Changes in DNA superhelical density monitored by polarized light scattering

Circular Intensity Differential Scattering for Label-Free Chromatin Characterization: A Review for Optical Microscopy

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.

High order DNA structure as inferred by optical fluorimetry and scanning calorimetry

New quantitative insights on the native high order chromatin-DNA structure existing within interphase nuclei are obtained by monitoring the effects of two common well-characterized fixatives, glutaraldehyde and ethanol/acetic acid mixture, at the level of the intranuclear DNA distribution and structures. Reproducible distinct levels of DNA fluorescence intensity and their intranuclear distribution are apparent in unfixed and fixed thymocytes by using DAPI and quantitative optical microscopy based on a charge coupled device. The fluorescent histograms correlated with the calorimetric thermograms on the very same thymocytes fixed and unfixed, establish an unequivocal baseline for the different levels of structural organization of the chromatin within the intact nucleus; namely their number, DNA packing ratio and fiber diameter. A systematic comparison among all the numerous models, being so far proposed for the quinternary and quaternary levels of DNA folding, to identifies the rope or ribbon-like and the chromonema as the ones that best fit with the in situ distribution.

Polarized light scattering: a biophysical method for studying bacterial cells

We outline a method which uses differentially polarized light scattering to study the properties of bacterial cell suspensions, i.e., spores of Bacillus Subtilis. The identification of bacterial cells of different strains, with and without plasmid insertion, was performed by means of differential polarization light scattering (DPLS). The samples displayed distinct angular behaviors for the S14 and S34 normalized scattering parameters of the Mueller matrix depending on the strain and on a plasmid insertion in the chromosomal unit. These experiments, performed blindly, point out the possibility of achieving real time identification of micro-organisms by DPLS spectrometry.