Permeation of a beta-heptapeptide derivative across phospholipid bilayers

Based on a number of experiments it is concluded that the fluorescein labeled beta-heptapeptide fluoresceinyl-NH-CS-(S)-beta(3)hAla-(S)-beta(3)hArg-(R)-beta(3)hLeu-(S)-beta(3)hPhe-(S)-beta(3)hAla-(S)-beta(3)hAla-(S)-beta(3)hLys-OH translocates across lipid vesicle bilayers formed from DOPC (1,2-dioleoyl-sn-glycero-3-phosphocholine). The conclusion is based on the following observations: (i) addition of the peptide to the vicinity of micrometer-sized giant vesicles leads to an accumulation of the peptide inside the vesicles; (ii) if the peptide is injected inside individual giant vesicles, it is released from the vesicles in a time dependent manner; (iii) if the peptide is encapsulated within sub-micrometer-sized large unilamellar vesicles, it is released from the vesicles as a function of time; (iv) if the peptide is submitted to immobilized liposome chromatography, the peptide is retained by the immobilized DOPC vesicles. Furthermore, the addition of the peptide to calcein-containing DOPC vesicles does not lead to significant calcein leakage and vesicle fusion is not observed. The finding that derivatives of the beta-heptapeptide (S)-beta(3)hAla-(S)-beta(3)hArg-(R)-beta(3)hLeu-(S)-beta(3)hPhe-(S)-beta(3)hAla-(S)-beta(3)hAla-(S)-beta(3)hLys-OH can translocate across phospholipid bilayers is supported by independent measurements using Tb(3+)-containing large unilamellar vesicles prepared from egg phosphatidylcholine and wheat germ phosphatidylinositol (molar ratio of 9:1) and a corresponding peptide that is labeled with dipicolinic acid instead of fluorescein. The experiments show that this dipicolinic acid labeled beta-heptapeptide derivative also permeates across phospholipid bilayers. The possible mechanism of the translocation of the particular beta-heptapeptide derivatives across the membrane of phospholipid vesicles is discussed within the frame of the current understanding of the permeation of certain oligopeptides across simple phospholipid bilayers.