The biaxial nematic phase was recently observed in different thermotropic liquid crystals, namely bent-core compounds, side-chain polymers, bent-core dimers, and organosiloxane tetrapodes. In this work, a series of experiments with a nematic organosiloxane tetrapode where nuclear magnetic resonance (NMR) spectra are collected while the sample is continuously rotating around an axis perpendicular to the magnetic field, are discussed in conjunction with the analysis of a deuterium NMR experiment on the same system reported earlier. The sample used is a mixture of a deuterated probe with the tetrapode. The mixture exhibits a nematic range between -40 degrees C and 37 degrees C. The results of the two independent, but complementary deuterium NMR experiments confirm the existence of a biaxial nematic phase for temperatures below 0 degrees C with high values of the asymmetry parameter at low temperatures. The presence of slow movements of the tetrapode mesogenic units in the low-temperature regime could also be detected through the analysis of the NMR spectra. Simulations indicate that these movements are mainly slow molecular reorientations of the mesogenic units associated with the presence of collective modes in the nematic phases of this compound. In the case of tetrapodes, recent investigations attribute the origin of biaxiality to the hindering of reorientations of the laterally attached mesogenic units which constitute the tetrapode. This study relates the molecular movements with the nematic biaxial ordering of the system.

Saturation transfer difference NMR (STD-NMR) spectroscopy has emerged as a powerful screening tool and a straightforward way to study the binding epitopes of active compounds in early stage lead discovery in pharmaceutical research. Here we report the application of STD-NMR to characterize the binding of the anti-inflammatory drugs ibuprofen, diclofenac, and ketorolac to COX-1 and COX-2. Using well-studied COX inhibitors and by comparing STD signals with crystallographic structures, we show that there is a relation between the orientations of ibuprofen and diclofenac in the COX-2 active site and the relative STD responses detected in the NMR experiments. On the basis of this analysis, we propose that ketorolac should bind to the COX-2 active site in an orientation similar to that of diclofenac. We also show that the combination of STD-NMR with competition experiments constitutes a valuable tool to address the recently proposed behavior of COX-2 as functional heterodimers and complements enzyme activity studies in the effort to rationalize COX inhibition mechanisms.