Aggregation/disaggregation of chlorophyll a in model phospholipid-detergent vesicles and micelles.

The photosynthetic pigments of higher plants exist in complex oligomeric states, which are difficult to study in vivo. To investigate aggregation processes of chlorophyll a (Chl a), we used an in vitro reconstitution procedure, with this pigment incorporated into liposomes of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), micelles and pre-micelle media of the detergent n-dodecyltrimethylammonium chloride (DTAC), and mixed, spontaneous, DMPC-DTAC vesicles and micelles. Chl a oligomers were characterized by UV-visible absorption, steady-state and time-resolved fluorescence, and fluorescence lifetime imaging microscopy. Equivalent diameters of the colloidal structures were obtained by fluorescence correlation spectroscopy. In DMPC liposomes and DMPC-DTAC vesicles and micelles, three fluorescence lifetimes indicated the coexistence of Chl a monomers (≈5 ns) and oligomers (≈1-2 to ≈0.1 ns). The increase in DTAC amount, in the mixed system, induces a progressive solubilization of DMPC liposomes (from vesicles to micelles) and simultaneous disruption of Chl a aggregates; in pure DTAC micelles, mostly monomers were found. The present work aims for a better understanding of chlorophyll-chlorophyll (Chl-Chl), Chl-lipid, and Chl-detergent interactions in spontaneous colloidal micro- and nanostructures.

Spontaneous vesicles, disks, threadlike and spherical micelles found in the solubilization of DMPC liposomes by the detergent DTAC.

The spontaneous colloidal nanostructures formed in water by the zwitterionic phospholipid DMPC (1,2-dimyristoyl-sn-glycero-3-phosphocholine) with the cationic detergent DTAC (n-dodecyltrimethylammonium chloride) were investigated at a fixed DMPC concentration and variable detergent:lipid total molar ratios (D:L). Apparent (neutral-sphere-equivalent) hydrodynamic diameters (Φ(e)) of liposomes and micelles were obtained by dynamic light scattering (DLS). Fluorescence lifetime imaging microscopy (FLIM), using chlorophyll-a as a probe, showed the morphology of giant vesicles and threadlike micelles. Micro-differential scanning calorimetry (micro-DSC) detected the presence of bilayers, in vesicles and discoidal micelles (disks). Pure DMPC liposomes are multilamellar and polydisperse (Φ(e)≈100-10,000 nm). As D:L increased, smaller vesicles were found, due to the bigger spontaneous curvature of the bilayer: at D:L=1, ULVs (unilamellar vesicles; Φ(e)≈100 nm) appeared and, at D:L=2-10, ULVs coexisted with disks (Φ(e)≈30 nm). Bilayers totally disappeared at D:L≥15, giving rise to spheroidal (Φ(e)≈2-16 nm) and threadlike (Φ(e)≈100-10,000 nm) micelles. A quasi-equilibrium structural diagram for the DMPC-DTAC-water system shows equivalent diameters of the scattering nanoparticles as a function of D:L. The results obtained herein for the system DMPC-DTAC show the role of electrostatic interactions in the formation of the mixed structures.

Time evolution of the thermotropic behavior of spontaneous liposomes and disks of the DMPC-DTAC aqueous system.

In this work, solubilization of the phospholipid 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) by the cationic detergent n-dodecyltrimethylammonium chloride (DTAC) was studied in aqueous solution, at a fixed DMPC concentration and variable detergent:lipid (D:L) molar ratios. The colloidal nanostructures present in different stages of the solubilization process were characterized using micro-differential scanning calorimetry (DSC) and dynamic light scattering (DLS) techniques. For total (analytical) D:L molar ratios below approximately 1, DTAC monomers incorporate into the DMPC liposome bilayers, forming smaller and more fluid vesicles than pure DMPC liposomes. At D:L approximately 1-2, vesicles begin to rupture, coexisting with intact vesicles and bilayer fragments. At D:L approximately 2-12.5, discoidal and spherical micelles are formed and coexist with vesicles; a slow structural rearrangement of the system, monitored in successive DSC heating/cooling cycles, was observed, and is reported for the first time. Finally, for D:L above approximately 15-20, the bilayers are completely dissolved, and the main aggregates in solution become spherical micelles, which slowly evolve to cylindrical (threadlike) micelles. Based on the dependence of the temperature and enthalpy of transitions on the total D:L molar ratio, at constant DMPC concentration, a schematic model, showing the different colloidal nanostructures present in the solubilization process, is proposed.

Unfolding kinetics of beta-lactoglobulin induced by surfactant and denaturant: a stopped-flow/fluorescence study.

The beta-->alpha transition of beta-lactoglobulin, a globular protein abundant in the milk of several mammals, is investigated in this work. This transition, induced by the cationic surfactant dodecyltrimethylammonium chloride (DTAC), is accompanied by partial unfolding of the protein. In this work, unfolding of bovine beta-lactoglobulin in DTAC is compared with its unfolding induced by the chemical denaturant guanidine hydrochloride (GnHCl). The final protein states attained in the two media have quite different secondary structure: in DTAC the alpha-helical content increases, leading to the so-called alpha-state; in GnHCl the amount of ordered secondary-structure decreases, resulting in a random coil-rich final state (denatured, or D, state). To obtain information on both mechanistic routes, in DTAC and GnHCl, and to characterize intermediates, the kinetics of unfolding were investigated in the two media. Equilibrium and kinetic data show the partial accumulation of an on-pathway intermediate in each unfolding route: in DTAC, an intermediate (I(1)) with mostly native secondary structure but loose tertiary structure appears between the native (beta) and alpha-states; in GnHCl, another intermediate (I(2)) appears between states beta and D. Kinetic rate constants follow a linear Chevron-plot representation in GnHCl, but show a more complex mechanism in DTAC, which acts like a stronger binding species.