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A new method for rotational diffusion measurements of proteins using fluorescent correlation spectroscopy based on superconducting nanowire single-photon detector

 On 22 December 2015, it was announced that the collaborative research group of National Institute of Information and Communications Technology (NICT), Hokkaido University and Osaka University has developed a new method for rotational diffusion measurements of fluorescent molecules in solution. The details were published in Optics Express*.

 Fluorescence correlation spectroscopy (FCS) is often used to observe fluorescence intensity fluctuations of fluorescent target molecules, in order to investigate biomolecule dynamics. The rotational diffusion coefficient calculated from the relaxation time of fluorescence is a tool for measurement of small changes in volume such as dimerization of proteins or formation of small aggregation of particles. However, the relaxation time is usually in the submicrosecond range, and the signal is buried among the after-pulse noise of avalanche photodiode (APD) detector used in the conventional FCS.

 NICT developed a superconductive nanowire single-photon detector (SSPD) with 95% absorption at visible light wavelength of 635 nm (VW-SSPDs) and the detector was used in FCS instead of APDs. In contrast to APDs, VW-SSPDs are free from the after-pulse noise, and the FCS system installed with VW-SSPDs can detect the signal of the relaxation time from molecular rotational diffusion. In the experiments, the rotational diffusion was successfully measured for rod-like shaped CdSe/CdS core/shell quantum dot (22 nm in length and 7 nm in diameter) to simulate protein.

 The research group expects that this result will open the way to measurements of rotational diffusion of proteins in living cells and will be used for early checkup of prion diseases.

*Johtaro Yamamoto, Makoto Oura, Taro Yamashita, Shigehito Miki, Takashi Jin, Tokuko Haraguchi, Yasushi Hiraoka, Hirotaka Terai, and Masataka Kinjo, "Rotational diffusion measurements using polarization-dependent fluorescence correlation spectroscopy based on superconducting nanowire single-photon detector", Optics Express Vol. 23, Issue 25, pp. 32633-32642 (2015), doi: 10.1364/OE.23.032633; Published 14 December 2015