Terminated PAMAM Dendrimers - American Chemical Society

Article pubs.acs.org/JPCB

Study of the Complexation of Oxacillin in 1‑(4-Carbomethoxypyrrolidone)-Terminated PAMAM Dendrimers Jon S. Hansen, Mario Ficker, Johannes F. Petersen, Bjarne E. Nielsen, Saira Gohar, and Jørn B. Christensen* Department of Chemistry, University of Copenhagen, Thorvaldsensvej 40, DK-1871 Frederiksberg, Copenhagen, Denmark S Supporting Information *

ABSTRACT: The complexation of oxacillin to three generations of 1-(4carbomethoxypyrrolidone)-terminated PAMAM dendrimers was studied with NMR in CD3OD and CDCl3. The stochiometries, which were determined from Job plots, were found to be both solvent- and generation-dependent. The dissociation constants (Kd) and Gibbs energies for complexation of oxacillin into the 1-(4-carbomethoxypyrrolidone)-terminated PAMAM dendrimer hosts were determined by 1H NMR titrations and showed weaker binding of oxacillin upon increasing the size (generation) of the dendrimer.

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INTRODUCTION Dendrimers are a class of synthetic macromolecules of nanoscale dimensions, which are characterized by having a structure based on repetitive branching from a core. Depending on the repetitive branching unit, dendrimers can have interior voids in the structure capable of hosting smaller molecules.1−3 The potential uses of dendrimers in nanomedicine and nanotechnology are areas that have been growing rapidly in recent years, and the applications cover a broad area from drug delivery, to cosmetics, to extraction of oil, to catalysis.4−6 Dendrimers are characterized by a large number of surface groups, which are responsible for properties such as solubility. It is thus possible by proper modifications of the surface to create dendrimers that are unimolecular micelles having an interior with completely different physical properties than the surrounding solvent. This was originally described by Newcome and co-workers7 and Tomalia and co-workers,8 and the formation of guest−host complexes between small molecules and dendrimers has since been studied by a number of other groups.9−21 Water-soluble dendrimers have also been studied as potential vehicles for drug delivery or for improving pharmacokinetics of drugs and compounds studied covering the range from anticancer drugs such as cis-platin14 and Campthotecin,22,23 guanosine monophosphate,16 dexamethasone phosphate,18 NSAIDs,21,24 and iminodiacetic acid derivatives25 to nerve gas antidotes.26,27 Recently we discovered that PAMAM dendrimers having 1-(4-carbomethoxypyrrolidone) groups (Figure 1) as surface groups are soluble in a number of very different solvents ranging from water to toluene, making it possible to study the formation and physical properties of guest−host complexes in solvents of very different properties. This is interesting because © 2013 American Chemical Society

the tertiary structure of dendrimers (and possibly the size of the internal voids) in solution is known to depend on the polarity of the solvent. Amis and co-workers studied the size and shape of PAMAM dendrimers in different alcohols (methanol to 1-butanol) using neutron diffraction and found that the changes in the shape and size became smaller for larger generation dendrimers, indicating that the tertiary structure of the dendrimers became less flexible.28 Later, Rinaldi and coworkers did extensive NMR studies on polypropylene imine (PPI) dendrimers, which gave detailed information on the tertiary structure, showing that the structure of PPI dendrimers changes considerably upon changing solvent from CDCl3 to C6D6, becoming more compact.29,30 Knowing that the pyrrolidone-terminated PAMAM dendrimers are virtually nontoxic31,32 led us to choose the β-lactam antibiotic oxacillin (Figure 2) as the guest molecule because such a system could eventually be interesting for bypassing the efflux-pump-mediated resistance in methicillin-resistant Staphylococcus aureus (MRSA) bacteria. Oxacillin has a free carboxylic acid group that could facilitate binding to the interior tertiary amines in the dendrimers. Previous work with acetamideterminated PAMAM dendrimers showed that PAMAM dendrimers with a neutral amide surface were capable of binding ionic compounds such as Congo red and Indocyanine green.33 Three generations of 1-(4-carbomethoxypyrrolidone)terminated PAMAM dendrimers (G2, G3, and G4) were chosen as the dendrimer hosts together with oxacillin, and the Received: August 28, 2013 Revised: November 12, 2013 Published: November 12, 2013 14865

dx.doi.org/10.1021/jp408613z | J. Phys. Chem. B 2013, 117, 14865−14874

The Journal of Physical Chemistry B

Article

Figure 1. Illustration of a G4 1-(4-carbomethoxypyrrolidone) PAMAM dendrimer with encapsulated oxacillin molecules.

