Studying single living cells and chromosomes by confocal Raman microspectroscopy

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Abstract
MANY indirect methods have been developed to study the constitu-tion and conformation of macromolecules inside the living cell. Direct analysis by Raman spectroscopy is an ideal complement to techniques using directly labelled fluorescent probes or of indirect labelling with mono- and polyclonal antibodies. The high information content of Raman spectra can characterize biological macromolecules both in solution and in crystals1,2. The positions, intensities and linewidths of the Raman lines (corresponding to vibrational energy levels) in spectra of DNA–protein complexes yield information about the composition, secondary structure and interactions of these molecules, including the chemical micro-environment of molecular subgroups. The main drawback of the method is the low Raman scattering cross-section of biological macromolecules, which until now has prohibited studies at the level of the single cell with the exception of (salmon) sperm heads, in which the DNA is condensed to an exceptionally high degree3. Ultraviolet-resonance Raman spectroscopy has been used to obtain single cell spectra (ref. 4; and F. Sureau and P. Y. Turpin, personal communication), but in this method absorption of laser light may impair the integrity of the sample. We have avoided this problem in developing a novel, highly sensitive confocal Raman microspec-trometer for nonresonant Raman spectroscopy. Our instrument makes it possible to study single cells and chromosomes with a high spatial resolution ( ≲ 1 μm3).