Hydrophobic Interfacing of Fluorescent Membrane Probes

Fluorescent flippers have been introduced as small-molecule probes to image membrane tension in living systems. While the hydrophilic headgroup region has been modified extensively for intracellular targeting, little is known about the hydrophobic interfacing with the surrounding membrane. To tackle this challenge, the design, synthesis and evaluation of a glutamine-derived flipper collection is reported. Considering the importance of tension-induced phase separation for tension imaging, this study is focused on how to modulate the distribution of functional flippers between ordered and disordered microdomains. Also of interest was control over intermembrane transfer without loss of function for the specific labeling of plasma and intracellular membranes. Evidence is presented for a two-step insertion mechanism through more accessible disordered domains into better matching ordered domains. This process also explains differences between partition coefficients and bioimaging. It is further demonstrated that interdomain and intermembrane distribution can be regulated by hydrophobic interfacing to control brightness in fluorescence lifetime imaging microscopy and responsiveness to membrane tension. Irreversible partitioning inhibits intermembrane transfer and coincides with internalization into cells. These results demonstrate that hydrophobic interfacing can improve probe performance and provide guidelines on how to proceed.