In Silico Study
Interaction of HL with DNA
It was shown that HL could intercalate into DNA and form stable complex when bound in a minor DNA groove via AGC-GCTA nucleotides (Fig. 3a). In this case, the stacking interaction between the nitrogenous base of A nucleotide and HL phenyl group as well as electrostatic interaction between CCl3 group and C nucleotide was formed. After MD simulation, the RMSD values for DNA double helix and HL were found to be 3.3 and 1.62 Å, respectively. Nucleotide environment of HL (GCA-CTA) was partly changed and stacking interaction was lost, while a hydrogen bond was formed between G nucleotide and CO group of HL.
Binding of HL with a major DNA groove occurred via TCG-AT nucleotides (Fig. 3b). In this case, a hydrogen bond between the CO group of HL and nitrogenous base of A nucleotide was formed and stacking interaction between both HL phenyls and nitrogenous bases of C and G nucleotides occurred. Besides, a probability of electrostatic bond between HL CCl3 group and nitrogenous bases of AT nucleotides appeared.
After MD simulation, no changes in the nucleotide environment of HL were detected. The RMSD values for DNA and HL were 2.77 and 1.58 Å, respectively. Due to it the stacking interaction between C nucleotide and phenyl group disappeared.
In the case of HL intercalation into DNA, its environment consisted of CG-CG nucleotides (Fig. 3c). The stacking interaction with CG nucleotides appeared: one phenyl group was clamped between the nitrogenous bases and the other formed a stacking interaction with C nucleotide.
After MD simulation, the RMSD values for DNA double-helix and HL were 1.71 and 1.89 Å, respectively. The nucleotide environment of HL was not changed. One of the phenyl group formed a ion-π interaction with the nitrogenous base of G nucleotide, which appeared to be shifted by 1.12 Å.
The obtained energy parameters testified that the energy of steric clashes between DNA and HL as well as within HL itself was insignificant (Table 1).
Table 1 Energy parameters (kJ/mol) of diphenyl-N-(trichloroacetyl)-amidophosphate (HL) interaction alone and in combination with C60 fullerene with DNA double-helix We have done the comparative analysis of energy parameters of HL binding with minor and major DNA grooves or its intercalation into minor DNA groove. Bump values were shown to be 6.2 kJ/mol in the case of HL intercalation into DNA and 2.5 kJ/mol in the case of its binding with minor and major DNA grooves (Table 1). Int values were 8.7 kJ/mol in the case of HL intercalation into DNA, 6.3 kJ/mol in the case of its binding with a major DNA groove, and 3.6 kJ/mol in the case of its binding with a minor DNA groove. These data showed that HL binding with a minor groove of DNA was the most stable.
Combined Interaction of HL and C60 Fullerene with DNA
Previously with the use of computer simulation, we have demonstrated that C60 molecule could interact with DNA and form a stable C60+DNA complex when binding with DNA minor groove [15]. As it is shown in Fig. 4, C60 fullerene could also form ion-π bonds with CCl3 groups of HL molecule.
We have used two versions of molecular modeling of HL, C60 fullerene and DNA interaction, which were proposed in [16] and proved to be useful for interpretation of MD simulation results. We have used 1XRW PDB structure of DNA molecule in the version, when initially HL and then C60 molecule intercalate into DNA (HL+C60) and 2MIW PDB structure of DNA molecule - in the version, when initially C60 molecule and then HL intercalate into DNA(C60+HL).
The binding with a minor DNA groove in the case of HL+C60 version occurred via GCTA-GCAT nucleotides (Fig. 4a). Phenyl groups filled a minor groove and entered into stacking interactions: one group with C60 fullerene and the other one with the nitrogenous base of G nucleotide. The electrostatic interactions arose between CCl3 group of HL and both the nitrogenous base of G nucleotide and C60 fullerene.
According to MD simulation results, a DNA double helix in this case was characterized by considerable mobility (RMSD value is 3.08 Å), RMSD value for HL was 2.04 Å, while C60 fullerene remained virtually immovable. As a result, the nucleotide environment of HL+C60 structure was changed by TGC-GCATG. Besides, a hydrogen bond between amino group of HL and DNA and stacking interactions between the nucleotides of nitrogenous GC bases and C60 fullerene appeared.
