Insights on the Auxochromic Properties of the Guanidinium Group

Wavelength dependent photochemistry of BODIPY-phenols and their applications in the fluorescent labeling of proteins

A series of BODIPY dyes were synthesized, that were at the 3, or 3 and 5 positions, substituted by photochemically reactive quinone methide (QM) precursor moieties. Fluorescence properties of the molecules were investigated and we demonstrated that the molecules undergo wavelength dependent photochemistry. Photodeamination to deliver QMs takes place only upon excitation to higher excited singlet states, showing unusual anti-Kasha photochemical reactivity. The findings were corroborated by TD-DFT computations. Laser flash photolysis experiments could not reveal QMs due to the low efficiency of their formation, but enabled the detection of phenoxyl radicals. The applicability of the molecules for the fluorescent labeling of bovine serum albumin as a model protein upon photoexcitation at 350 nm was demonstrated.

1,2,5,6-Tetrakis(guanidino)-Naphthalenes: Electron Donors, Fluorescent Probes and Redox-Active Ligands

New redox-active 1,2,5,6-tetrakis(guanidino)-naphthalene compounds, isolable and storable in the neutral and deep-green dicationic redox states and oxidisable further in two one-electron steps to the tetracations, are reported. Protonation switches on blue fluorescence, with the fluorescence intensity (quantum yield) increasing with the degree of protonation. Reactions with N-halogenosuccinimides or N-halogenophthalimides led to a series of new redox-active halogeno- and succinimido-/phthalimido-substituted derivatives. These highly selective reactions are proposed to proceed via the tri- or tetracationic state as the intermediate. The derivatives are oxidised reversibly at slightly higher potentials than that of the unsubstituted compounds to dications and further to tri- and tetracations. The integration of redox-active ligands in the transition-metal complexes shifts the redox potentials to higher values and also allows reversible oxidation in two potentially separated one-electron steps.

The photodynamic activities of dimethyl 131-[2-(guanidinyl)ethylamino] chlorin e6 photosensitizers in A549 tumor

Effective photosensitizers are particularly important factor in clinical photodynamic therapy (PDT). However, there is a scarcity of photosensitizers for simultaneous cancer photo-diagnosis and targeted PDT. Herein, two novel dimethyl 2-(guanidinyl)ethylamino chlorin e₆ photosensitizers were synthesized and their efficacy in PDT in A549 tumor was investigated. It was shown that compounds 3 and 4 have a long absorption wavelength in the near infrared region and strong fluorescence emission with slow photo-bleaching rate and markedly strong ability of ¹O₂ generation. They exhibited lower cytotoxicity and higher photo-cytotoxicity in vitro compared to the known anticancer drug m-THPC in MTT assay in A549 lung cancer cell lines. Compound 4 exhibit better inhibition effect than compound 3 and the IC₅₀ value of compound 4 was 0.197 μM/L under 2 J/cm² laser irradiation, while compound 3 showed better anti-tumor effects compared to compound 4 in vivo. Intracellular ROS generation was found to be responsible for apoptotic cell death in DCFDA assay. Subcellular localization confirmed the damage site of compounds 3 and 4 in PDT. These findings suggest that the two novel photosensitizers might serve as potential photosensitizers for improved therapeutic efficiency of PDT.

Benzoylguanidines as Anion-Responsive Systems

A series of benzoylguanidinium salts was prepared and the changes in UV/Vis spectra, triggered by the presence of anions, were investigated. All compounds undergo deprotonation with basic anions like dihydrogenphosphate and acetate in acetonitrile. The most pronounced spectral changes were obtained by deprotonation of N¹ -benzoyl-N³ -(p-nitrophenyl) guanidinium chloride which shows the naked-eye visible color change from colorless to yellow. Measured pK_a (BH⁺ ) in acetonitrile ranges from 12-16, which is comparable to the pyridinium cations. The proton transfer equilibria were also tested in acetonitrile/water mixture where all but the most acidic derivatives showed pK_a (BH⁺ ) of 4-6 units which corresponds to apparent association constants of 10⁴ -10⁶  dm³  mol^-1 . UV/Vis spectra of neutral and protonated forms were modelled by the TD-DFT approach using CAM-B3LYP and PBE0 functionals and compared to CC2 results. In the case of CAM-B3LYP, a parameter ω, defining amount of long-range exchange correction, was varied to achieve the best agreement with the experimental spectra. The optimized ω parameters are 0.10 a₀ ^-1 for neutral benzoylguanidines and 0.20 a₀ ^-1 for neutral nitrobenzoyl and protonated systems. The larger ω parameter in the latter is ascribed to more pronounced charge transfer character of the HOMO-LUMO transition - the one responsible for the lowest energy absorption band.

Biguanides, anion receptors and sensors

Biguanides are strong bases (pK_a > 10), their protonated forms bind anions and may therefore act as receptors for anions. We report on easy-to-make anion receptors and fluorescence-based sensors utilizing the biguanide moieties that respond to the presence of anions with a change in fluorescence. The observed changes in fluorescence are anion-specific and even though the biguanide receptors are cross-reactive, these sensors may be used to identify various anions (halides, carboxylates, phosphates). Paper-based analytical arrays were used to assess the discriminatory ability of the sensors in the qualitative and quantitative analysis of multiple anions.