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

We demonstrate the efficient trapping of myoglobin (Mb), which is a small protein, in aqueous solution using a 1064 nm laser beam. Conventional optical tweezers exert barely sufficient radiation force (RF) on nanometer-sized dielectric objects, such as small proteins and organic molecules, for manipulation of them. 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 target object: resonant optical trapping (ROT). The near-infrared absorbance dependence of ROT of Mb and optical trapping of Mb without the heme cofactor (apomyoglobin) clearly indicates that the heme cofactor plays a crucial role in the ROT of Mb. Furthermore, confocal Raman microspectroscopy suggested the structural conformational change of trapped Mb. Such an ROT technique would open up new channels in the development of nanobioscience, such as transportation/crystallization techniques and a selective molecular sorting technique for biomolecules.