The binding of HL+C60 with a major DNA groove occurred via C-ATCC nucleotides (Fig. 4b). Hydrogen bonds were formed between HL CO group and the nitrogenous base of A nucleotide. Phenyl groups filled a major DNA groove and entered into stacking interaction with C60 fullerene.
According to MD simulation results, the nucleotide environment of HL+C60 structure was not changed: the values of RMSD for DNA and HL were 3.14 and 2.24 Å, respectively, C60 fullerene remained virtually immovable. In this case, all interactions between HL and C60 fullerene disappeared and HL sterically interacted only with DNA. It is supposed that as a result C60 fullerene as well as HL would be pushed out from the major DNA groove.
When HL+C60 were intercalated into a minor DNA groove (Fig. 4c), the binding with CGT-GAG nucleotides occurred. C60 fullerene occurred to be built into a minor DNA groove and to interact with it sterically. HL phenyl groups formed stacking interactions with the nitrogenous bases of CG-CG nucleotides. According to MD simulation, the values of RMSD for DNA and HL were 2.29 and 2.13 Å, respectively, C60 fullerene remained virtually immovable, and CC13 group of HL entered into electrostatic interaction with the nitrogenous base of G nucleotide.
In the case of C60+HL version, the binding with a minor DNA groove (Fig. 4d) occurred via CGC-GCC nucleotides. One of the HL phenyl groups formed a stacking with C60 fullerene and the other with G nucleotide. The CC13 group of HL appeared to form electrostatic bond with the nitrogenous base of C nucleotide. According to MD analysis, the values of RMSD for DNA and HL in this case were 2.35 and 2.75 Å, respectively, C60 fullerene stayed immovable, and the nucleotide environment of C60+HL structure was changed by CGCT-GC. In addition, the C60 molecule penetrated deeper into DNA and formed a stacking interaction with the nitrogen base of C nucleotide.
According to the calculated energy parameters, the complex formed in the case of C60+HL intercalation into a minor DNA groove was the most rigid (the Bump value 20.0 kJ/mol) (Table 1). In contrast in the case of HL+C60 interaction with DNA, this parameter was only 6.2 kJ/mol when HL+C60 was intercalated into a minor DNA groove, 7.8 kJ/mol when it was bound with a minor DNA groove, and 8.8 kJ/mol when it was bound with a major DNA groove. Besides, the energy parameters showed that formation of strong hydrogen bonds inside HL+C60+DNA complex was possible only in the case of binding with a major groove of DNA, when the value of Hbnd was − 2.3 kJ/mol, in other types of interaction it was equal zero or − 1.0 kJ/mol (Table 1). Moreover, according to MD results in this case both C60 fullerene and HL appeared to be displaced from a major DNA groove.
Therefore, C60+HL binding with a minor DNA groove is suggested to be the most probable version of C60 fullerene, HL, and DNA combined interaction.
In vitro Study of HL Biological Effects
CCRF-CЕM Cells Viability
In in vitro experiments, the long-term influence of diphenyl-N(trichloroacetyl)-amidophosphate (HL) in the range 2.5–10 μM on the viability of human leukemic CCRF-CЕM cells was estimated by MTT test. Cells were incubated for 24, 48, and 72 h in RPMI 1640 medium with either 16 μM C60 fullerene or HL alone or their combination. Viability of cells incubated without C60 or HL was considered to be 100%.
At 24 h of incubation, no effect of HL on CCRF-CЕM cells viability was observed (Fig. 5). Still at more prolonged incubation, the cytotoxic activity of HL in 5 and 10 μM concentration became obvious, cell viability at 48 h was inhibited by 25 and 33%, respectively, and continued to fall at 72 h.
