In normal human cells, the telomere region of chromosomes is shortened with each mitosis, causing the cells to enter the stage of aging and apoptosis. However, in tumor cells, with the activation of the telomerase, the telomere of chromosomes is continuously lengthened, which makes the tumor cell extend to achieve “eternal life” [[1], [2], [3]]. In the presence of Na+ and K+, the DNA sequence at the end of the telomere may form a G-quadruplex (Gq) structure, which is composed of π–π stacking G-quartets formed by four guanines connected via Hoogsteen hydrogen bonding (Fig. 1A). The formation of the Gq structure prevents telomerase from recognizing telomere DNA. Therefore, tumor cells may be selectively induced to death through the binding of small molecule with the Gq and “lock” the telomere in the Gq structure [4].

Considering that the Gq stabilizer might have the potential to treat the cancer in a new way with fewer side effects, Gq binding ligands should be designed with attention to their structural characteristics leading them to selectively stabilize the Gq DNA structure in the presence of normal double-stranded DNA structures [5,6]. Based on previous research, small molecules may bind with Gq by three modes: end-stacking, intercalation, and outside groove binding [7] (Fig. 1A). There are mainly two kinds of binding forces, one of which is π-π interaction formed between aromatic rings of the small molecules and the base pairs of DNA and the other is electrostatic interactions formed between the positive charge center of the small molecules and the phosphate anions on the DNA backbone. It is also worth mentioning that Gq DNA has four grooves compared to the two grooves of the normal double-stranded DNA structure, so binding to the grooves located on the backbones may be beneficial to improve the selectivity of Gq binding.

Porphyrins refer to compounds that are comprised of four modified pyrrole subunits connected via methine bridges and form an aromatic macrocyclic structure (Fig. 1B). The aromatic plane of porphyrins has a similar size to the aromatic ring structure of Gq, and thus may stabilize Gq by strong π-π stacking interactions [8]. In addition, various substituents attached to the methyl carbon, may extend to the phosphate backbone of Gq DNA, forming electrostatic interactions with it, while affecting the electron cloud density distribution of the entire molecule. So far, porphyrin-related Gq binding reagents have attracted considerable attention as potential antitumor drugs, and continuous efforts have been paid to improve the binding force and selectivity to Gq DNA [9]. To this end, in this work, phenyl carbamate side chains, which are common anti-cancer pharmacophores [10,11] were attached to the porphyrin ring in order to introduce a potential amino positive charge center and more aromatic interactions, thus enhancing binding ability as well as Gq versus duplex selectivity (Fig. 1B).

The coordination of metal ions with porphyrins can also be used to tune the binding ability of the Gq binding ligands, as the metal center may influence both the steric and electronic properties of the molecule [[12], [13], [14], [15], [16]]. In fact, the importance and therapeutic applications of metal ions in nucleic acid chemistry have been known and studied for a long time [[17], [18], [19], [20], [21]]. While, to date, most of the studies have focused on metal complexes of Pt(II), Ru(II), Au(I/II), Ag(II), Pd(II), etc. [22]. In this work, copper was chosen as the metal center, based on the fact that, compared to the above metal ions with relatively larger size, it may form stronger interactions with the oxygen atoms of the G-quartets guanine molecules according to the hard-soft-acid-base theory.

In view of the above, in this work, two porphyrin carbamate derivatives (1 and 2) and a Cu(II) metalloporphyrin (1-Cu) were designed and synthesized, which were studied as antitumor agents targeting telomere Gq formation sequence. The single crystal of both 1 and 1-Cu were obtained and their structural characterization were analyzed in detail regarding to the influence on their Gq binding ability. The binding ability were further studied with both in vitro and in silico methods. Both 1 and 1-Cu showed high selectively toward Gq DNA compared with double-stranded DNA. The anti-proliferation activities on cancer cells were also tested and results showed that 1 and 1-Cu showed high anti-tumor potency while maintain low cytotoxicity on normal cells.