Small-Angle X-ray Scattering as a Powerful Tool for Phase and Crystallinity Assessment of Monoclonal Antibody Crystallites in Support of Batch Crystallization.

Crystalline suspensions of monoclonal antibodies (mAbs) have great potential to improve drug substance isolation and purification on a large scale and to be used for drug delivery via high-concentration formulations. Crystalline mAb suspensions are expected to have enhanced chemical and physical properties relative to mAb solutions delivered intravenously, making them attractive candidates for subcutaneous delivery. In contrast to small molecules, the development of protein crystalline suspensions is not a widely used approach in the pharmaceutical industry. This is mainly due to the challenges in finding crystalline hits and the suboptimal physical properties of the resulting crystallites when hits are found. Modern advances in instrumentation and increased knowledge of mAb crystallization have, however, resulted in higher probabilities of discovering crystal forms and improving their particle properties and characterization. In this regard, physical, analytical characterization plays a central role in the initial steps of understanding and later optimizing the crystallization of mAbs and requires careful selection of the appropriate tools. This contribution describes a novel crystal structure of the antibody pembrolizumab and demonstrates the usefulness of small-angle X-ray scattering (SAXS) for characterizing its crystalline suspensions. It illustrates the advantages of SAXS when used to (i) confirm crystallinity and crystal phase of crystallites produced in batch mode; (ii) confirm crystallinity under various conditions and detect variations in crystal phases, enabling fine-tuning of the crystallizations for phase control across multiple batches; (iii) monitor the physical response and stability of the crystallites in suspension with regard to filtration and washing; and (iv) monitor the physical stability of the crystallites upon drying. Overall, this work highlights how SAXS is an essential tool for mAb crystallization characterization.

Hysteresis in the gas sorption isotherms of metal-organic cages accompanied by subtle changes in molecular packing.

Structural deformation in response to gas sorption is rarely observed for porous molecular solids, when compared to porous framework materials. Here, we describe the effect of chemical modification on the exterior of lantern-type metal-organic cages on the emergence and then disappearance of cooperative gas uptake. The results suggest that supramolecular design of ligands can be used to reveal this behaviour.

A Coordinative Solubilizer Method to Fabricate Soft Porous Materials from Insoluble Metal-Organic Polyhedra.

Porous molecular cages have a characteristic processability arising from their solubility, which allows their incorporation into porous materials. Attaining solubility often requires covalently bound functional groups that are unnecessary for porosity and which ultimately occupy free volume in the materials, decreasing their surface areas. Here, a method is described that takes advantage of the coordination bonds in metal-organic polyhedra (MOPs) to render insoluble MOPs soluble by reversibly attaching an alkyl-functionalized ligand. We then use the newly soluble MOPs as monomers for supramolecular polymerization reactions, obtaining permanently porous, amorphous polymers with the shape of colloids and gels, which display increased gas uptake in comparison with materials made with covalently functionalized MOPs.

Postsynthetic Covalent and Coordination Functionalization of Rhodium(II)-Based Metal-Organic Polyhedra.

Metal-organic polyhedra (MOP) are ultrasmall (typically 1-4 nm) porous coordination cages made from the self-assembly of metal ions and organic linkers and are amenable to the chemical functionalization of its periphery; however, it has been challenging to implement postsynthetic functionalization due to their chemical instability. Herein, we report the use of coordination chemistries and covalent chemistries to postsynthetically functionalize the external surface of ≈2.5 nm stable Rh(II)-based cuboctahedra through their Rh-Rh paddlewheel units or organic linkers, respectively. We demonstrate that 12 N-donor ligands, including amino acids, can be coordinated on the periphery of Rh-MOPs. We used this reactivity to introduce new functionalities (e.g., chirality) to the MOPs and to tune their hydrophilic/hydrophobic characteristics, which allowed us to modulate their solubility in diverse solvents such as dichloromethane and water. We also demonstrate that all 24 organic linkers can be postsynthetically functionalized with esters via covalent chemistry. In addition, we anticipate that these two types of postsynthetic reactions can be combined to yield doubly functionalized Rh-MOPs, in which a total of 36 new functional molecules can be incorporated on their surfaces. Likewise, these chemistries could be synergistically combined to enable covalent functionalization of MOPs through new linkages such as ethers. We believe that both reported postsynthetic pathways can potentially be used to engineer Rh-MOPs as scaffolds for applications in delivery, sorption, and catalysis.

Molecular tectonics: from a rigid achiral organic tecton to 3D chiral Co and Fe coordination networks.

An achiral organic tecton bearing four coordinating sites of the pyridyl type leads to the formation of iso-structural 3D helical coordination polymers when combined with Co(SCN)2 and Fe(SCN)2 achiral neutral complexes. Their formation occurs during the self-assembly process in the solid state, which leads to crystals composed of homochiral coordination polymers.

Switchable gate-opening effect in metal-organic polyhedra assemblies through solution processing.

