This Thesis in biological physics has two components describing the use of X-ray scattering
techniques to study the structure of two different stacked lipid membrane systems.The first
part focuses on the interaction between a short 11-mer peptide Tat which is part of the Tat
protein in the HIV-1 virus. Although highly positively charged the Tat protein has been shown
to translocate through hydrocarbon lipid bilayers easily without requiring the cell's energy
which is counter to its Born self-energy. In this work Tat's location in the headgroup region
was demonstrated using a combined X-ray scattering and molecular dynamics approach. Bilayer
thinning was observed as well as softening of different membrane mimics due to Tat. It was
concluded that Tat's headgroup location which increases the area lipid and its bilayer
softening likely reduce the energy barrier for passive translocation.The second part is a
rigorous investigation of an enigmatic phase in the phase diagram of the lipid
dimyristoylphosphatidylcholine (DMPC). The ripple phase has fascinated many researchers in
condensed matter physics and physical chemistry as an example of periodically modulated phases
with many theoretical and simulation papers published. Despite systematic studies over the past
three decades molecular details of the structure were still lacking. By obtaining the highest
resolution X-ray data so far this work revealed the complex nature of the chain packing as
well as confirming that the major side is thicker than the minor side of the saw-tooth ripple
structure. The new model shows that the chains in the major arm are tilted with respect to the
bilayer normal and that the chains in the minor arm are slightly more disordered than all-trans
gel-phase chains i.e. the chains in the minor arm are more fluid-like. This work provides the
highest resolution X-ray structure of the ripple phase to-date.
