Abstract
Circular Intensity Differential Scattering (CIDS) provides a differential measurement of the circular right and left polarized light and has been proven to be a gold standard label-free technique to study the molecular conformation of complex biopolymers, such as chromatin. In early works, it has been shown that the scattering component of the CIDS signal gives information from the long-range chiral organization on a scale down to 1/10th–1/20th of the excitation wavelength, leading to information related to the structure and orientation of biopolymers in situ at the nanoscale. In this paper, we review the typical methods and technologies employed for measuring this signal coming from complex macro-molecules ordering. Additionally, we include a general description of the experimental architectures employed for spectroscopic CIDS measurements, angular or spectral, and of the most recent advances in the field of optical imaging microscopy, allowing a visualization of the chromatin organization in situ.
Highlights
Chromatin is a complex chiral macromolecule composed by the DNA double helix and proteins located in the nucleus of eukaryotic cells [1]
The main reason is that extracting the localized polarimetric signature of a biological medium is difficult due to the extremely low Signal-to-Noise Ratio (SNR) and the mix of numerous different structures confined in a small volume
Using the Stokes–Mueller formalism, it is shown that the differential circular polarized light, coupled with a demodulation detection scheme from a Lock-in Amplifier (LA) at the reference frequency of the Photoelastic Modulators (PEM), can analyze the differential intensity between the right and left circular polarization states in a few microseconds, allowing extracting the CD/Circular Intensity Differential Scattering (CIDS) amplitude in a fast and robust way [80,81]
Summary
Chromatin is a complex chiral macromolecule composed by the DNA double helix and proteins located in the nucleus of eukaryotic cells [1]. The main reason is that extracting the localized polarimetric signature of a biological medium is difficult due to the extremely low Signal-to-Noise Ratio (SNR) and the mix of numerous different structures confined in a small volume It is the main reason why applications of this technique are widely dedicated to arranged and patterned inorganic material media. Early works have put more emphasis on the study of only a few elements of the MM, more sensitive to particular configurations of the medium This is typically the case of Circular Dichroism (CD), defined using the intensities of the circular left and right polarization of the light scattered from the sample, arising from media exhibiting an optical activity. We show the capability of this technique to be incorporated in an imaging configuration, demonstrated from early works to the most recent advances in the optical scanning microscopy field for chromatin imaging
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