Efficient characterization for protein crystals using confocal Raman spectroscopy

Confocal Raman spectroscopy was applied to the characterization of various states emerging in the screening of protein crystallization. Four main characterized states, namely single crystals, microcrystals, precipitates, and clear drops without solid materials, appear in a droplet for crystallization; the first three states should be critically distinguished and characterized because of the limitations of visual observation under an optical microscope. Using lysozyme and other proteins, crystallization was performed by the hanging drop vapor diffusion technique and was monitored through an automated confocal Raman system. Prior to the spectroscopic analysis, an optical microscope with a charge-coupled device (CCD) camera and associated image processing software were used to rapidly identify the XY locations to be measured spectroscopically by focusing the laser beam on a test sample. Instead of the current image analysis by optical microscopy, confocal Raman spectroscopy with a high spatial resolution was used to identify the state of protein crystallization. Such real-time Raman monitoring also distinguished real protein crystals from pseudo-protein crystals emerging in a crystallization droplet.