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EXPERIMENTAL SECTION Unless otherwise stated, all starting materials were obtained from commercial suppliers and used as received. Solvents were HPLC grade and used as received. 1H NMR and 13C NMR spectra were recorded on a 500 MHz NMR (Bruker) apparatus (500 MHz for 1H NMR and 125 MHz for 13C NMR) equipped with a cryo-probe. Chemical shifts are reported in ppm downfield of TMS (tetramethylsilane) using the resonance of the residual solvent as the internal standard. Preparation of Dendrimers. The PAMAM dendrimers of the generations G2−G4 were synthesized by published procedures,3 starting from 1,4-diaminobutane as the core. The 1-(4-carbomethoxypyrrolidone) surface functionalization was done by reacting the amino-terminated dendrimers with

Figure 2. The molecular structure of the antibiotic oxacillin.

solvents used were CD3OD (dielectric constant: 32.6) and CDCl3 (dielectric constant: 4.8).34 The binding of oxacillin to the dendrimers was studied by NMR, which is probably the best all-round technique for studying guest−host complexation in dendrimers.35 14866



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dimethylitaconate (Figure 2).31,36 The completion of the reaction was tested with the Kaiser test, which is highly sensitive to primary and secondary amines. NOE Spectroscopy. NOESY experiments were conducted on a 500 MHz NMR (Bruker) apparatus equipped with a cryoprobe. The concentration of dendrimer was 4.8 mM, and the concentration of oxacillin was 47.3 mM in CDCl3; for the CD3OD solutions, the same concentrations were used. The dendrimer sample was incubated at room temperature with the oxacillin for 2 h prior to starting the experiment. The experiments were performed at 25 °C with a 2 s relaxation delay, 205 ms acquisition time, 300 ms mixing time, and 8.2 μs 1H 90° pulse width. Eight transients were averaged for 1024 t1 increments. Preparation of NMR Samples for Job Plots. The Five mL stock solutions in CDCl3 and CD3OD were prepared with 10 mM of each generation of 1-(4-carbomethoxypyrrolidone) dendrimers from G2 to G4 and 10 mM oxacillin as the free acid. The samples were prepared by injecting a total volume of 500 μL in 15 NMR tubes for each Job plot, keeping a constant total concentration of 10 mM ([den] + [oxa]), where the ratio [den]/[oxa] was varied. Preparation of NMR Samples for Dissociation Constant Determination. The 5 mL stock solutions of (1) and (2) in CDCl3 and CD3OD were prepared, (1) containing 1 mM of a given 1-(4-carbomethoxypyrrolidone) dendrimer and (2) containing 1 mM of a given 1-(4-carbomethoxypyrrolidone) dendrimer and 150 mM oxacillin. Samples were prepared for each dendrimer generation and for each solvent, keeping the dendrimer concentration constant while varying the oxacillin concentration from 0 to 100 mM.37 Binding saturation curves were fitted employing Origin 9.0. Preparation of NMR Samples for Aggregation Constant Determination. The 150 mM oxacillin solutions were prepared in CDCl3 and CD3OD. These stock solutions were diluted with either CDCl3 or CD3OD to obtain oxacillin concentrations similar to those obtained for the determination of dissociation constants. The aggregation curves were fitted employing Origin 9.0.

A 1-(4-carbomethoxypyrrolidone)-terminated G4 dendrimer is illustrated in Figure 1, and the synthetic preparation of a G3 4carbomethoxypyrrolidone dendrimer by modification of a G3 PAMAM-NH2 dendrimer with dimethylitaconate is depicted in Scheme 1. Scheme 1. Synthesis of a G3 1-(4-Carbomethoxypyrrolidone)Terminated PAMAM Dendrimer with 32 Surface Groups

