Gels formed via metal–ligand coordination typically have very low branch functionality, f, as they consist of ∼2–3 polymer chains linked to single metal ions that serve as junctions. Thus, these materials are very soft and unable to withstand network defects such as dangling ends and loops. We report here a new class of gels assembled from polymeric ligands and metal-organic cages (MOCs) as junctions. The resulting ‘polyMOC’ gels are precisely tunable and may feature increased branch functionality. We show two examples of such polyMOCs: a gel with a low f based on a M2L4 paddlewheel cluster junction and a compositionally isomeric one of higher f based on a M12L24 cage. The latter features large shear moduli, but also a very large number of elastically inactive loop defects that we subsequently exchanged for functional ligands, with no impact on the gel's shear modulus. Such a ligand substitution is not possible in gels of low f, including the M2L4-based polyMOC.

The structure of two protected amino acids, FMOC-l-leucine and FMOC-l-valine, and a dipeptide, N-acetyl-l-valyl-l-leucine (N-Ac-VL), were studied via one- and two-dimensional solid-state nuclear magnetic resonance (NMR) spectroscopy. Utilizing O magic-angle spinning (MAS) NMR at multiple magnetic fields (17.6-35.2 T/750-1500 MHz forH) the O quadrupolar and chemical shift parameters were determined for the two oxygen sites of each FMOC-protected amino acids and the three distinct oxygen environments of the dipeptide. The one- and two-dimensional,O, N-O, C-O, and H-O double-resonance correlation experiments performed on the uniformly C,N and 70% O-labeled dipeptide prove the attainability ofO as a probe for structure studies of biological systems. N-O and C-O distances were measured via one-dimensional REAPDOR and ZF-TEDOR experimental buildup curves and determined to be within 15% of previously reported distances, thus demonstrating the use of O NMR to quantitate interatomic distances in a fully labeled dipeptide. Through-space hydrogen bonding of N-Ac-VL was investigated by a two-dimensionalH-detected O R-R-INEPT experiment, furthering the importance of O for studies of structure in biomolecular solids.

Direct 17O NMR structural studies of bound water in crystalline hydrates and biological macromolecules are challenging due to the low natural abundance and quadrupolar nature of 17O nuclei. However, the advent of high field NMR positions 17O NMR to become an important tool to address structural problems in biological solids. We show herein that the NMR properties of 17O in a series of amino acids and dipeptides can be determined by a combination of non-spinning and magic-angle spinning experiments using a range of magnetic field strengths from 9.4 to 21.1 T. Furthermore, we propose a 17O chemical shift fingerprint region for bound water molecules in biological solids that is well outside the previously determined ranges for carbonyl, carboxylic, and hydroxyl oxygens, thereby offering the ability to resolve multiple 17O environments using rapid one-dimensional NMR techniques. Finally, we compare our experimental data against quantum chemical calculations using GIPAW and hybrid-DFT, finding intriguing discrepancies between the electric field gradients calculated from structures determined by x-ray and neutron diffraction.

The structure and dynamics of the bound water in barium chlorate monohydrate were studied with 17O nuclear magnetic resonance (NMR) spectroscopy in samples that are stationary and spinning at the magic-angle in magnetic fields ranging from 14.1 to 21.1 T. 17O NMR parameters of the water were determined, and the effects of torsional oscillations of the water molecule on the 17O quadrupolar coupling constant (CQ) were delineated with variable temperature MAS NMR. With decreasing temperature and reduction of the librational motion, we observe an increase in the experimentally measured CQ explaining the discrepancy between experiments and predictions from density functional theory. In addition, at low temperatures and in the absence of 1H decoupling, we observe a well-resolved 1H–17O dipole splitting in the spectra, which provides information on the structure of the H2O molecule. The splitting arises because of the homogeneous nature of the coupling between the two 1H–17O dipoles and the 1H–1H dipole.

We report star polymer metal-organic cage (polyMOC) materials whose structures, mechanical properties, functionalities, and dynamics can all be precisely tailored through a simple three-component assembly strategy. The star polyMOC network is composed of tetra-arm star polymers functionalized with ligands on the chain ends, small molecule ligands, and palladium ions; polyMOCs are formed via metal-ligand coordination and thermal annealing. The ratio of small molecule ligands to polymer-bound ligands determines the connectivity of the MOC junctions and the network structure. The use of large M12L24 MOCs enables great flexibility in tuning this ratio, which provides access to a rich spectrum of material properties including tunable moduli and relaxation dynamics.

Polymer gels are often very soft due to their low branch functionality ( f) and the inevitable presence of defects (e.g., primary loops or dangling chains). Polymer metal−organic cage (polyMOC) gels are a relatively new class of supramolecular gels with precisely defined junction structures made possible by subcomponent assembly of nanoscale MOCs connected by polymer chains. Herein, we report that variation of the molecular weight and architecture of the polymer component of polyMOCs provides an entry into gels with ultra-high f. For example, materials with f ∼ 9− 12, i.e., ∼ 9−12 polymer chains connect each MOC within the gel network, are realized. As a consequence of their increased f, these gels display exemplary mechanical properties at low concentrations (down to 240 μM) of metal ions and only 5.4−5.9 wt % of polymer. Furthermore, X-ray and neutron scattering pointed to an additional level of structural hierarchy that arises from the assembly of M 12 L 24 MOCs into clusters. The relationships between polymer and polyMOC network structure revealed here will facilitate the design of high-performance polyMOCs.

The importance of studying site-specific interactions of structurally similar water molecules in complex systems is well known. We demonstrate the ability to resolve four distinct bound water environments within the crystal structure of lanthanum magnesium nitrate hydrate via 17O solid state nuclear magnetic resonance (NMR) spectroscopy. The approach utilizes high-resolution multi-dimensional 17O NMR experiments at high magnetic fields (18.8 – 35.2 T) where each individual water environment was resolved. The quadrupolar coupling constants and asymmetry parameters of the 17O of each water were determined to be between 6.6 and 7.1 MHz, and 0.83 and 0.90. The resolution of the four unique, yet similar, structural waters within a hydrated crystal via 17O NMR spectroscopy demonstrates the ability to decipher the unique electronic environment of structural water within a single hydrated crystal structure.

We present an improved and general approach for implementing echo train acquisition (ETA) in magnetic resonance spectroscopy, particularly where the conventional approach of Carr-Purcell-Meiboom-Gill (CPMG) acquisition would produce numerous artifacts. Generally, adding ETA to any N-dimensional experiment creates an N + 1 dimensional experiment, with an additional dimension associated with the echo count, n, or an evolution time that is an integer multiple of the spacing between echo maxima. Here we present a modified approach, called phase incremented echo train acquisition (PIETA), where the phase of the mixing pulse and every other refocusing pulse, φ(P), is incremented as a single variable, creating an additional phase dimension in what becomes an N + 2 dimensional experiment. A Fourier transform with respect to the PIETA phase, φ(P), converts the φ(P) dimension into a Δp dimension where desired signals can be easily separated from undesired coherence transfer pathway signals, thereby avoiding cumbersome or intractable phase cycling schemes where the receiver phase must follow a master equation. This simple modification eliminates numerous artifacts present in NMR experiments employing CPMG acquisition and allows "single-scan" measurements of transverse relaxation and J-couplings. Additionally, unlike CPMG, we show how PIETA can be appended to experiments with phase modulated signals after the mixing pulse.