We have studied the structure of two lipopeptides based on the simple dipeptide building block L-Phe-D-Oxd. These peptides have been reported previously to form fiber-like materials. The lipopeptides synthesized here had the structures C(n)(2)H((2n+1))CO-L-Phe-D-Oxd-OBn or C(n)(2)H((2n+1))CO-D-Phe-L-Oxd-OBn with n = 5 or 11. Addition of the N-terminal lipid modification did not cause a major disturbance of the structures these molecules form. The lipid modifications themselves showed highly rigid structures as inferred from solid-state (2)H NMR. The peptide backbone showed (13)C NMR chemical shifts in agreement with β-sheet secondary structure. Addition of a lipid modification to the N-terminus is a common motif in biology to attach proteins to the membrane. Therefore, we also investigated the lipopeptides in the presence of synthetic POPC bilayers. Two different molecular species were detected under these circumstances: (i) lipopeptide monomers that showed chain order parameters similar to those of the host membrane, (ii) lipopeptide aggregates that exhibited very similar structures and dynamics as the crystalline aggregates. Overall, the lipopeptides showed a well defined and rigid secondary structure that is in agreement with fibrillar aggregates previously detected for those peptides without the lipid modification.

Quantitative three dimensional analysis is possible, in principle, by PIXE tomography. But, the inherent problems in quantitation, restrictions on the sample geometry and preparation, and specimen damage due to high fluences make this method unsuitable for many biological samples. The specimen under investigation, a murine artery, was around a millimeter in diameter and the induced atherosclerotic lesion was spread few hundreds of micrometers across the length of the artery. Since no tomographic experiments were possible, we chose to do the 3D quantitative analysis by means of PIXE Stacking. Herein, thin serial sections of the specimen are prepared and measured by conventional ion beam techniques. The resultant two dimensional quantitative element maps are stacked and aligned to reconstruct a quantitative volume of the specimen. Although the reconstructed dimension has poorer spatial resolution as compared with the measured dimensions, new information can still be gained from it. The three dimensional element distribution of the atherosclerotic lesion shows calcification on the outer surface of the artery, which otherwise would not have been easily visible in the two dimensional analysis.