In Situ and Real-Time SFG Measurements Revealing Organization and Transport of Cholesterol Analogue 6-Ketocholestanol in a Cell Membrane.

Cholesterol organization and transport within a cell membrane are essential for human health and many cellular functions yet remain elusive so far. Using cholesterol analogue 6-ketocholestanol (6-KC) as a model, we have successfully exploited sum frequency generation vibrational spectroscopy (SFG-VS) to track the organization and transport of cholesterol in a membrane by combining achiral-sensitive ssp (ppp) and chiral-sensitive psp polarization measurements. It is found that 6-KC molecules are aligned at the outer leaflet of the DMPC lipid bilayer with a tilt angle of about 10°. 6-KC organizes itself by forming an α-ß structure at low 6-KC concentration and most likely a ß-ß structure at high 6-KC concentration. Among all proposed models, our results favor the so-called umbrella model with formation of a 6-KC cluster. Moreover, we have found that the long anticipated flip-flop motion of 6-KC in the membrane takes time to occur, at least much longer than previously thought. All of these interesting findings indicate that it is critical to explore in situ, real-time, and label-free methodologies to obtain a precise molecular description of cholesterol's behavior in membranes. This study represents the first application of SFG to reveal the cholesterol-lipid interaction mechanism at the molecular level.

In situ molecular-level insights into the interfacial structure changes of membrane-associated prion protein fragment [118-135] investigated by sum frequency generation vibrational spectroscopy.

Protein aggregation is associated with many "protein deposition diseases". A precise molecular detail of the conformational transitions of such a membrane-associated protein structure is critical to understand the disease mechanism and develop effective treatments. One potential model peptide for studying the mechanism of protein deposition diseases is prion protein fragment [118-135] (PrP118-135), which shares homology with the C-terminal domain of the Alzheimer's ß-amyloid peptide. In this study, sum frequency generation vibrational spectroscopy (SFG-VS) has been applied to characterize interactions between PrP118-135 and 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (POPG) lipid bilayer in situ. The conformation change and orientation of PrP118-135 in lipid bilayers have been determined using SFG spectra with different polarization combinations. It is found that low-concentration PrP118-135 predominantly adopts α-helical structure but with tiny ß-sheet structure. With the PrP118-135 concentration increasing, the molecular number ratio of parallel ß-sheet structure increases and reaches about 44% at a concentration of 0.10 mg/mL, indicating the formation of abnormally folded scrapie isoforms. The α-helical structure inserts into the lipid bilayer with a tilt angle of ~32° versus the surface normal, while the ß-sheet structure lies down on the lipid bilayer with the tilt and twist angle both of 90°. The 3300 cm(-1) N-H stretching signal in psp spectra arises from α-helical structure at low PrP concentration and from the ß-sheet structure at high PrP concentration. Results from this study will provide an in-depth insight into the early events in the aggregation of PrP in cell membrane.

Intramolecular vibrational coupling in water molecules revealed by compatible multiple nonlinear vibrational spectroscopic measurements.

The complex structure of water and its interactions with solid surfaces require the development of multiple vibrational spectroscopic measurements to study the molecular structure of interfacial water and bulk water near the solid surface. In this study, a newly developed compatible multiple nonlinear vibrational spectroscopy system has been applied to investigate the molecular structure of water in the interfacial region of an ionic solid (CaF(2) substrate) and bulk isotopic D(2)O-HOD-H(2)O mixtures. Using this compatible system, the sum frequency generation (SFG) vibrational spectra and infrared-infrared-visible three-pump-field four-wave-mixing (IIV-TPF-FWM) spectra of the same water molecules can be collected at the same experimental geometry. It is found that SFG and IIV-TPF-FWM can be used to characterize the molecular structures of interfacial water and bulk water molecules at an interfacial distance below 42 nm, respectively. SFG and IIV-TPF-FWM results both suggest an intramolecular vibrational coupling dominates the spectra. The results achieved by this method are helpful to clarify the origination of the vibrational coupling of the interfacial water as well as the bulk water near the solid surface.

Expression of brain prolactin releasing peptide (PrRP) changes in the estrous cycle of female rats.

Prolactin releasing peptide (PrRP) is a neuropeptide with 31 or 20 amino acid residues and regarded as a potent and specific stimulator of pituitary prolactin. PrRP immunoreactive (PrRP-ir) neurons and mRNA are found in medulla oblongata and hypothalamus and the fibers containing PrRP are widely distributed in rat brains. Therefore, it is postulated that PrRP might act as a neurohormone or a neurotransmitter as well as a neuromodulator in the brain. In the present study, we probed the expression of brain PrRP in the estrous cycle of female rats and the relationship between brain PrRP and GnRH. Female rats were divided into four groups: the diestrus, the proestrus, the estrus and the metaestrus, which were identified by the vaginal cytological examination. Immunohistochemistry, reverse transcriptase-polymerase chain reaction (RT-PCR) and immunofluorescent double labeling histochemistry combining confocal laser scanning microscope (CLSM) were used. The results showed that PrRP immunoreactive neurons in nucleus of solitary tract (NTS) and ventrolateral reticular nucleus (VLRN) in the proestrus were less than those in the diestrus, the estrus and the metaestrus. Similarly, the relative optical density of PrRP-ir fibers of the bed nucleus of stria terminalis (BST) in the proestrus was decreased compared with those in other three groups. However, the brain PrRPmRNA level was higher in the proestrus and estrus than those in the metaestrus and diestrus. We also observed the co-localization of GPR10-immunoreactive (GPR10-ir) and GnRH-immunoreactive (GnRH-ir) neurons in hypothalamic medial preoptic area (MPO). The present results provide morphological evidences that PrRP in the female rat brains might participate in the regulation of the rat estrous cycle at least in a direct way.