Raman imaging of alkyne as a small tag for biological molecules

Role of small molecules such as drugs or metabolites in cells is commonly studied by fluorescence microscopy in which a fluorescent label is attached to the molecule. However, fluorescent labels are typically large that often interfere with the normal cellular function of the molecule. To avoid the use of bulky fluorescent labels, we introduce a technique that uses a simple small chemical tag called alkyne consisting of two carbons connected by a triple bond. The alkyne-tagged molecule is imaged using Raman microscopy that detects the strong Raman signal from the CC triple bond stretching vibration (~2120 cm ^-1). Because the alkyne signal is located in the silent region of the cell (1800-2700 cm ^-1), it does not interfere with any intrinsic cellular Raman signals. Here, we demonstrate this technique by showing Raman images of an alkyne-tagged component of DNA in a living cell using a slit-scanning confocal Raman microscope. This fast imaging technique is based on a line-shaped focus illumination and simultaneous detection of the Raman spectra from multiple points of the sample. Using this microscope, we obtained time-course Raman images of the incorporation of EdU in the DNA of HeLa cells in just several tens of minutes.