The fusion of biological membranes is mediated by integral membrane proteins with Alpha-helical transmembrane segments. Additionally, those proteins are often modified by the covalent attachment of hydrocarbon chains. Previously, a series of de novo designed Alpha-helical peptides with mixed Leu/Val sequences was presented, mimicking fusiogenically active transmembrane segments in model membranes (Hofmann et al., Proc. Natl. Acad. Sci. USA 101 (2004) 14776–14781). From this series, we have investigated the peptide LV16 (KKKW LVLV LVLV LVLV LVLV KKK), which was synthesized featuring either a free N-terminus or a saturated N-acylation of 2, 8, 12, or 16 carbons. We used 2H and 31P NMR spectroscopy to investigate the structure and dynamics of those peptide lipid modifications in POPC and DLPC bilayers and compared them to the hydrocarbon chains of the surrounding membrane. Except for the C2 chain, all peptide acyl chains were found to insert well into the membrane. This can be explained by the high local lipid concentrations the Nterminal lipid chains experience. Further, the insertion of these peptides did not influence the membrane structure and dynamics as seen from the 2H and 31P NMR data. In spite of the fact that the longer acyl chains insert into the membrane, they do not adapt their lengths to the thickness of the bilayer. Even the C16 lipid chain on the peptide, which could match the length of the POPC palmitoyl chain, exhibited lower order parameters in the upper chain, which get closer and finally reach similar values in the lower chain region. 2H NMR square law plots reveal motions of slightly larger amplitudes for the peptide lipid chains compared to the surrounding phospholipids. In spite of the significantly different chain lengths of the acylations, the fraction of gauche defects in the inserted chains is constant.