Fluorescence imaging probes for specific G-quadruplex (G4) conformations are of considerable interest in biomedical research. Herein, we present the synthesis and the binding properties of a new water-soluble near-infrared (NIR) amphiphilic squaraine dye (CAS-C1) which is capable of selective detection of parallel over non-parallel and non G4 topologies. The striking changes in its linear optical response upon binding to parallel G4s give rise to high fluorescence quantum yields (Φ_f ≈ 0.7) and one-photon molecular brightness in the far-red-NIR region. The outstanding recognition process of CAS-C1 for parallel G4s via end-stacking provides binding constants in the nanomolar regime (K_b = 10⁷ to 10⁸ M⁻¹) awarding it as one of the most potent parallel G4 binders currently available. Moreover, the CAS-C1-parallel G4 system exhibits large two-photon absorption (TPA) cross-sections and molecular brightness in the second NIR biological transparency window (λ ≈ 1275 nm), making it an ideal candidate for NIR-to-NIR ultrasensitive two-photon procedures.

中文翻译：

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近红外到近红外双光子方酸的选择性并行 G 四链体识别†

用于特定 G 四链体 (G4) 构象的荧光成像探针在生物医学研究中引起了极大的兴趣。在此，我们介绍了一种新型水溶性近红外 (NIR) 两亲性方酸菁染料 ( CAS-C1 ) 的合成和结合特性，该染料能够选择性检测平行而非平行和非 G4 拓扑。与平行 G4 结合后，其线性光学响应发生显着变化，从而产生高荧光量子产率 ( Φ _f ≈ 0.7) 和远红近红外区域的单光子分子亮度。CAS-C1通过末端堆积对平行 G4的出色识别过程提供了纳摩尔范围内的结合常数（K _b = 10 ⁷至 10 ⁸ M ^-1），使其成为目前可用的最有效的平行 G4 结合剂之一。此外，CAS-C1并行G4系统在第二个NIR生物透明窗口（ λ ≈ 1275 nm）中表现出大的双光子吸收（TPA）截面和分子亮度，使其成为NIR-to-NIR的理想候选者超灵敏双光子程序。