Computational Design, Synthesis, and Photochemistry of Cy7-PPG, an Efficient NIR-Activated Photolabile Protecting Group for Therapeutic Applications

Abstract

Photolabile Protecting Groups (PPGs) are molecular tools used, for example, in photopharmacology for the activation of drugs with light, enabling spatiotemporal control over their potency. Yet, red-shifting of PPG activation wavelengths into the NIR range, which penetrates the deepest in tissue, has often yielded inefficient or insoluble molecules, hindering the use of PPGs in the clinic. To solve this problem, we report herein a novel concept in PPG design, by transforming clinically-applied NIR-dyes with suitable molecular orbital configurations into new NIR-PPGs using computational approaches. Using this method, we demonstrate how Cy7, a class of NIR dyes possessing ideal properties (NIR-absorption, high molecular absorptivity, excellent aqueous solubility) can be successfully converted into Cy7-PPG. We report a facile synthesis towards Cy7-PPG from accessible precursors and confirm its excellent properties as the most redshifted oxygen-independent NIR-PPG to date (λ_max =746 nm).

Keywords: Density Functional Theory; NIR Light; Photochemistry; Photolabile Protecting Groups; Photopharmacology.

Figure 1

PPGs in photocaging, current PPG designs and the undiscovered ideal PPG for use in tissue.

Figure 2

An overview of the function of PPGs and our novel DFT‐based workflow for designing ideal NIR‐PPGs.

Scheme 1

Attempted synthetic approaches to Cy7‐PPG.

Scheme 2

An efficient synthesis toward Cy7‐PPG, with two methods for loading ZS‐OH with a payload.

Figure 3

Cy7‐PPG‐OAc ideal photocage properties and payload uncaging with 760 nm light. a) Ideal wavelength λ_max, molecular absorptivity ϵ, low fluorescence and singlet oxygen photosensitization quantum yields Φ_fluor and Φ_P.S., and high stability of Cy7‐PPG‐OAc. b) UV/Vis monitored uncaging of Cy7‐PPG‐OAc upon irradiation with 760 nm light (*1 μM in degassed 99 : 1 milli‐Q H₂O/DMSO). c) NMR monitored uncaging of Cy7‐PPG‐OAc and comparison of its uncaging cross‐section ϵ×Φ_het to BODIPY‐PPG‐OAc (see Supporting Information).

Figure 4

Cy7‐PPG‐OAc uncaging with 760 nm light inside of tissue phantoms. a) Preparation of 1 cm cuboid tissue phantoms of Hollandse Nieuwe (raw herring) and Speklap (raw pork belly). b) Mounted tissue phantoms containing a sample of Cy7‐PPG‐OAc (0.5 ml, 2 mM in 1 : 1 D₂O/d₆‐DMSO). c) Rate of PL uncaging upon 760 nm irradiation through the tissue phantoms (*given in nmol/min).