Conformational Analysis of Triazine Dendrimers: Using NMR Spectroscopy To Probe the Choreography of a Dendrimer’s Dance

Abstract

One-dimensional (1D) and two-dimensional (2D) NMR studies are used to probe the conformation of a melamine dendrimer bearing unique NMR signals from the core to the periphery. Four conceptual anchors for dendrimer conformation emerge from these experiments. First, changes in isomer populations observed by 1 H NMR reveal the onset of globular structure. Second, NOE complexity emerges with globular structure: variable temperature NOESY studies show that the peripheral groups, BOC-protected aliphatic amines, fold back into the globular core of the macromolecule at 75 °C in DMSO-d 6 . Third, variable temperature coefficients measured for NH protons suggest that solvent is largely excluded from the interior of the dendrimer: the carbamate NH groups of the periphery are most sensitive to temperature while the NHs nearest the core show little temperature dependence. Conformation is influenced by solvent choice: backfolding is observed in DMSO-d 6 , but not in either CDCl 3 or CD 3 OD. Finally, relaxation studies show that peripheral groups are more dynamic than groups at the core. These anchors consolidate observations made by many groups on disparate systems within a common architecture.

Simulation of a Symmetric Binary Mixture of Charged Dendrimers Under Varying Electrostatic Interactions: Static and Dynamic Aspects

“…Such a correlation function essentially relaxes via the reorientational motion of the entire molecule, 42,63,64 and can be accessed experimentally with spectroscopic techniques such as neutron magnetic resonance measurements 64,65 and dielectric relaxation spectroscopy 66 (the latter technique requires the existence of a permanent dipole moment and probes the first order Legendre polynomial, but the essence of the information obtained remains the same). The reorientational spectra calculated separately for the two different generations in the examined electrostatic regimes are presented in Figure 11.…”

Design, synthesis and structure of novel G-2 melamine-based dendrimers incorporating 4-(n-octyloxy)aniline as a peripheral unit

“…According to H. Kessler (in 1982, [ 62 ]), this parameter is appropriate for describing amide protons solvation in p→π conjugated systems, such as –N(H)–C(=O)– ↔ –N + (H)=C(–O − )– in peptides and proteins, in water. Later on (in 2008), X. K. Moreno and E. E. Simanek [ 47 ] demonstrated the validity of the above TGs in the case of amino- s -triazines, –N(H)–C(=N–)– ↔ –N + (H)=C(–N − –)–, as well, in DMSO- d 6 . Following this new insight, if the TG values of “amidine-like” protons in amino- s -triazines are more negative than −4 ppb K −1 in strong hydrogen bond acceptor solvents, such as DMSO- d 6 [ 45 ], then the NH groups are exposed to the solvent rather than developing intramolecular hydrogen bonds.…”

“…A well-documented restricted rotation effect that causes diastereomerism is thus induced [ 43 – 45 ]. In the case of some dendritic melamines, this intrinsic feature can promote specific spatial arrangements at room temperature in solution, for example, asymmetric vs propeller ( C 3 -symmetric) [ 30 , 36 , 45 – 46 ], “dendritic choreography” [ 47 ], “open-gates closed-gates” frontier rotamerism [ 31 ] and “in out” axial chirality [ 33 ].…”

Polyester Dendrimers

“…For example, at low generations, a dendrimer typically has a floppy, flat structure, but at higher generations (usually > G-4), the polymer adopts a more globular or even spherical conformation [112] and rigidity increases with generation [113]. The behaviour of these compounds is complex with backfolding being significant [114][115][116].…”