The protons for the G3 1-(4-carbomethoxypyrrolidone) PAMAM dendrimer and the protons for oxacillin have been assigned in Figures 3 and 4, respectively The dendrimer proton signals k−m belong to the 1-(4-carbomethoxypyrrolidone) surface groups, while the signals a−j belong to the inner branches of the dendrimer. The periodical change of dendrimer signals in ppm and the signal shape can provide information about where the guest molecules are situated (binding site). This technique also enables estimation of the stoichiometry of bound guest molecules and provides information about binding strength, which can be pictured by calculation of the respective Gibbs energies and dissociation constants, Kd’s.38 The 1H NMR titration of a G3 1-(4-carbomethoxypyrrolidone) dendrimer with oxacillin in CD3OD (Figure 5) shows significant ppm shifts for both dendrimer and oxacillin proton signals. Especially the dendrimer proton signal c, which is positioned next to the tertiary amino groups in the dendrimer’s interior, is affected by a large downfield shift of almost 0.5 ppm. This is most likely due to protonation of the tertiary amino groups by oxacillin, which induces the strong electrostatic shift of the dendrimer signal. Signals farther away from the protonation site also exhibit a significant shift. This observation also supports the hypothesis of the strong electrostatic binding of oxacillin within the interior of the dendrimer branches. The surface signal n from the 1-(4-carbomethoxypyrrolidone) groups does not undergo a significant shift for low oxacillin concentrations. Thus, oxacillin is most likely initially bound within the dendrimer and not on the surface. However, upon addition of large amounts of oxacillin, that is, above 20 equiv, the surface groups undergo an upfield shift. This shift is most likely attributed to favorable lipophilic interactions between aromatic units of oxacillin and the 1-(4-carbomethoxypyrrolidone) dendrimer surface. The 1H NMR titration of the G3 1-(4-carbomethoxypyrrolidone) dendrimer with oxacillin in CDCl3 (Figure 6) also shows a large shift for some of the dendrimer proton signals. Similar to the methanol experiment, the proton signal, c, next to the tertiary amino groups is shifting downfield by almost 0.4 ppm. This shift may likely be caused by protonation and electrostatic interaction with the carboxylate of oxacillin. While the surface groups exhibited a lipophilic shift at very high oxacillin concentrations in methanol, this shift is already starting at a concentration of 3.7 mM oxacillin in chloroform, showing a difference in binding. From the above analysis, it can be stated that already at low oxacillin concentrations, the interior of the dendrimer is saturated with guest molecules. Further addition of oxacillin results in weaker binding

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RESULTS AND DISCUSSION In order to elucidate the possibility of complex formation between 1-(4-carbomethoxypyrrolidone)-terminated PAMAM dendrimers and oxacillin, a number of 1H NMR titrations of different generations of 1-(4-carbomethoxypyrrolidone)terminated PAMAM dendrimers with oxacillin were performed. The NMR samples were prepared by incubation of oxacillin with the corresponding dendrimer, for example, generations from G2 (16 surface groups) to G4 (64 surface groups). It is advantageous to employ 1H NMR to measure the dendrimer− oxacillin interaction because the 1H NMR method extends the range of measurable interactions into the mM range, a region not covered by traditional biochemical assays.37 The binding behavior of oxacillin was found to depend both on the employed solvent and on the generation of the 1-(4carbomethoxypyrrolidone) dendrimers in a significant manner. NOESY experiments revealed an interaction between oxacillin and the interior of the dendrimers. This interaction may likely be electrostatic, that is, an interaction between a protonated tertiary amine in the dendrimer and the carboxylate of oxacillin. Favorable lipophilic interactions between the aromatic units in oxacillin and the ethylene branches and the butylene core in the PAMAM dendrimers were also found to contribute. 14867



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Figure 3. Assignment of the 1H NMR spectra of the G3 dendrimer in CD3OD (top) and CDCl3 (bottom).

Figure 4. Assignment of the 1H NMR spectra of oxacillin in CD3OD (top) and CDCl3 (bottom).

interactions with the 1-(4-carbomethoxypyrrolidone) surface groups. NOE experiments were performed in methanol and chloroform in order to confirm the above analysis of the oxacillin binding mode. The protons located at positions h, d, f, k, c, g, e, i, and j assigned to be in the interior of the G3 1-(4carbomethoxypyrrolidone) dendrimer were found to couple with the β-lactam proton 6 and also the aromatic protons 8−10 in oxacillin both in methanol and in chloroform. The NOESY spectrum for binding in methanol is shown in Figure 7.

