Red-emitting benzo[e]indolium probes for HSA based on the TICT characteristics

Human serum albumin (HSA) is the most abundant protein in human blood plasma. It plays an important role in many physiological functions, and the increase or decrease of HSA can exhibit the conditions of human health. Therefore, the accurate detection of HSA with high selectivity and sensitivity has a great clinical importance. We herein developed four red-emitting benzo[e]indolium fluorescent dyes, L1-L4, for probing HSA in a physiological buffer or human urine. Satisfactorily, L2 and L4 showed high selectivity and excellent sensitivity for HSA in both PBS buffer and human urine. L2 was completely embedded in the α-helices from subdomain IIA (Sudlow site I) of HSA, and the limit of detection (LOD) was 13.43 mg L−1 in PBS and 8.91 mg L−1 in urine, respectively. However, L4 inserted into the α-helices from subdomain IIA and IIIA (Sudlow site II) of HSA, and exhibited a lower LOD (1.75 mg L−1 in PBS, 1.50 mg L−1 in urine). Furthermore, the high tolerance of environmental pH and anti-interference of L2 and L4 enabled us to develop the promising molecular probes for HSA.

Four red-emitting benzo[e]indolium fluorescent dyes with TICT characteristics, L1-L4, were synthesized and used to probe HSA in a physiological buffer or human urine. L1, L2 and L4 were stable in the range of pH 4.0-10.0, which are suitable for bioprobe applications. L2 and L4 show high sensitivity and excellent selectivity for HSA. The binding of probes in the hydrophobic interior of the HSA backbone resulted in the restriction of the TICT state and an emission intensity enhancement. L2 is completely embedded in the α-helices from subdomain IIA of HSA, whereas L4 inserts into the α-helices from subdomains IIA and IIIA. The LOD of both L2 (13.43 mg L−1 in PBS, 8.91 mg L−1 in urine) and L4 (1.75 mg L−1 in PBS, 1.50 mg L−1 in urine) meet the requirements of a traditional HSA assay in PBS or urine.189