Proton sponges cations

Once internalized, the essential step for the polyplex is to escape rapidly the endosomal vesicle in order to release the nucleic acid in the cytosol and prevent its lysosomal degradation. As the endosomal and lysosomal pH presents values between 4.5 and 6.5 and therefore differs from the neutral pH of 7.4 in other biological compartments [58], some polycations containing protonable residues like PEI facilitate this step by the proton sponge effect [59, 60]. As not all cationic polymers display this attribute, another effective method for enhanced endosomal polyplex release is incorporation of specific endosomal membrane disrupting or pore-forming domains, such as lytic lipid moieties or endosomolytic peptides. 

Recently, there has been research activity in the area of chiral proton sponges, in particular, those containing nitrogens with pairs of different substituents. The molecular asymmetry of such sponges, especially well pronounced in the cationic forms, arises as a result of the arrangement of bulkier functions at opposite sides of the naphthalene ring plane. Synthesis of these compounds, e.g. 5248, 5333 and 5449, is also achieved via alkylation of the corresponding A,A -dialkyl- or A,A,A -trialkyl-l,8-diaminonaphthalenes (Scheme 3). 

FIGURE 4. Possible types of IHB potential energy profiles in proton sponge cations... 

Theoretical calculations of an [H3N H- NH3]+ system have shown that when the N - N distance is 2.75 A, the potential curve has two minima with a barrier of 10.9 klrnol with a decrease in the distance to ca 2.50 A, the barrier disappears. As seen in Table 5, the minimal N N distance for proton sponge cations is 2.54-2.55 A, i.e. none of the compounds has a symmetrical barrier-free H-bond. Indeed, in 80% of cases, the X-ray studies on salts of proton sponge 1 have revealed an asymmetric hydrogen bridge with the distances N—H and N - H in the ranges 0.84-1.27 A and 1.42-1.86 A, respectively. It is believed that such strong variations are due to the influence of the anion, either by an electric field effect or by structural changes in the crystal lattice. 

An interesting situation occurs in the dication of the doubly protonated proton sponge 35 2H+ and in the zwitterion of 4,5-dihydroxy-l,8-bis(dimethylamino)naphthalene (111). The asymmetry of both hydrogen bridges in these systems is clearly controlled by electrostatic factors in order to obtain maximal separation of the two positively charged centres in 35 2H+ and, in reverse, to gain an attraction between the cationic and anionic centres in 111. 

There are very specific changes of the hydrogen bridge geometry in the cations of 2,7-disubstituted proton sponges. The measurements of such systems were performed for five salts with cations 44 H+, 101-II1,102 H+, 105 H+ and 112 H+. The ortfio-substituents in these cations display a distinct steric pressure on the adjacent A-methyl groups, pushing... 

C and 15N NMR spectra of the proton sponges and their cations were investigated both in solutions and in the solid state. These were used to establish structural and electronic characteristics of the molecules and to investigate the proton exchange between the cations and bases94. 

Selective displacement of one CIT3 by the CD3 group was employed to establish the structure of IF1B in the cations of non-symmetrically substituted naphthalene proton sponges by FI NMR1". This direct method is simple and does not require NMR on other nuclei such as deuterium, carbon or nitrogen. 

The oxidation of compound 32 occurs in a similar manner but even more easily136. Its radical cation can be stored unchanged in acetonitrile for months. The UV spectrum of 32+ (7.max = 480 nm, lg e = 3.1) is very close to that of neutral 32 itself, from which one can conclude that the naphthalene moiety does not take part in the oxidation. As found by cyclovoltammetry, the double proton sponge 35, unlike its isomer 79, is oxidized with reversible two-electron transition, composed of two superimposed one-electron steps at Eli2 —0.50 V (Table 13)45. Apparently, the driving force for this process is the formation of the resonance-stabilized dication 133 (equation 8), which was isolated in the form of black crystals with I3- anions (kmax = 643 nm, lg e = 4.02) and investigated in... 

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