Donor-donor energy migration (DDEM) as a tool for studying aggregation in lipid phases

Nanoscale packing differences in sphingomyelin and phosphatidylcholine revealed by BODIPY fluorescence in monolayers: physiological implications

Phosphatidycholines (PC) with two saturated acyl chains (e.g., dipalmitoyl) mimic natural sphingomyelin (SM) by promoting raft formation in model membranes. However, sphingoid-based lipids, such as SM, rather than saturated-chain PCs have been implicated as key components of lipid rafts in biomembranes. These observations raise questions about the physical packing properties of the phase states that can be formed by these two major plasma membrane lipids with identical phosphocholine headgroups. To investigate, we developed a monolayer platform capable of monitoring changes in surface fluorescence by acquiring multiple spectra during measurement of a lipid force-area isotherm. We relied on the concentration-dependent emission changes of 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY)-labeled PC to detect nanoscale alterations in lipid packing and phase state induced by monolayer lateral compression. The BODIPY-PC probe contained an indacene ring with four symmetrically located methyl (Me) substituents to enhance localization to the lipid hydrocarbon region. Surface fluorescence spectra indicated changes in miscibility even when force-area isotherms showed no deviation from ideal mixing behavior in the surface pressure versus cross-sectional molecular area response. We detected slightly better mixing of Me4-BODIPY-8-PC with the fluid-like, liquid expanded phase of 1-palmitoyl-2-oleoyl-PC compared to N-oleoyl-SM. Remarkably, in the gel-like, liquid condensed phase, Me4-BODIPY-8-PC mixed better with N-palmitoyl-SM than dipalmitoyl-PC, suggesting naturally abundant SMs with saturated acyl chains form gel-like lipid phase(s) with enhanced ability to accommodate deeply embedded components compared to dipalmitoyl-PC gel phase. The findings reveal a fundamental difference in the lateral packing properties of SM and PC that occurs even when their acyl chains match.

GLTP-fold interaction with planar phosphatidylcholine surfaces is synergistically stimulated by phosphatidic acid and phosphatidylethanolamine

Among amphitropic proteins, human glycolipid transfer protein (GLTP) forms a structurally-unique fold that translocates on/off membranes to specifically transfer glycolipids. Phosphatidylcholine (PC) bilayers with curvature-induced packing stress stimulate much faster glycolipid intervesicular transfer than nonstressed PC bilayers raising questions about planar cytosol-facing biomembranes being viable sites for GLTP interaction. Herein, GLTP-mediated desorption kinetics of fluorescent glycolipid (tetramethyl-boron dipyrromethene (BODIPY)-label) from lipid monolayers are assessed using a novel microfluidics-based surface balance that monitors lipid lateral packing while simultaneously acquiring surface fluorescence data. At biomembrane-like packing (30-35 mN/m), GLTP uptake of BODIPY-glycolipid from POPC monolayers was nearly nonexistent but could be induced by reducing surface pressure to mirror packing in curvature-stressed bilayers. In contrast, 1-palmitoyl-2-oleoyl-phosphatidylethanolamine (POPE) matrices supported robust BODIPY-glycolipid uptake by GLTP at both high and low surface pressures. Unexpectedly, negatively-charged cytosol-facing lipids, i.e., phosphatidic acid and phosphatidylserine, also supported BODIPY-glycolipid uptake by GLTP at high surface pressure. Remarkably, including both 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphate (5 mol%) and POPE (15 mol%) in POPC synergistically activated GLTP at high surface pressure. Our study shows that matrix lipid headgroup composition, rather than molecular packing per se, is a key regulator of GLTP-fold function while demonstrating the novel capabilities of the microfluidics-based film balance for investigating protein-membrane interfacial interactions.

