Chiara Nicolini

Characterization of the Temperature- and Pressure-Induced Inverse and Reentrant Transition of the Minimum Elastin-Like Polypeptide GVG(VPGVG) by DSC, PPC, CD, and FT-IR Spectroscopy

We investigated the temperature- and pressure-dependent structure and phase behavior of a solvated oligopeptide, GVG(VPGVG), which serves as a minimalistic elastin-like model system, over a large region of the thermodynamic phase field, ranging from 2 to 120 degrees C and from ambient pressure up to approximately 10 kbar, applying various spectroscopic (CD, FT-IR) and thermodynamic (DSC, PPC) measurements. We find that this octapeptide behaves as a two-state system which undergoes the well-known inverse-temperature folding transition occurring at T approximately 36 degrees C, and, in addition, a slow trend reversal at higher temperatures, finally leading to a reentrant unfolding close to the boiling point of water. Furthermore, the pressure-dependence of the folding/unfolding transition was studied to yield a more complete picture of the p, T-stability diagram of the system. A molecular-level picture of these processes, in particular on the role of water for the folding and unfolding events of the peptide, presented with the help of molecular-dynamics simulations, is presented in a companion article in this issue.

Small-scale composition fluctuations and microdomain formation in lipid raft models as revealed by small-angle neutron scattering

Substantial evidence has accumulated recently that in cell membranes composition fluctuations and microdomains exist, also referred to as rafts. The structure of ternary model raft lipid mixtures, such as sphingomyelin, cholesterol and DOPC (1,2-dioleoyl-sn-glycero-3-phosphatidylcholine) or POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine), was studied by small-angle neutron scattering using the H/D contrast variation technique. The lipid mixtures exhibit a Q−3 power-law scattering, indicating the existence of composition fluctuations, which correspond to space-filling droplets with a broad size-distribution ranging from about 20 to 200 nm. No indications of critical-like concentration fluctuations are visible in the concentration and temperature range (15–60 °C) covered. The data clearly demonstrate the existence of microdomains in the model raft mixtures not only in the μm range as revealed by fluorescence microscopy, but also their existence over a large length scale down to the range of several nanometres. In addition, we were able to directly visualize the resistance of part of the lipid domains against non-ionic detergent (Triton X-100) in situ.