Fluorescent and Photosensitizing Conjugates of Cell-Penetrating Peptide TAT(47-57): Design, Microwave-Assisted Synthesis at 60 °C, and Properties

Conjugates based on Cell-penetrating Peptides (CPPs) are scientifically relevant owing to their structural complexity; their ability to enter cells and deliver drugs, labels, antioxidants, bioactive compounds, or DNA fragments; and, consequently, their potential for application in research and biomedicine. In this study, carboxyamidated fluorescently labeled conjugates FAM-GG-TAT(47-57)-NH₂ and FAM-PEG₆-TAT(47-57)-NH₂ and photosensitizer-labeled conjugate Ch_k-PEG₆-TAT(47-57)-NH₂ [where TAT(47-57) is the CPP, 5(6)-carboxyfluorescein is the (FAM) fluorophore, chlorin k (Ch_k) is the photosensitizer, and the dipeptide glycyl-glycine (GG) or hexaethylene glycol (PEG₆) is the spacer] were originally designed, prepared, and fully characterized. Practically, all chemical reactions of the synthetic steps (peptide synthesis, spacer incorporation, and conjugation) were microwave-assisted at 60 °C using optimized protocols to give satisfying yields and high-quality products. Detailed analyses of the conjugates using spectrofluorimetry and singlet oxygen detection showed that they display photophysical properties typical of FAM or Ch_k. Anticandidal activity assays showed that not only this basic property of TAT(47-57) was preserved in the conjugates but also that the minimal inhibitory concentration was slightly reduced for cells incubated with PS-bearing conjugate Ch_k-PEG₆-TAT(47-57)-NH₂. Overall, these results indicated that the synthetic approach on-resin assisted by microwaves at 60 °C is simple, straightforward, selective, metal-free, sufficiently fast, cleaner, and more cost-effective than those previously used for preparing this type of macromolecule. Furthermore, such new data show that microwaves at 60 °C and/or conjugation did not harm the integrity of the conjugates' constituents. Therefore, FAM-GG-TAT(47-57)-NH₂, FAM-PEG₆-TAT(47-57)-NH₂, and Ch_k-PEG₆-TAT(47-57)-NH₂ have high potential for practical applications in biochemistry, biophysics, and therapeutics.