Localisation of BODIPY-labelled phosphatidylcholines in lipid bilayers

A series of sn-2 acyl-labelled phosphatidyl-cholines (PC), bearing 4,4-difluoro-1-3-5-7-tetra-methyl-4-bora-3a,4a-diaza-s-indacene-8-yl (Me(4)-BODIPY) at the end of the C(n)-acyl chains were solubilised in unilamellar vesicles and studied with respect to the order and location of the Me(4)-BODIPY (denoted: B) group. The obtained results are based on time-resolved electronic energy transfer from donors (2-(9-anthroyloxy)-stearic acid) localised in the lipid-water interface to acceptors BnPC (n = 3, 5, 7, 9, 11, 13, 15), as well as the energy migration among the Me(4)-BODIPY groups of BnPC:s. The donor-acceptor and the donor-donor experiments strongly suggest that the Me(4)-BODIPY group in BnPC tends to loop back close to the lipid-water interface. The Me(4)-BODIPY groups, residing in the two bilayer leaflets, are located at approximately the same depth, and transversally separated by ca. 27 A for all n-values. Close to the interface, the optimal transversal distribution widens somewhat with increasing length of the sn-2 acyl chain. The obtained order parameter profile of the BnPC:s is also compatible with such a location.

Self-aggregation--an intrinsic property of G(M1) in lipid bilayers

We demonstrate that the ganglioside G(M1) in lipid bilayers of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) exhibits a non-uniform lateral distribution, i.e., enriched regions of GM(1) molecules are formed, which is an argument in favour of self-aggregation of G(M1) being an intrinsic property of G(M1) ganglioside. This was concluded from energy transfer/migration studies of BODIPY-labelled gangliosides by means of time-resolved fluorescence lifetime and depolarization experiments. Three fluorophore-labelled gangliosides were synthesized to include either of two spectroscopically different BODIPY groups. These were specifically localized either in the polar headgroup region or in the non-polar region of the lipid bilayer. An eventual ganglioside-ganglioside affinity/aggregation induced by the BODIPY groups was experimentally excluded, which suggests their use in examining the influence of G(M1) in more complex systems.

Utility and considerations of donor-donor energy migration as a fluorescence method for exploring protein structure-function

This review aims at surveying the use of electronic energy transport between chemically identical fluorophores (i.e. donors) in studies of various protein systems. Applications of intra- and interprotein energy migration are presented that make use of polarised steady-state and time-resolved fluorescence spectroscopic techniques. The donor-donor energy migration (DDEM) and the partial donor-donor energy migration (PDDEM) models for calculating distances between donor groups are exposed together with the most recent development of an extended Forster theory (EFT). Synthetic fluorescence depolarisation data that mimic time-correlated single photon counting experiments were generated using the EFT, and then further re-analysed by the different models. The results obtained were compared with the known parameters used to generate EFT data. Aspects on how to adopt the EFT in the analyses of time-correlated single photon counting experiments are also presented, as well as future aspects on using energy migration for examining protein structure.

Dimers of dipyrrometheneboron difluoride (BODIPY) with light spectroscopic applications in chemistry and biology

A ground-state dimer (denoted D(I)) exhibiting a strong absorption maximum at 477 nm (epsilon = 97 000 M(-1)cm(-1)) can form between adjacent BODIPY groups attached to mutant forms of the protein, plasminogen activator inhibitor type 1 (PAI-1). No fluorescence from excited D(I) was detected. A locally high concentration of BODIPY groups was also achieved by doping lipid phases (micelles, vesicles) with BODIPY-labeled lipids. In addition to an absorption band located at about 480 nm, a new weak absorption band is also observed at ca. 570 nm. Both bands are ascribed to the formation of BODIPY dimers of different conformation (D(I) and D(II)). Contrary to D(I) in PAI-1, the D(II) aggregates absorbing at 570 nm are emitting light observed as a broad band centered at about 630 nm. The integrated absorption band of D(I) is about twice that of the monomer, which is compatible with exciton coupling within a dimer. The Förster radius of electronic energy transfer between a BODIPY excited monomer and the ground-state dimer (D(I)()) is 57 +/- 2 A. A simple model of exciton coupling suggests that in D(I) two BODIPY groups are stacked on top of each other in a sandwich-like configuration with parallel electronic transition dipoles. For D(II) the model suggests that the S(0) --> S(1) transition dipoles are colinear. An explanation for the previously reported (J. Am. Chem. Soc. 1994, 116, 7801) exceptional light spectroscopic properties of BODIPY is also presented. These are ascribed to the extraordinary electric properties of the BODIPY chromophore. First, changes of the permanent electric dipole moment (Delta(mu) approximately -0.05 D) and polarizability (-26 x 10(-40) C m(2) V(-1)) between the ground and the first excited states are small. Second, the S(0) <--> S(1) electronic transition dipole moments are perpendicular to Delta(mu).