Abstract
SummarySecond harmonic generation (SHG)-based probes are useful for nonlinear optical imaging of biological structures, such as the plasma membrane. Several amphiphilic porphyrin-based dyes with high SHG coefficients have been synthesized with different hydrophilic head groups, and their cellular targeting has been studied. The probes with cationic head groups localize better at the plasma membrane than the neutral probes with zwitterionic or non-charged ethylene glycol-based head groups. Porphyrin dyes with only dications as hydrophilic head groups localize inside HEK293T cells to give SHG, whereas tricationic dyes localize robustly at the plasma membrane of cells, including neurons, in vitro and ex vivo. The copper(II) complex of the tricationic dye with negligible fluorescence quantum yield works as an SHG-only dye. The free-base tricationic dye has been demonstrated for two-photon fluorescence and SHG-based multimodal imaging. This study demonstrates the importance of a balance between the hydrophobicity and hydrophilicity of amphiphilic dyes for effective plasma membrane localization.
Highlights
JF-1 and JF-2 are more hydrophilic than JR-2 and JR-3 because of the presence of extra triethylene glycol (TEG)-substituted aryl groups attached at the meso positions of the porphyrins
While synthesizing AK-1, we found that the reaction completes successfully in dimethylacetamide (DMA); if the alkylation is performed in other solvents such as dimethylformamide (DMF), decomposition predominates
We have synthesized a library of far-red to NIR light absorbing and emitting donor-acceptorbased porphyrin dyes with different live cell localization properties depending on the type of hydrophilic head groups
Summary
Nonlinear optical microscopies based on two-photon excited fluorescence (TPEF) and second harmonic generation (SHG) offer various advantages over linear optical microscopy, such as deep light penetration, less photodamage, and reduced background signal (Campagnola and Dong, 2011; Denk and Svoboda, 1997; Helmchen and Denk, 2006; Khadria et al, 2017; Pantazis et al, 2010; Pawlicki et al, 2009; Rau and Kajzar, 2008) Both TPEF and SHG have been established as robust tools for biological imaging, as well as for measuring membrane potentials of neurons in vitro and ex vivo (Benoren et al, 1996; Campagnola et al, 1999; Campagnola and Loew, 2003; Dombeck et al, 2005, 2004; Helmchen and Denk, 2006; Jiang et al, 2007; Kuhn et al, 2008; Nuriya et al, 2016, 2005; Zoumi et al, 2002).
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