Gate-opening gas sorption is known for metal-organic frameworks, and is associated with structural flexibility and advantageous properties for sensing and gas uptake. Here, we show that gate-opening is also possible for metal-organic polyhedra (MOPs), and depends on the molecular organisation in the lattice. Thanks to the solubility of MOPs, several interchangeable solvatomorphs of a lantern-type MOP were synthesised via treatment with different solvents. One phase obtained through use of methanol induced a gate-opening effect in the lattice in response to carbon dioxide uptake. The sorption process was thoroughly investigated with in situ powder X-ray diffraction and simultaneous adsorption experiments. Meanwhile, solution processing of this flexible phase using THF led to a permanently porous phase without a gate-opening effect. Furthermore, we find that we can change the metallic composition of the MOP, and yet retain flexibility. By showing that gate-opening can be switched on and off depending on the solvent of crystallisation, these findings have implications for the solution-based processing of MOPs.

Self-assembly of metal-organic polyhedra into supramolecular polymers with intrinsic microporosity.

Designed porosity in coordination materials often relies on highly ordered crystalline networks, which provide stability upon solvent removal. However, the requirement for crystallinity often impedes control of higher degrees of morphological versatility, or materials processing. Herein, we describe a supramolecular approach to the synthesis of amorphous polymer materials with controlled microporosity. The strategy entails the use of robust metal-organic polyhedra (MOPs) as porous monomers in the supramolecular polymerization reaction. Detailed analysis of the reaction mechanism of the MOPs with imidazole-based linkers revealed the polymerization to consist of three separate stages: nucleation, elongation, and cross-linking. By controlling the self-assembly pathways, we successfully tuned the resulting macroscopic form of the polymers, from spherical colloidal particles to colloidal gels with hierarchical porosity. The resulting materials display distinct microporous properties arising from the internal cavity of the MOPs. This synthetic approach could lead to the fabrication of soft, flexible materials with permanent porosity.

Molecular tectonics: gas adsorption and chiral uptake of (l)- and (d)-tryptophan by homochiral porous coordination polymers.

Combinations of two enantiomerically pure organic tectons 1 and 3 with either Zn(ii) or Cu(ii) cations lead to the formation of four homochiral 3D networks among which two, 1-Cu and 3-Cu, are robust porous crystals displaying homochiral cavities and permanent microporosity. 3-Cu porous crystals capture 66% and 20% of l- and d-tryptophan, respectively, after 30 min of adsorption.

Rhodium-Organic Cuboctahedra as Porous Solids with Strong Binding Sites.

The upbuilding of dirhodium tetracarboxylate paddlewheels into porous architectures is still challenging because of the inertness of equatorial carboxylates for ligand-exchange reaction. Here we demonstrate the synthesis of a new family of metal-organic cuboctahedra by connecting dirhodium units through 1,3-benzenedicarboxylate and assembling cuboctahedra as porous solids. Carbon monoxide and nitric oxide were strongly trapped in the internal cavity thanks to the strong affinity of unsaturated axial coordination sites of dirhodium centers.

A luminescent molecular turnstile.

A molecular turnstile based on a hydroquinone luminescent hinge bearing two divergently oriented pyridyl units behaving as a rotor and equipped with a handle composed of a tridentate coordinating site considered as the stator was synthesized and its structure was studied in the solid state by X-ray diffraction on single crystal. Its dynamic behaviour in solution was investigated by 1- and 2-D NMR experiments which revealed the free rotation of the rotor around the stator. The rotational movement was locked upon addition of Pd(ii) simultaneously complexed by the tridentate moiety of the stator and one of the two monodentate pyridyl sites of the rotor. Interestingly, whereas the open state of the turnstile was luminescent, for its closed state the emission was quenched by the heavy atom effect of Pd(ii).

Optical reading of the open and closed states of a molecular turnstile.

A molecular turnstile composed of a hydroquinone based rotor and a stator bearing a tridentate coordinating site can be reversibly switched between open and closed states. The locking and unlocking processes may be read optically.

Molecular tectonics: homochiral coordination networks based on combinations of a chiral neutral tecton with Hg(II), Cu(II) or Ni(II) neutral complexes as metallatectons.

The use of compound 1 as an enantiomerically pure neutral and rigid organic linear tecton bearing two divergently oriented monodentate coordinating sites appended with two chiral centres of the same (S) configuration leads, in the presence of neutral metal complexes behaving either as a linear (Cu(hfac)2, Cu(OAc)2) or a V-shaped (HgCl2) 2-connecting node or a 4-connecting square node (NiCl2), to the formation of four homochiral 1- and 2-D coordination polymers.

Molecular tectonics: enantiomerically pure 1D stair-type mercury coordination networks based on rigid bismonodentate C2-chiral organic tectons.

Combinations of three enantiomerically pure organic chiral linear tectons bearing two divergently oriented pyridyl units as coordinating poles with HgCl2 as a two-connecting V-shape metallatecton offering two free coordination sites lead to the formation of stair-type 1D enantiomerically pure mercury coordination networks.

Molecular tectonics: homochiral 3D cuboid coordination networks based on enantiomerically pure organic tectons and ZnSiF6.

Upon combining enantiomerically pure bis-monodentate organic tectons with ZnSiF6, homochiral 3D cuboid architectures displaying chiral channels are formed.