It should be noted that 50% decrease of CCRF-CЕM cells viability (IC50) was detected at 72 h under the action of HL in 10 μM concentration (Fig. 5). Recently, we have shown that another CAPh representative dimorfolido-N-trichloracetylphosphorylamid caused 50% decrease of CCRF-CЕM cells viability at 72 h in concentration 1 mM [16]. The comparative analysis of these data demonstrates that introduction of phenoxy instead of morfolido groups into the structure of CAPh derivative allowed to decrease by two orders its effective toxic concentration against leukemic cells. We assume that conformational flexibility of –P(O)(OC6H5) core ensured more effective interaction of this compound with DNA.
No influence of C60 fullerene used alone on cell viability during incubation period was detected (data not presented). At the same time, the results shown on Fig. 6 demonstrate that C60 fullerene intensified HL cytotoxic activity against CCRF-CЕM cells. Under combined action of C60 fullerene and HL, 50% decrease of cell viability was observed at lower HL concentration (5 μM) and at earlier period (48 h) of incubation than under the action of HL by itself. Moreover at 72 h of combined action of C60 fullerene and HL, the cytotoxic effect of HL was detected in a low 2.5 μM concentration at which HL by itself had no influence on cell viability (Fig. 6).
Thus, C60 fullerene was shown to potentiate cytotoxic effects of HL and to increase significantly leukemic cells sensitivity to its action in a low concentration. Taking into account that C60 fullerene is able to accumulate inside leukemic cells over 24 h [28] and to localize in intracellular compartments [29,30,31], particularly in the nucleus [32, 33], its interaction with nuclear DNA of actively proliferated cancer cells should not be excluded.
So the data obtained in vitro are in accordance with the results of in silico study and demonstrate that initial binding of C60 molecule and then of HL with minor DNA groove with formation of a stable complex could be one of the possible reasons of their synergistic inhibition of CCRF-CЕM cells proliferation.
Erythrocytes Resistance to Hemolysis
On estimation of anticancer potential of HL and C60 fullerene combination, it is important to take into account its possible effects on nonmalignant cells, in particular on blood cells.
Study of erythrocytes resistance to acidic hemolysis allows to elucidate the influence of pharmacological agent at the membrane level. Dynamics of hemolysis reflects the dynamics of erythrocyte plasma membrane disruption and hence the stability of its structural organization. In Fig. 7, the dependence of the percentage of hemolysis erythrocytes were incubated for 1 h in NaCl solution without additions (control) and with either HL or C60 fullerene alone or in combination. No influence of 16 μM C60 fullerene on erythrocytes hemolysis was detected (not shown). HL in 2.5 μM concentration alone or in combination with C60 affected erythrocytes resistance to hemolysis (Fig. 7). Meanwhile, under the action of HL in 10 μM concentration acceleration of hemolysis with maximum at 20 s was detected. Combined action of 10 μM HL and C60 fullerene was followed by further hemolysis intensification with 60% of hemolysed erythrocytes at 20 s.
The data obtained showed that application of C60 fullerene in combination with 2.5 μM HL had no harmful effect on structural stability of blood erythrocytes membrane and at the same time allow to increase significantly cytotoxic activity of HL in this low concentration against leukemic cells. Despite the synergetic cytotoxic effect of C60 fullerene and HL in 10 μM concentration against leukemic cells, the application of their combination appeared to be limited by intensification of hemolytic effect.
Finally, in the context of the above in vitro study, it is important to emphasize that a water/solid interface have confined water which also can affect both the transport, thermodynamic properties of nanostructures [34, 35], and their interaction with cell membranes [36].
There are literature data on the intracellular localization of carbacylamidophosphates derivatives, in particular, penetration of phosphoramidates into the cells. Thus, phosphoramidate derivatives can penetrate throw the membrane of MDA-231 breast cancer and lung cancer (H460, H383, and H2009) cell lines [37]. Nitrobenzyl phosphoramide mustards were shown to permeate across cell membranes and to be localized in mitochondria of NTR+ mammalian cells [10]. It is not excluded that C60 fullerene, which is able to penetrate the membrane of the cancer cell due to passive diffusion or endocytosis [28, 30, 32] with accumulation in the nucleus and mitochondria [30, 32, 33], could be a transporter of small antitumor molecules [38,39,40].