Luidmila S. Yakimova

The subject of this article is a new monosubstituted pillar[5]arene with a terminal carboxylic group. Previously, it was shown that monosubstituted pillar[5]arene forms solid lipid nanoparticles with various morphologies depending on the nature of the terminal group. The present work is devoted to the synthesis of monosubstituted pillar[5]arene with an anionic carboxylic group for the further creation of drug–carrier systems based on them. The chemical structure of the product obtained was established based on ¹H and ¹³C NMR, IR spectroscopy, MALDI TOF mass spectrometry, and elemental analysis.

Pillar[5]arene with l -phenylalanine fragments retains the native form of BSA. Betaine pillar[5]arenes recognize the specific pocket of serum albumin.

Sensitive systems with controlled release of drugs or diagnostic markers are attractive for solving the problems of biomedicine and antitumor therapy. In this study, new decasubstituted pillar[5]arene derivatives containing L-Tryptophan and L-Phenylalanine residues have been synthesized as pH-responsive drug nanocarriers. Fluorescein dye (Fluo) was loaded into the pillar[5]arene associates and used as a spectroscopic probe to evaluate the release in buffered solutions with pH 4.5, 7.4, and 9.2. The nature of the substituents in the pillar[5]arene structure has a huge influence on the rate of delivering. When the dye was loaded into the associates based on pillar[5]arene derivatives containing L-Tryptophan, the Fluo release occurs in the neutral (pH = 7.4) and alkaline (pH = 9.2) buffered solutions. When the dye was loaded into the associates based on pillar[5]arene with L-Phenylalanine fragments, the absence of release was observed in every pH evaluated. This happens as the result of different packing of the dye in the structure of the associate. This fact was confirmed by different fluorescence mechanisms (aggregation-caused quenching and aggregation-induced emission) and association constants. It was shown that the macrocycle with L-Phenylalanine fragments binds the dye more efficiently (lgK_a = 3.92). The experimental results indicate that the pillar[5]arene derivatives with amino acids fragments have a high potential to be used as a pH-responsive drug delivery devices, especially for promoting the intracellular delivering, due to its nanometric size.

New water-soluble derivatives of thiacalix[4]arene, p-tert-butylcalix[4]arene, and p-tert-butylthiacalix[4]arene with sulfonatopropyl- and sulfonatobutyl moieties on the lower rim were synthesized. The template effect of alkali metal cations (Na⁺, K⁺, Cs⁺) on the configuration of products formed in the alkylation reaction of lower rim with 1,3-propane- and 1,4-butanesultone was revealed. In the case of Na⁺, the cone stereoisomer was formed exclusively, while with K⁺ and Cs⁺, mixtures of stereoisomers (cone, partial cone, and 1,3-alternate) were obtained.

Novel monosubstituted pillar[5]arenes with one or two terminal carboxyl groups were synthesized by the reaction of succinic anhydride with pillar[5]arene derivative containing a diethylenetriamine function. The ability for non-covalent self-assembly in chloroform, dimethyl sulfoxide, as well as in tetrahydrofuran-water system was studied. The ability of the synthesized macrocycles to form different types of associates depending on the substituent nature was established. The formation of stable particles with average diameter of 192 nm in chloroform and of 439 nm in DMSO was shown for pillar[5]arene containing two carboxyl fragments. Solid lipid nanoparticles (SLN) based on monosubstituted pillar[5]arenes were synthesized by nanoprecipitation in THF-water system. Minor changes in the structure of the macrocycle substituent can dramatically influence the stability and shape of SLN (spherical and rod-like structures) accordingly to DLS and TEM. The presence of two carboxyl groups in the macrocycle substituent leads to the formation of stable spherical SLN with an average hydrodynamic diameter of 364–454 nm. Rod-like structures are formed by pillar[5]arene containing one carboxyl fragment, which diameter is about of 50–80 nm and length of 700–1000 nm. The synthesized stable SLN open up great prospects for their use as drug storage systems.

The therapeutic application of serum albumin is determined by the relative content of the monomeric form compared to dimers, tetramers, hexamers, etc. In this paper, we propose and develop an approach to synthesize the cone stereoisomer of p-tert-butylthiacalix[4]arene with sulfobetaine fragments stabilization of monomeric bovine serum albumin and preventing aggregation. Spectral methods (UV-vis, CD, fluorescent spectroscopy, and dynamic light scattering) established the influence of the synthesized compounds on the content of monomeric and aggregated forms of BSA even without the formation of stable thiacalixarene/protein associates. The effect of thiacalixarenes on the efficiency of protein binding with the antibiotic ciprofloxacin was shown by fluorescence spectroscopy. The binding constant increases in the presence of the macrocycles, likely due to the stabilization of monomeric forms of BSA. Our study clearly shows the potential of this macrocycle design as a platform for the development of the fundamentally new approaches for preventing aggregation.

Novel water-soluble pillar[5]arenes containing both quaternary ammonium fragments and amino acid residues (Gly, l-Ala) were synthesized in good yields. Two-dimensional ¹H–¹H NOESY NMR spectroscopy and IR spectroscopy showed that the synthesized pillar[5]arenes tend to form different types of β-structures. The macrocycle containing glycine residues is characterized by the formation of β-sheets, while the pillar[5]arene containing L-alanine residues forms β-loops. Dynamic light scattering and transmission electron microscopy showed that the pillar[5]arene containing glycine fragments formed nanosized associates with an average hydrodynamic diameter of 150 nm, which was explained by a more compact packing of the substituents in this macrocycle.

Sulfobetaine derivatives of p-tert-butylthiacalix[4]arene were synthesized in good yields by the reaction of p-tert-butylthiacalix[4]arenes containing terminal N,N-dimethyl fragments with 1,4-butane sultone. An effect of the stereoisomeric form of the macrocyclic platform (cone, partial cone, and 1,3-alternate) on their reactivity was established. The ability of the synthesized macrocycles to form stable nanosized associates with the model protein, human serum albumin, was studied. The partial cone stereoisomer was found to increase the thermal stability of the model protein.

Regulating the structure of macrocyclic host molecules and supramolecular assemblies is crucial because the structure–activity relationship often plays a role in governing the properties of these systems. Herein, we propose and develop an approach to the synthesis of the family of sulfobetaine functionalized thiacalix[4]arenes with regulation of the self-assembly and cytotoxic effect against cancer cell lines. The dynamic light scattering method showed that the synthesized macrocycles in cone, partial cone and 1,3-alternate conformations form submicron-sized particles with Ag⁺ in water, but the particle size and polydispersity of the systems studied depend on the macrocycle conformation. Based on the results obtained by ¹H and ¹H-¹H NOESY NMR spectroscopy and transmission electron microscopy for the macrocycles and their aggregates with Ag⁺, a coordination scheme for the Ag⁺ and different conformations of p-tert-butylthiacalix[4]arene functionalized with sulfobetaine fragments was proposed. The type of coordination determines the different shapes of the associates. Cytotoxic properties are shown to be controlled by the shape of associates, with the highest activity demonstrated by thiacalix[4]arenes in partial cone conformation. This complex partial cone/Ag⁺ is two times higher than the reference drug imatinib mesylate. High selectivity against cervical carcinoma cell line indicates the prospect of their using as components of new anticancer system.