The chromophores of protein secondary structure
In Section 12.2, we saw that the peptide bond in proteins possesses UV absorption
bands in the area of 220–190 nm. The carbon atom vicinal to the peptide bond (the Ca
atom) is asymmetric and a chiral centre in all amino acids except glycine. This
chirality induces asymmetry into the peptide bond chromophore. Because of the serial
arrangement of the peptide bonds making up the backbone of a protein, the individual
chromophores couple with each other. The (secondary) structure of a polypeptide thus
induces an ‘overall chirality’ which gives rise to the CD phenomenon of a protein in
the wavelength interval 260–190 nm.
With protein circular dichroism, the molar ellipticity yalso appears asmean
residue ellipticityyres, owing to the fact that the chromophores responsible for the
chiral absorption phenomenon are the peptide bonds. Therefore, the number of
chromophores of a polypeptide in this context is equal to the number of residues.
Because of the law of Beer–Lambert (Equation 12.2), the number of chromophores is
proportional to the magnitude of absorption, i.e. in order to normalise the spectrum of
an individual polypeptide for reasons of comparison, the CD has to be scaled by the
number of peptide bonds.12.5.2 Instrumentation
The basic layout of a CD spectrometer follows that of a single-beam UV absorption
spectrometer. Owing to the nature of the measured effects, an electro-optic modulator,
as well as a more sophisticated detector are needed, though.Mean residue ellipticity (1° cm2 dmol-^1
)190–60x10^3
200 210 220 230 240 250–40x10^3–20x10^320x10^3040x10^360x10^380x10^3100x10^3(nm)Fig. 12.18Circular dichroism spectra for three standard secondary structures according to Fasman. Ana-helical
peptide is shown in dark green, a peptide adoptingb-strand structure ingrey,anda random coil peptide in lightgreen.512 Spectroscopic techniques: I Photometric techniques