Phospholipid headgroups govern area per lipid and emergent elastic properties of bilayers

Phospholipid bilayers are liquid-crystalline materials whose intermolecular interactions at mesoscopic length scales have key roles in the emergence of membrane physical properties. Here we investigated the combined effects of phospholipid polar headgroups and acyl chains on biophysical functions of membranes with solid-state ²H NMR spectroscopy. We compared the structural and dynamic properties of phosphatidylethanolamine and phosphatidylcholine with perdeuterated acyl chains in the solid-ordered (s_o) and liquid-disordered (l_d) phases. Our analysis of spectral lineshapes of 1,2-diperdeuteriopalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE-d₆₂) and 1,2-diperdeuteriopalmitoyl-sn-glycero-3-phosphocholine (DPPC-d₆₂) in the s_o (gel) phase indicated an all-trans rotating chain structure for both lipids. Greater segmental order parameters ($S_{\text{CD}}$) were observed in the l_d (liquid-crystalline) phase for DPPE-d₆₂ than for DPPC-d₆₂ membranes, while their mixtures had intermediate values irrespective of the deuterated lipid type. Our results suggest the $S_{\text{CD}}$ profiles of the acyl chains are governed by methylation of the headgroups and are averaged over the entire system. Variations in the acyl chain molecular dynamics were further investigated by spin-lattice ($R_{1\text{Z}}$) and quadrupolar-order relaxation ($R_{1\text{Q}}$) measurements. The two acyl-perdeuterated lipids showed distinct differences in relaxation behavior as a function of the order parameter. The R_1Z rates had a square-law dependence on $S_{\text{CD}}$, implying collective mesoscopic dynamics, with a higher bending rigidity for DPPE-d₆₂ than for DPPC-d₆₂ lipids. Remodeling of lipid average and dynamic properties by methylation of the headgroups thus provides a mechanism to control the actions of peptides and proteins in biomembranes.     

Crowding-induced protein destabilization in the absence of soft attractions

It is generally assumed that volume exclusion by macromolecular crowders universally stabilizes the native states of proteins and destabilization suggests soft attractions between crowders and protein. Here we show that proteins can be destabilized even by crowders that are purely repulsive. With a coarse-grained sequence-based model, we study the folding thermodynamics of two sequences with different native folds, a helical hairpin and a β-barrel, in a range of crowder volume fractions, ${\phi }_{\text{c}}$. We find that the native state, N, remains structurally unchanged under crowded conditions, while the size of the unfolded state, U, decreases monotonically with ${\phi }_{\text{c}}$. Hence, for all ${\phi }_{\text{c}}>0$, U is entropically disfavored relative to N. This entropy-centric view holds for the helical hairpin protein, which is stabilized under all crowded conditions as quantified by changes in either the folding midpoint temperature, $T_{\text{m}}$, or the free energy of folding. We find, however, that the β-barrel protein is destabilized under low-T, low-${\phi }_{\text{c}}$ conditions. This destabilization can be understood from two characteristics of its folding: 1) a relatively compact U at $T<T_{\text{m}}$, such that U is only weakly disfavored entropically by the crowders; and 2) a transient, compact, and relatively low-energy nonnative state that has a maximum population of only a few percent at ${\phi }_{\text{c}}=0$, but increasing monotonically with ${\phi }_{\text{c}}$. Overall, protein destabilization driven by hard-core effects appears possible when a compaction of U leads to even a modest population of compact nonnative states that are energetically competitive with N.