MAS 1 H NMR Probes Freezing Point Depression of Water and Liquid-Gel Phase Transitions in Liposomes

Abstract

The lipid bilayer typical of hydrated biological membranes is characterized by a liquid-crystalline, highly dynamic state. Upon cooling or dehydration, these membranes undergo a cooperative transition to a rigidified, more-ordered, gel phase. This characteristic phase transition is of significant biological and biophysical interest, for instance in studies of freezing-tolerant organisms. Magic-angle-spinning (MAS) solid-state NMR (ssNMR) spectroscopy allows for the detection and characterization of the phase transitions over a wide temperature range. In this study we employ MAS 1 H NMR to probe the phase transitions of both solvent molecules and different hydrated phospholipids, including tetraoleoyl cardiolipin (TOCL) and several phosphatidylcholine lipid species. The employed MAS NMR sample conditions cause a previously noted substantial reduction in the freezing point of the solvent phase. The effect on the solvent is caused by confinement of the aqueous solvent in the small and densely packed MAS NMR samples. In this study we report and examine how the freezing point depression also impacts the lipid phase transition, causing a ssNMR-observed reduction in the lipids' melting temperature (T m ). The molecular underpinnings of this phenomenon are discussed and compared with previous studies of membrane-associated water phases and the impact of membrane-protective cryoprotectants.

Water Distribution, Dynamics, and Interactions with Alzheimer’s β-Amyloid Fibrils Investigated by Solid-State NMR

“…Thus, the majority of water is frozen by 255 K, leaving only a small percentage of water that experiences significant freezing-point depression due to tight association with the peptide. 41,42 …”

On the use of ultracentrifugal devices for routine sample preparation in biomolecular magic-angle-spinning NMR

“…We first examine the application of this approach to the ssNMR study of lipid vesicles, or vesicle-bound proteins, using our recent work as an example (Mandal et al 2015; Mandal and Van der Wel 2016). Unilamellar lipid vesicles of varying sizes are frequently used to mimic biological membranes in studies of protein-lipid or peptide-lipid interactions (Scalise et al 2013; Mandal et al 2015; Stepanyants et al 2015; LeBarron and London 2016; Veshaguri et al 2016).…”

“…Experiments were performed at a spinning rate of 13 kHz, with a set temperature of 275 K. Based on systematic temperature calibration experiments with external samples (Mandal and Van der Wel 2016), we estimate this to reflect a similar sample temperature (within several degrees). The 2D spectrum was acquired with 50 ms of proton-driven spin diffusion recoupling, a 1.5 ms CP contact time, a recycle delay (RD) of 3 s and an applied TPPM decoupling on 1 H at 83 KHz.…”

“…Additionally, the proper folding and function of membrane proteins requires not only a lipid bilayer environment, but also a certain degree of hydration (Zhang et al 2014). Dehydration can have adverse impacts on membranes, as it can cause lateral separation of membrane proteins from lipid-rich domains, formation of non-bilayer lipid phases and an increased propensity for gel phase formation (Wolfe and Bryant 1999; Bryant et al 2001; Mandal and Van der Wel 2016). Some of these dehydration-induced changes can be directly observed via ssNMR (Ulrich and Watts 1994; Gawrisch et al 2007; Zhang et al 2014).…”

“…It is designed to balance the need to maximize the signal/noise, maintain biological relevance and achieve easy reproducibility. We note that such devices are successfully used by ourselves (Van der Wel et al 2007; Li et al 2011; Hoop et al 2012; Hoop et al 2014; Mandal et al 2015; Mandal and Van der Wel 2016) and various other groups (Böckmann et al 2009; Gardiennet et al 2012; Bertini et al 2013; Kunert et al 2014; Wiegand et al 2015; Wiegand et al 2016b), but that there are many others that have not adopted this approach.…”