptide or protein. The signs and intensities of the CD signals induced into the achiral aromatic side chain transitions are dependent on their environment. It is a useful fingerprint of protein identity and conformation, but seldom leads to direct structural analysis. Disulfide bonds are often counted as honorary aromatic groups since their CD may be apparent between 250 and 270 nm. Wavelengths of relevant residues are given in Table 1.

When peptides with aromatic residues assemble (such as in a membrane), bands in the 230–240 nm region of the CD spectrum appear. They arise as a result of π– π* exciton coupling of aromatic transitions, where one of the components occur at about 235 nm and the shorter wavelength component is masked by the backbone transitions (Woody 2007).