Optical-trapping Raman microscopy detection of single unilamellar lipid vesicles

Raman spectra of individual unilamellar phospholipid vesicles ( approximately 0.6 microm in size) have been acquired by optical-trapping confocal Raman microscopy over the 900-3200-cm(-)(1) region. Raman scattering from the phospholipid bilayer of a single, trapped liposome could be detected, along with molecular species trapped within the vesicle. The Raman spectra of vesicles prepared from four different phosphatidylcholine lipids, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC), and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), could be readily distinguished by evaluating differences in the skeletal C-C and C-H stretching modes of the acyl hydrocarbon tails. These differences correlate with changes in lipid organization for different gel to liquid-crystal transition temperatures (T(m)): 41, 24, 7, and -20 degrees C for DPPC, DMPC, DLPC, and DOPC, respectively. The spectra could be acquired on the same trapped vesicle for several hours, which allowed the permeability of the bilayer to be investigated by monitoring the leakage of perchlorate anions from the vesicle. Vesicles prepared from pure DPPC or DOPC, with gel to liquid-crystal transition temperatures well above and well below room temperature, exhibited no detectable anion transfer. DLPC and DMPC vesicles permitted rapid ion transfer across the bilayer. The lengths of hydrocarbon tails were shorter in these two lipids, which could indicate that shorter chains lower the hydrophobic barrier of a membrane to ion transport. While the DMPC chains were longer than DLPC with a correspondingly higher T(m), the temperature of the experiment corresponds to the T(m) of DMPC, and domain boundaries between gel and liquid-crystal phases could contribute to high membrane permeability.