However, the NOE signals in chloroform were found to be significantly weaker than the ones recorded in methanol. These are shown in the Supporting Information. These NOE experiments are in good agreement with the results obtained from the 1H NMR titrations. Determination of the binding stoichiometry for the respective dendrimer−oxacillin complex was achieved by the method of continuous variation.39,40 Job plots were performed for the dendrimer generations G2−G4 in deuterated methanol and chloroform. The obtained Job plots are shown 14868



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Figure 5. The stacked 1H NMR spectra of the G3 1-(4-carbomethoxypyrrolidone) PAMAM dendrimer (1 mM) in CD3OD incubated with different concentrations of oxacillin, showing a significant change in chemical shift for both the dendrimer (left) and oxacillin (right).

in Figure 8, and the determined stoichiometries are listed in Table 1. As a general trend, the higher generations of dendrimers were capable of forming complexes with a higher number of oxacillin molecules than the lower dendrimer generations. In deuturated methanol, the dendrimer capable of binding the highest number of oxacillin molecules was the G4 dendrimer, where a 3:1 oxacillin/dendrimer complex was calculated. The stoichiometry decreased to 1:1 for the G3 and the G2 dendrimers. In CDCl3, the larger dendrimer generations, G3 and G4, were capable of binding two oxacillin molecules, whereas the G2 dendrimer was only able to bind a single oxacillin molecule. It is important to emphasize that the maxima determined from the Job plots do not provide information on the highest possible binding stoichiometry but instead on which of the complexes contributes most to the observed chemical shift. Thus, the determined stoichiometry is the one of the predominant species, even though there may be higher stoichiometries existing in small concentrations.

Figure 6. The stacked 1H NMR spectra of the G3 1-(4carbomethoxypyrrolidone) PAMAM dendrimer (1 mM) in CDCl3 incubated with different concentrations of oxacillin.

Figure 7. (Left) The full NOESY spectra of the G3 1-(4-carbomethoxypyrrolidone) dendrimer incubated with oxacillin in CD3OD, where the relevant cross peaks between the oxacillin and the dendrimer are highlighted. (Right) An enlarged section of the low-field cross peaks. 14869



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Figure 8. Job plots for binding of oxacillin with 1-(4-carbomethoxypyrrolidone) dendrimers G2−G4 in CDCl3 and CD3OD.

to study the complexation of different guest molecules to ureaadamantyl-modified PPI dendrimers.43 Due to the self-aggregation of oxacillin, the available monomeric oxacillin is not equal to the added oxacillin concentration; it is thus necessary to take this aggregation into account to determine a more precise binding constant. This is done by first determining the degree of aggregation for oxacillin and then adjusting the binding model to factor the degree of aggregation into the determination of dissociation constant. One way to determine the extent of self-aggregation is to use the isodesmic model (eq 2) as done by Baxter and coworkers.41,42 The isodesmic model works under the assumption that the monomeric units aggregate into stacks, and the aggregation constant for each monomer added to the aggregate is the same (independent of the stack length).

Table 1. The binding stoichiometries for the complexes formed between the tested generations of 1-(4-carbomethoxypyrrolidone) dendrimers and oxacillin in CDCl3 and CD3OD respectively dendrimer/solvent

CD3OD

CDCl3

pyrrolidone G2 pyrrolidone G3 pyrrolidone G4

1:1 1:1 3:1

1:1 2:1 2:1

The difference in stochiometries of the complexes in CD3OD and CDCl3 for the G2 and G3 dendrimers could be explained by better solvation of the oxacillin in CD3OD than that in CDCl3 and therefore a smaller gain in free energy upon binding to the dendrimer. This is also supported by the difference in aggregation constants for oxacillin, 3.19 M−1 in CD3OD versus 4.67 M−1 in CDCl3. In order to determine the binding constants for the binding of oxacillin to the respective 1-(4-carbomethoxypyrrolidone) dendrimer, a suitable binding model was needed. Because dendrimers are large macromolecules capable of binding multiple guest molecules, some assumptions are necessary in order to determine the binding constant; (1) each dendrimer molecule has a number n of independent binding sites, and (2) the binding sites all have equal dissociation constants. Such a system can be described by the equilibrium given in eq 1. Furthermore, it was necessary to correct for aggregation of the oxacillin, and this led to eq 1, which is derived in the Supporting Information. Δδ = − =

−

⎛ Δδ = ΔδmaxK a[oxa]0 ⎜⎜ ⎝1 +

This model described by Baxter makes use of H NMR to follow the chemical shift change in a dilution experiment. In eq 2, Δδ is the change in observed chemical shift relative to the pure monomeric substance, Δδmax is the maximum chemical shift observed for a theoretical completely stacked species, [oxa]0 is the starting concentration of oxacillin, and Ka is the aggregation constant. The aggregation constant can be determined by fitting the model described by eq 2 to the obtained data set from a 1H NMR dilution experiment. The obtained data sets are shown in Figure 9, and the aggregation constants are listed in Table 2. Having determined the aggregation constants for oxacillin, the binding model (eq 5) can now be rewritten to take the oxacillin aggregation into account. This is done by substitution of the initial oxacillin concentration [oxa]0 for the available oxacillin concentration [oxa]avail (eq 3). The available oxacillin concentration can, according to the isodesmic model,41,42 be expressed by eq 4; substituting this into eq 3 yields the final binding model (eq 5).

