Circular dichroism beamline celebrates first birthday with a technical “première”

One year into its operation, scientists from Diamond have used the UV-visible Circular Dichroism beamline (B23) to record Circular Dichroism spectra using capillary cells. These allow researchers to use only a minute volume of often precious sample solution (1-2 μlitre). The success of this proof-of-concept experiment could lead to capillary cells being used with any biological sample in solution, such as DNA employed as a scaffold for nano-biotechnology.

Circular Dichroism (CD) is a versatile and fast spectroscopic technique which is used to obtain low-resolution structural information about a wide range of chiral materials in solution, such as proteins that do not crystallise well or not at all and, as such, are unsuitable for X-ray crystallography or alternative techniques. CD can be performed on bench-top instruments but synchrotron light brings two main advantages to the technique. Firstly, the high photon flux of the synchrotron beam in the UV region can extend the utility of the method towards lower wavelengths with a good signal-to-noise ratio over a wide wavelength range (140 to 700 nm). Secondly, the cross section of the collimated beam produced by B23 is less than 1 mm, considerably smaller than for a bench-top instrument (1 cm). These features have been exploited by Diamond scientists who successfully recorded CD spectra for solution samples encapsulated in capillary cells: this technique requires only a small volume (1-2 μlitre) of the samples that are often precious and available in limited quantity, for instance because they are difficult to synthesise.

One user group that has already benefited greatly from B23’s first year in operation is Dr Eugen Stulz’s research group at the University of Southampton’s School of Chemistry, which is working in the area of DNA architectonics. This emerging field is dedicated to the use of DNA as a scaffold for nano-biotechnology. Because of its well known structure and its inherent flexibility, allowing loops, kinks, bends and junctions to be introduced, double-stranded DNA (dsDNA) is attractive as a new building block. Moreover, the organic chemist can now synthesise almost any modified nucleotide and substitute it on the DNA strand. The Southampton group is studying the properties of such modified DNA where porphyrins, which are chromophores derived from natural systems (e.g. chlorophyll), are attached onto the DNA. The porphyrins have specific optical properties and are known to be able to transfer electrons if they are aligned properly. This research could lead to the synthesis of new molecular electronic wires or mimics of the photosynthetic system. The Southampton group has used the CD spectrometer on the beamline to determine the structure of natural and modified DNA.

The figure shows the modelled structure of a porphyrin modified DNA and the CD spectra of various metallated porphyrins with zinc (magenta), cobalt (red) and copper (blue); the black line shows the CD spectrum of the unmetallated porphyrin-DNA for comparison

For more information on the Circular Dichroism beamline, please contact giuliano.siligardi@diamond.ac.uk