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. 2019 Feb 22;10:897. doi: 10.1038/s41467-019-08558-7

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© The Author(s) 2019

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Fig. 1 — Design of cell permeable sensors for calcium-dependent molecular fMRI. a The design consists of a cell permeable paramagnetic platform (black complex, Mn-PDA candidate complexes shown at top right), a BAPTA-based calcium chelator (dark blue), and a linker connecting them (green, R). Prior to cell entry (top) the BAPTA carboxylates are protected with cleavable AM esters (red), and water exchange (cyan spheres) is expected to take place at the paramagnetic metal center (purple), leading to T₁-weighted MRI signal enhancement (labeled MRI). When the agent enters cells (bottom left), the AM esters are cleaved, liberating the sensor in its calcium-free “off” state; in this state, water exchange is expected to be blocked by interactions between the BAPTA moiety and Mn³⁺, leading to low MRI signal. When calcium binds (bottom right), the MRI signal could increase again as interactions between the BAPTA and paramagnetic platform are reduced. b Evaluation of interactions between untethered BAPTA and Mn-PDA contrast agents. In the absence of BAPTA, the Mn³⁺ complexes (40 µM) do not produce calcium-dependent T₁-weighted MRI contrast changes (top), but in the presence of 1.1 equiv BAPTA, both MnL1 and MnL3 display sensitivity to addition of 1 mM calcium (bottom). c Longitudinal relaxivity (r₁) values corresponding to the conditions in panel b. Error bars denote SD of four measurements. d Optical spectra of contrast agents MnL1 and MnL3 in the presence of BAPTA after incubation for 0 h (solid) or 24 h (dashed), indicating comparative stability of MnL3