(Kd + [oxa]0 + n[den]0 )2 − 4[den]0 n[oxa]0 ) ⎛ Δδmax ⎜ Kd [oxa]0 + 1+ ⎜ 2 n[den]0 n[den]0 ⎝ ⎞ ⎟ ⎟ ⎠

(2)

1

Δδmax (Kd + [oxa]0 + n[den]0 2n[den]0

2 ⎛ Kd [oxa]0 ⎞ [oxa]0 + ⎜1 + ⎟ −4 n[den]0 n[den]0 ⎠ n[den]0 ⎝

⎞2 ⎟ ⎟ 4K a[oxa]0 + 1 ⎠ 2

Δδ = (1)

This equation correlates the change in chemical shift, which can be extracted from the 1H NMR titration experiments, with the desired dissociation constant and the number of binding sites per dendrimer, using the known starting concentrations. This method for determining dissociation constants as well as binding stoichiometries has previously been used by Baxter and co-workers.42 Later, a similar model was applied by Meijer et al.

−

⎛ Δδmax ⎜ Kd [oxa]avail + 1+ 2 ⎜ n[den]0 n[den]0 ⎝ 2 ⎛ Kd [oxa]avail ⎞ [oxa]avail + ⎜1 + ⎟ −4 n[den]0 n[den]0 ⎠ n[den]0 ⎝

[oxa]avail 14870

⎛ = [oxa]0 ⎜⎜ ⎝ (1 +

⎞2 ⎟ ⎟ 4K a[oxa]0 + 1 ) ⎠

⎞ ⎟ ⎟ ⎠

(3)

2

(4)



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Figure 9. Aggregation of oxacillin in CDCl3 (left), where the peak at 6.10 ppm is followed, and for CD3OD (right), where the peak at 5.58 ppm is followed.

⎛ Kd 4[oxa]0 Δδmax ⎜ Δδ = + ⎜1 + 2 ⎜ n[den]0 n[den]0 (1 + 4K a[oxa]0 + 1 )2 ⎝ −

⎛ ⎞2 Kd 4[oxa]0 16[oxa]0 ⎜1 + ⎟ − + ⎜ ⎟ 2 n[den]0 n[den]0 (1 + 4K a[oxa]0 + 1 ) ⎠ n[den]0 (1 + 4K a[oxa]0 + 1 )2 ⎝

This model was then fitted to the experimental data obtained from the 1H NMR titration; this was achieved using the leastsquares fitting routine implemented in Origin 9.0 while fixing the number of binding sites n to the previously determined stoichiometry. This was required as the fitting model depends on three variables (n, Δδmax, Kd), and because simultaneously fitting all of them was not possible, n was fixed at the previously determined values. The obtained data with the fitted curves are shown in Figure 10, and the obtained dissociation constants are listed in Table 3. The fitting of the binding model to the 1H NMR revealed that the binding strength is correlated to the dendrimer size and the applied solvent. The observed trend is that each oxacillin molecule exhibits a weaker binding interaction with the large dendrimer generations, that is, the values of Kd increase as the dendrimer generation increases. An explanation might be steric repulsion between oxacillin molecules in the dendrimer. With just one guest molecule, the smallest dendrimers are able to provide a well-suited binding pocket. Upon increasing generation, the dendrimer interior has to be shared by more than one guest molecule, resulting in unfavorable steric repulsion between the oxacillin guests and thus weaker binding.

