Rational Design of a Near‐infrared Fluorescence Probe for Ca<sup>2+</sup> Based on Phosphorus‐substituted Rhodamines Utilizing Photoinduced Electron Transfer
Shodai Takahashi, Kenjiro Hanaoka, Yohei Okubo, Honami Echizen, Takayuki Ikeno, Toru Komatsu, Tasuku Ueno, Kenzo Hirose, Masamitsu Iino, Tetsuo Nagano, Yasuteru Urano
Abstract
Abstract Fluorescence imaging in the near‐infrared (NIR) region (650–900 nm) is useful for bioimaging because background autofluorescence is low and tissue penetration is high in this range. In addition, NIR fluorescence is useful as a complementary color window to green and red for multicolor imaging. Here, we compared the photoinduced electron transfer (PeT)‐mediated fluorescence quenching of silicon‐ and phosphorus‐substituted rhodamines (SiRs and PRs) in order to guide the development of improved far‐red to NIR fluorescent dyes. The results of density functional theory calculations and photophysical evaluation of a series of newly synthesized PRs confirmed that the fluorescence of PRs was more susceptible than that of SiRs to quenching via PeT. Based on this, we designed and synthesized a NIR fluorescence probe for Ca 2+ , CaPR‐1 , and its membrane‐permeable acetoxymethyl derivative, CaPR‐1 AM , which is distributed to the cytosol, in marked contrast to our previously reported Ca 2+ far‐red to NIR fluorescence probe based on the SiR scaffold, CaSiR‐1 AM , which is mainly localized in lysosomes as well as cytosol in living cells. CaPR‐1 showed longer‐wavelength absorption and emission (up to 712 nm) than CaSiR‐1 . The new probe was able to image Ca 2+ at dendrites and spines in brain slices, and should be a useful tool in neuroscience research.