Phase Behavior of Polar Porcine Brain Lipid Extract Using Multinuclear Solid-State NMR

English. Polar lipid extract, often sourced from brain matter, are commonly used in permeability assays. The phase behavior of their constituent phospholipids have long been widely studied at various levels of hydration, both in isolation and in prepared mixtures with other lipids commonly present in extracts, such as cholesterol. In this study, the phase behavior of whole porcine polar brain lipid extract evaluated using 13C-, 31P- and 1H-MAS NMR as well as static 31P- and 2H-NMR, the latter enabled by D2O addition. Focus was put on phase behavior evaluation using comparative signal enhancements from 13C DP, CP and INEPT techniques, a procedure called dynamics based spectral editing due to the techniques’ different dependences on fatty acid chain segment reorientation time and C-H bond angles. By supporting results from dynamics based spectral editing with results from the long-established static NMR-experiments, it was found that dynamics based spectral editing alone can distinguish four phases in porcine polar brain lipid extract. A phase map spanning 264.0-348.3 K and 5-90% water content by total molar water content was established for the whole extract using the primary apparent phases at each temperature and water content. The main known limitation of the phase map is the presence of unresolved NMR information in the results. This information may pertain to individual phase behaviors of the constituent phospholipids in the extract. Recoupling NMR experiments are recommended for future work to obtain phospholipid-by-phospholipid phase behavior information.

English. The brain is largely made up of fat. Specifically, much of the fat is cholesterol and phospholipids. These are molecules that mostly contain atoms that can be analyzed magnetically using Nuclear Magnetic Resonance (NMR) spectroscopy (hydrogen, carbon and phosphorous specifically). This study aims to tell if NMR applied only to carbon can be used to reveal the structure of fats. Fats notoriously have several solid, liquid and semi-solid forms rather than the usual trio of solid, liquid and gas. NMR relies on the precessing (spinning-at-an-angle) motion that the magnetic moments of atomic cores perform when they return to magnetic equilibrium after exposure to a resonant magnetic field. The spinning happens at a frequency specific to the set of atomic bonds that the atom finds itself in. With NMR, molecules are an enormous cohort of hobby analog radio broadcasters to tune into. Chemists can gain a picture of how the molecules in a sample look by checking which NMR frequencies exist and how intense their signals are compared to each other. Normally, atoms in solids experience too many competing magnetic interactions other than atomic bonds for NMR to work. As is, NMR must be done on samples in liquid solution where fast molecular motions average-out these interactions when viewed across time, like spinning car tires seeming to lose their side patterns. In fact, spinning solids rapidly at a specific angle can average-out the interactions as well. This way, solids can also be studied with NMR. Spin fast and almost all solid interactions go away; the side patterns simply appear as circles. Spin slowly and some interactions remain; potentially revealing patterns re-emerge. Many specialized NMR techniques rely on transferring polarization (basically transferring magnetic moment) from one type of atom to another to improve signal. INEPT is one such technique (NMR and abbreviations get along well). It transfers polarization through atomic bonds and relies on fast molecular motion to frequently orient the bond properly for transfer. Imagine it as a township that is heavily into astrology and can somehow communicate telepathically every time the planets align. The faster the solar system spins, the more often that happens. In contrast, CP (Cross Polarization), improves signal by transferring polarization from one atom to another directly through space. It is favored by slow molecular motion that lets atoms remain physically close over long periods of time. Think of a quiet countryside village where every neighborly encounter is a monologue face-off; at least you learn something. Finally, experiments without enhancement are called DP (direct polarization). This is more akin to workday stairwell greetings; polite and comfortably distanced.