Resonant Excitation Effect on Optical Trapping of Myoglobin: The Important Role of a Heme Cofactor

Tatsuya Shoji , Noboru Kitamura , and Yasuyuki Tsuboi
We demonstrate the efficient trapping of myoglobin (Mb), which is a small protein, in aqueous solution. Conventional optical tweezers exert barely sufficient radiation force (RF) to manipulate nanometer-sized dielectric objects such as small proteins and organic molecules. One possible candidate to enhance the RF is to use laser light with a wavelength that is electronically resonant with the electronic transition of the object to be trapped; resonant optical trapping (ROT). In this study, ROT of Mb was investigated in aqueous solution using a 1064 nm laser beam. We found that trapping of Mb was accompanied by an increase in the near-infrared absorbance of a heme cofactor in Mb. By contrast, trapping of Mb without the heme cofactor (apomyoglobin) was not detected at all. This clearly indicates that the heme cofactor plays a crucial role in the ROT of Mb. Furthermore, confocal Raman microspectroscopy indicated structural conformational changes to the trapped Mb. Such a ROT technique would open up new channels in the development of nano-bioscience, such as transportation/ crystallization techniques and a selectively molecular sorting technique for biomolecules.
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