Fluorescence-detected circular dichroism
Fluorescence-detected circular dichroism (FDCD) is the measurement of the differential emission intensities from an optically active sample that is excited by left and right circularly polarized light. This method takes advantage of the chiral specificities and the structural sensitivities of CD, combined with the selectivity and sensitivity of enhanced fluorescence detection.
Because FDCD can selectively measure the CD of a specific fluorescent chromophore, it is particularly useful for the study of proteins, which have multiple chromophores. FDCD can be measured with the standard CD detector when paired with the PTC-510, PTC-517 or MPTC-513 cell holder. When samples are free from fluorescence anisotropy, this method is effective because the photoselection artifact is small. However, when the sample has a larger fluorescence anisotropy, the photoselection artifact will distort the FDCD spectrum. The FDCD-551 attachment is specially designed to reduce or eliminate this artifact while greatly enhancing sensitivity due to much more efficient light collection.
Comparison with CD
FDCD offers a significant sensitivity advantage over absorption CD spectroscopy. The figures show CD and FDCD spectra of the same sample in concentrations varying by 10×. The sample is (1S,2S)-trans-cyclohexanediol bis(6-methoxy-2-naphthoate).
Data courtesy of Dr. Tatsuo Nehira, Graduate School of Integrated Sciences for Life, Hiroshima University, Japan, and Prof. Nina Berova, Department of Chemistry, Columbia University, USA.
The figure on the left shows photoselection artifacts with conventional FDCD measurement using the FDCD-550. The figure on the right shows artifact-free FDCD spectra using the FDCD-551 system. The sample is RS-1,1’-Bi-2-naphthol 0.4 ppm / Glycerin.
FDCD-550 FDCD Unit
FDCD measurements are possible with this attachment, which comprises a sample holder for 90° detection, focusing lens, filter holder and a short wavelength cut-off filter. The sample can be maintained at a constant temperature with circulating water. For these measurements, the J-1500 CD detector is repositioned to the emission side.
FDCD-551 Artifact-free FDCD Unit
The improvement in light collection efficiency and the ability to eliminate artifacts due to fluorescence anisotropy are two advantages of this FDCD unit. Using a unique design that includes a sandwiched elliptical cylinder mirror with two plane mirrors, all fluorescence light emitted in a circumferential direction from the cell is collected. The end result is a highly sensitive, artifact-free FDCD spectrum.