A new reference material for UV–visible circular dichroism spectroscopy

Abstract

To obtain accurate and consistent measurements from circular dichroism (CD) instruments over time and from different laboratories, it is important that they are properly calibrated. The characteristics of the available reference materials are not ideal to ensure proper calibration as they typically only give peaks in one or two spectral regions, and often have issues concerning purity and stability. Currently either camphor sulfonic acid or ammonium camphor sulfonate are used. The latter can be an unstable, slightly hygroscopic secondary standard compound with only one characterized CD band. The former is the very hygroscopic primary standard for which only one enantiomer is readily available. We have synthesized a new reference material for CD, Na[Co(EDDS)].H(2)O (EDDS = N,N-ethylenediaminedisuccinic acid) which addresses these problems. It is extremely stable and available in both enantiomeric forms. The CD spectrum of Na[Co(EDDS)].H(2)O has nine distinct peaks between 180 and 599 nm. It thus fulfils the principal requirements for CD calibration chemical standards and has the potential to be used to ensure good practice in the measurement of CD data, providing two spectra of equal magnitude and opposite sign for a given concentration and path length. We have carried out an interlaboratory comparison using this material and show how it can be used to improve CD comparability between laboratories. A fitting algorithm has been developed to assess CD spectropolarimeter performance between 750 and 178 nm. This could be the basis of a formal quality control process once criteria for performance have been decided.

Considerations of Noise and Measurement Reproducibility of Circular Dichroism Measurements Using Na[CoIII(EDDS)]

“…A xenon lamp spectrum was also obtained by fixing the alternating current voltage on the photomultiplier tube and recording the direct current using a JASCO J‐815 spectropolarimeter. Enantiomerically pure samples of Na[Co III ( R , R ‐EDDS)] and Na[Co III ( S , S ‐EDDS)] were synthesized according to the published procedures . Solvents and chemicals were obtained from Sigma‐Aldrich (Dorset, UK) and used without further purification.…”

“…To collect good quality circular dichroism (CD) data, it is important to know that the instrument being used is well calibrated and that it is performing to a sufficiently high standard for the application at hand. Calibration and standardization are long‐standing issues in CD spectroscopy . In a large part, the field has been relying on the calibration protocol that was developed from the 1969 join‐the‐dots spectrum of Lowry for the 291 nm camphorsulfonic acid peak .…”

“…Our work in collaboration with JASCO UK (Great Dunmow, UK) and Starna UK (Hainault, Essex, UK) has focused on the development of a new commercially available standard, Na[Co III (EDDS)] (where EDDS is ethylenediaminedisuccinato) (Fig. ), that is available in both enantiomeric forms . The exceptional long‐term stability of this compound allows for preparation of sealed “calibration kits” composed of three cuvettes, one of each enantiomer and a third containing the solvent.…”

“…), that is available in both enantiomeric forms . The exceptional long‐term stability of this compound allows for preparation of sealed “calibration kits” composed of three cuvettes, one of each enantiomer and a third containing the solvent. However, in developing this standard, we have come across several issues which are not usually apparent during typical measurements and that affect the quality of data collected.…”

Evaluation of instrumental errors built in circular dichroism spectrometers

“…32 More recently, the Wallace group has investigated the calibration problem of both conventional and synchrotron radiation CD spectrometers. 33,34 Two new standards have been recently proposed: a chlorine dimer with two sharp exciton doublets in the UV-Vis 35 range and a compound (Na[Co(EDDS)]Á H 2 O), 36 which has several bands in the wavelength range of interest: long-term stability is excellent and both enantiomeric forms are available. The latter compound will shortly be available commercially to any user in a calibration kit with sealed cells for the two enantiomers and the solvent.…”