CD3OD

CDCl3

3.19 ± 1.27 −2.88 ± 1.04

4.67 ± 0.82 −3.82 ± 0.44

(5)

Comparing the binding affinities in CDCl3 and CD3OD, oxacillin exhibits a stronger interaction with the smaller dendrimer generations G2 and G3 in chloroform. This observation may be ascribed to a stronger electrostatic interaction between oxacillin and the dendrimer as the carboxylate of oxacillin is more destabilized in a solvent with a low dielectric constant, such as chloroform in comparison to methanol. In methanol, competitive hydrogen bond interactions between hydroxyl groups and the carboxylate can occur. Furthermore, methanol is competing for hydrogen binding sites with oxacillin. For the largest dendrimer used (G4), the result was different as Table 3. Dissociation Constants and Molar Gibbs Energies for the Host−Guest Interaction between Oxacillin and the Different 1-(4-Carbomethoxypyrrolidone) Dendrimers As Determined by Fitting to the Titration Curvesa dendrimer Pyrrolidone G2 n Kd (mM) ΔG (kJ mol−1) Pyrrolidone G3 n Kd (mM) ΔG (kJ mol−1) Pyrrolidone G4 n Kd (mM) ΔG (kJ mol−1)

Table 2. Aggregation Constants, Ka, and the Corresponding Molar Gibbs Energies for the Aggregation of Oxacillin in CDCl3 and CD3OD, Calculated by Equation 2 Ka (M−1) ΔGm (kJ mol−1)

⎞ ⎟ ⎟ ⎟ ⎠

CD3OD

CDCl3

1 34.3 ± 3.5 −8.36 ± 0.25

1 23.7 ± 2.2 −9.28 ± 0.23

1 60.6 ± 12.7 −6.95 ± 0.53

2 47.2 ± 7.8 −7.57 ± 0.42

3 166.1 ± 38.1 −4.45 ± 0.58

2 292.2 ± 82.5 −3.05 ± 0.72

a

The numbers of guest molecules, n, were extracted from the previous calculated Job plots. 14871



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Figure 10. (Top) The fitted curves for the binding of oxacillin to the G2, G3, and G4 dendrimers in CDCl3, monitoring the shift of the dendrimer peak at 3.733, 2.337, and 2.633 ppm, respectively. (Bottom) The fitted curves for the binding of oxacillin to the G2, G3, and G4 dendrimers in CD3OD, monitoring the shift of the dendrimer peak at 2.627, 2.333, and 2.335 ppm, respectively.

tion are important aspects of using dendrimers for drug delivery, and our present study has shown that bigger is not necessarily better when it comes to the stoichiometry and stability of the complexes. Given that the pyrrolidone-terminated PAMAM dendrimers are essentially nontoxic and highly water-soluble makes studies of drug delivery in vitro the next logical step.

oxacillin showed a slightly higher affinity to the dendrimer in methanol than that in chloroform. In both cases, the binding strength was significantly weaker compared to the generations G2 and G3. This might be explained by a change in dendrimer structure going from G3 with 32 surface groups to G4 with 64 surface groups, where the surface density is increased significantly. This can lead to a new and more rigid structure, depending on the solvent, with less favorable oxacillin binding sites. Such a conterintuitive shift in guest-binding behavior of a dendrimer has been observed in triazine dendrimers,44 and a very recent EPR-study on binding of Cu(II) to pyrrolidoneterminated PAMAM dendrimers has also shown a change in binding sites upon going from G3 to G4.45

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ASSOCIATED CONTENT

S Supporting Information *

The preparation of the oxacillin carboxylic acid and additional figures (NMR titrations and NOE spectra of the G3 dendrimer in CDCl3) as well as the derivation of eq 1. This material is available free of charge via the Internet at http://pubs.acs.org.

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CONCLUSION AND OUTLOOK 1-(4-Carbomethoxypyrrolidone)-terminated PAMAM dendrimers of different generations (G2−G4) were found to form complexes with the antibiotic oxacillin in deuterated methanol and chloroform. The stoichiometries were determined by Job plots, which showed increased drug-carrying ability for the higher dendrimer generations but weaker binding per guest molecule. Fitting of a binding model to the obtained NMR data was used to study the binding strength of the formed complexes. The binding ability of the dendrimers was found to be solvent- and generation-dependent. This observation may be attributed to a higher degree of electrostatic interaction between the carboxylate of oxacillin and the protonated tertiary amines in the dendrimer in chloroform compared to that in methanol. The smaller dendrimer generations were found to bind fewer oxacillin molecules, but this interaction was found to be significantly stronger than the interaction with the larger generations. The loading capacity and ability to carry the load to its destina-

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +4520812771. Author Contributions

The article was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

The University of Copenhagen, Department of Chemistry is gratefully acknowledged for financial support. Christian G. Tortzen is gratefully acknowledged for his assistance in recording the NOE spectra. 14872



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Terminated PAMAM Dendrimers - American Chemical Society

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