Thin layers of water on biomolecular and other nanostructured surfaces can be supercooled to temperatures not accessible with bulk water. Chen et al. [PNAS 103, 9012 (2006)] suggested that anomalies near 220 K observed by quasi-elastic neutron scattering can be explained by a hidden critical point of bulk water. Based on more sensitive measurements of water on perdeuterated phycocyanin, using the new neutron backscattering spectrometer SPHERES, and an improved data analysis, we present results that show no sign of such a fragile-to-strong transition. The inflection of the elastic intensity at 220 K has a dynamic origin that is compatible with a calorimetric glass transition at 170 K. The temperature dependence of the relaxation times is highly sensitive to data evaluation; it can be brought into perfect agreement with the results of other techniques, without any anomaly. In contrast to bulk water, protein hydration water can be supercooled down to a glass transition at T g ≃ 170 K. Near T g translational degrees of freedom arrest, which induces discontinuities in the specific heat and the thermal expansion coefficient of the hydration water [1][2][3][4]. Due to the dynamic nature of the glass transition, freezing of microscopic degrees of freedom can already be observed far above T g .The protein dynamic transition is an abrupt onset of atomic displacements on the microscopic length and time scale probed by quasielastic neutron scattering (QENS). First observed twenty years ago in hydrated myoglobin and lysozyme at T ∆ ≃ 240 K [5], it is now known to be a generic property of hydrated proteins, while it is absent in dehydrated systems. It is therefore related to the dynamics of the hydration shell.In QENS, mean squared dispacements δx 2 are deduced from the elastic scattering intensity S(q, 0). Due to the finite spectrometer resolution (fwhm 2∆ω), one actually measures S(q, |ω| ∆ω). Full spectral measurements show that the anomalous decrease of the central peak is compensated by increasing inelastic wings [5,6]. These effects have been interpreted as precursors of the glass transition. Since QENS probes structural relaxation at ∆ω −1 ≃ 100 ps, while the calorimetric T g refers to a time scale of 100 s, it is natural that T ∆ is located far above T g : the protein dynamic transition is the microscopic manifestation of the glass transition in the hydration shell. The time-scale dependence of T ∆ also explains its variation with viscosity [7][8][9]. * Electronic address: wdoster@ph.tum.de Recently, these views have been challenged by the suggestion that there might be a time-scale independent transition. Support came mainly from QENS experiments on the backscattering spectrometer HFBS at NIST. In hydrated lysozyme, DNA and RNA, a kink was found not only in S(q, |ω| ∆ω), but also in the α relaxation time τ deduced from full spectra S(q, ω) [10][11][12][13]. This change of τ (T ) from high-T super-Arrhenius to low-T Arrhenius behavior at T L ≃ 220 K has been interpreted as a fragile-to-strong-transition (FST) from the...

We measured the effect of intrinsic lipid curvature, J, on structural properties of asymmetric vesicles made of palmitoyl-oleoyl-phosphatidylethanolamine (POPE; J<0) and palmitoyl-oleoyl-phosphatidylcholine (POPC; J∼0). Electron microscopy and dynamic light scattering were used to determine vesicle size and morphology, and x-ray and neutron scattering, combined with calorimetric experiments and solution NMR, yielded insights into leaflet-specific lipid packing and melting processes. Below the lipid melting temperature we observed strong interleaflet coupling in asymmetric vesicles with POPE inner bilayer leaflets and outer leaflets enriched in POPC. This lipid arrangement manifested itself by lipids melting cooperatively in both leaflets, and a rearrangement of lipid packing in both monolayers. On the other hand, no coupling was observed in vesicles with POPC inner bilayer leaflets and outer leaflets enriched in POPE. In this case, the leaflets melted independently and did not affect each other's acyl chain packing. Furthermore, we found no evidence for transbilayer structural coupling above the melting temperature of either sample preparation. Our results are consistent with the energetically preferred location of POPE residing in the inner leaflet, where it also resides in natural membranes, most likely causing the coupling of both leaflets. The loss of this coupling in the fluid bilayers is most likely the result of entropic contributions.

We addressed the onset of synergistic activity of the two well-studied antimicrobial peptides magainin 2 (MG2a) and PGLa using lipid-only mimics of Gram-negative cytoplasmic membranes. Specifically, we coupled a joint analysis of smallangle x-ray and neutron scattering experiments on fully hydrated lipid vesicles in the presence of MG2a and L18W-PGLa to allatom and coarse-grained molecular dynamics simulations. In agreement with previous studies, both peptides, as well as their equimolar mixture, were found to remain upon adsorption in a surface-aligned topology and to induce significant membrane perturbation, as evidenced by membrane thinning and hydrocarbon order parameter changes in the vicinity of the inserted peptide. These effects were particularly pronounced for the so-called synergistic mixture of 1:1 (mol/mol) L18W-PGLa/MG2a and cannot be accounted for by a linear combination of the membrane perturbations of two peptides individually. Our data are consistent with the formation of parallel heterodimers at concentrations below a synergistic increase of dye leakage from vesicles. Our simulations further show that the heterodimers interact via salt bridges and hydrophobic forces, which apparently makes them more stable than putatively formed antiparallel L18W-PGLa and MG2a homodimers. Moreover, dimerization of L18W-PGLa and MG2a leads to a relocation of the peptides within the lipid headgroup region as compared to the individual peptides. The early onset of dimerization of L18W-PGLa and MG2a at low peptide concentrations consequently appears to be key to their synergistic dye-releasing activity from lipid vesicles at high concentrations.SIGNIFICANCE We demonstrate that specific interactions of the antimicrobial peptides MG2a and PGLa with each other in POPE/POPG bilayers lead to the formation of surface-aligned parallel dimers, which already provide, at low peptide concentrations, the nucleus for the peptides' well-known synergistic activity.