Most of the analytical techniques covered in earlier articles on this blog — HPLC, mass spectrometry, UV-Vis spectrophotometry, amino acid analysis — address the question of how much peptide is in a sample and how pure it is. A quite different, and at least as relevant, question is what spatial form the peptide adopts in solution. Circular dichroism (CD) is one of the most widely used techniques for answering that question in a research setting.
What does circular dichroism actually measure?
CD spectroscopy exploits the fact that chiral molecules — and peptides are a textbook example, thanks to their L-amino acids — absorb left- and right-circularly polarised light differently. That difference in absorption is measured across a wavelength range, usually between 190 and 250 nm for the so-called “far-UV” region. The resulting spectrum says nothing about the amino acid sequence, but a great deal about how the peptide backbone folds in space.
From spectrum to secondary structure
Different secondary structures give characteristic CD signatures. An alpha helix usually shows two negative minima, around 208 and 222 nm, and a positive signal around 190-195 nm. A beta-sheet conformation gives a single negative minimum around 215-218 nm with a positive band slightly below it. An unstructured or “random coil” peptide shows a quite different pattern: a strongly negative signal just above 195 nm without the characteristic double dip of a helix. By comparing the measured spectrum with these reference patterns — often with the help of deconvolution software that resolves the spectrum into known basis spectra — a researcher can estimate the percentage of helix, sheet and coil in a sample.
Why this matters in peptide research
For researchers using synthetic peptides to study structure-function relationships, CD is a quick way to check whether a synthesised peptide adopts the expected conformation before time is invested in follow-up experiments. The technique also lends itself well to comparative studies: how does the conformation of a peptide change at a different pH, a different buffer composition, a different temperature, or in the presence of a membrane mimetic such as SDS micelles or TFE? Published literature on antimicrobial peptides shows, for example, that some sequences are largely unstructured in aqueous buffer but form a clear alpha helix in a membrane-like environment — a finding that comes to light only through this kind of spectroscopic comparison. Aggregation studies also make use of it: a shifting or distorting spectrum during a time-series measurement can indicate structural change or aggregation of the sample.
Practical considerations for a CD measurement
CD measurements are sensitive to sample composition. Chloride ions and some buffer components absorb strongly in the far-UV region and can disturb the signal; phosphate or sulphate buffers at low ionic strength are usually preferred over, for instance, high concentrations of NaCl. The cell path length must be matched to the peptide concentration: at typical concentrations of 0.1-1 mg/ml a quartz cuvette with a path length of 0.1-1 mm is often used to avoid detector saturation in the low-wavelength region. Because the signal is relatively weak, several scans are usually averaged to improve the signal-to-noise ratio.
The limits of the technique
CD is powerful for estimating the overall distribution of secondary structure, but it does not give atomic resolution the way NMR spectroscopy or X-ray crystallography do. The technique gives an average across the whole population of molecules in the cuvette and cannot say anything about local or tertiary structural detail. In practice CD is therefore often used as a quick, relatively inexpensive first screen, after which structurally interesting candidates are investigated further with additional techniques.
Circular dichroism thus remains a valuable part of the analytical toolkit in peptide research: not to determine what is in a vial, but to understand what form it takes once it goes into solution.
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