New spirit of an old technique: Characterization of lipid phase transitions via UV/Vis spectroscopy

Uncoated gold nanoparticles create fewer and less localized defects in model prokaryotic than in model eukaryotic lipid membranes

The rise of the populations of antibiotic resistant bacteria represents an increasing threat to human health. In addition to the synthesis of new antibiotics, which is an extremely expensive and time-consuming process, one of the ways to combat bacterial infections is the use of gold nanoparticles (Au NPs) as the vehicles for targeted delivery of therapeutic drugs. Since such a strategy requires the investigation of the effect of Au NPs (with and without drugs) on both bacterial and human cells, we investigated how the presence of coating-free Au NPs affects the physicochemical properties of lipid membranes that model prokaryotic (PRO) and eukaryotic (EU) cells. PRO/EU systems prepared as multilamellar liposomes (MLVs) and hybrid structures (HSs) from 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dipalmitoyl-sn-glycero-3-phosphatidylglycerol (DPPG)/1,2-dipalmitoyl-sn-glycero-3-phosphoserine (DPPS) in the absence (MLVs)/presence (HSs) of differently distributed Au NPs (sizes ∼20 nm) reported stabilization of the gel phase of PRO systems in comparison with EU one (DSC data of PRO/EU were Tm(MLVs) ≈ 41.8 °C/42.0 °C, Tm¯ (HSs) ≈ 43.1 °C/42.4 °C, whereas UV-Vis response Tm(MLVs) ≈ 41.5 °C/42.0 °C, Tm¯ (HSs) ≈ 42.9 °C/41.1 °C). Vibrational spectroscopic data unraveled a substantial impact of Au NPs on the non-polar part of lipid bilayers, emphasizing the increase of kink and gauche conformers of the hydrocarbon chain. By interpreting the latter as Au NPs-induced defects, which exert the greatest effect when Au NPs are found exclusively outside the lipid membrane, these findings suggested that Au NPs reduced the compactness of EU-based lipid bilayers much more than in analogous PRO systems. Since the uncoated Au NPs manifested adverse effects when applied as antimicrobials, the results obtained in this work contribute towards recognizing AuNP functionalization as a strategy in tuning and reversing this effect.

Optical and molecular features of negatively curved surfaces created by POPE lipids: A crucial role of the initial conditions

Membrane fusion is closely related to plasma membrane domains rich in cone-shaped phosphatidylethanolamine (PE) lipids that can reverse membrane curvature under certain conditions. The phase transition of PE-based lipid membranes from the lamellar fluid phase (Lα) to the inverse hexagonal phase (HII) is commonly taken as a general model in reconstructing the membrane fusion pathway, and whose structural features have been mostly described so far using structural and microscopic techniques. The aim of this paper is to decipher the optical and molecular features of Lβ → Lα and especially of Lα → HII transition of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) lipids at pH = 7.0 when they are initially prepared in the form of both multi- and unilamellar liposomes (MLVs and LUVs). The distinction between optical properties of MLS- and LUVs-derived HII phase, provided from turbidity-sensitive temperature-dependent UV-Vis spectra, was attributed to different formation mechanisms of HII phase. Most importantly, from FTIR spectroscopic data of POPE lipids in Lβ (15 °C), Lα (50 °C) and HII (85 °C) phases we identified the changes in molecular features of POPE lipids during phase transitions. Among the latter, by far the most significant is different hydration pattern of POPE lipids in MLVs- and LUVs-derived HII phase which extends from the polar-apolar interface all the way to the terminal amino group of the POPE lipid, along with the changes in the conformation of glycerol backbone as evidenced by the signature of α-methylene groups. Molecular dynamics simulations confirmed higher water penetration in HII phase and provided insight into hydrogen bonding patterns.

In Situ Quantitative Monitoring of Adsorption from Aqueous Phase by UV-vis Spectroscopy: Implication for Understanding of Heterogeneous Processes

The development of in situ techniques to quantitatively characterize the heterogeneous reactions is essential for understanding physicochemical processes in aqueous phase. In this work, a new approach coupling in situ UV-vis spectroscopy with a two-step algorithm strategy is developed to quantitatively monitor heterogeneous reactions in a compact closed-loop incorporation. The algorithm involves the inverse adding-doubling method for light scattering correction and the multivariate curve resolution-alternating least squares (MCR-ALS) method for spectral deconvolution. Innovatively, theoretical spectral simulations are employed to connect MCR-ALS solutions with chemical molecular structural evolution without prior information for reference spectra. As a model case study, the aqueous adsorption kinetics of bisphenol A onto polyamide microparticles are successfully quantified in a one-step UV-vis spectroscopic measurement. The practical applicability of this approach is confirmed by rapidly screening a superior adsorbent from commercial materials for antibiotic wastewater adsorption treatment. The demonstrated capabilities are expected to extend beyond monitoring adsorption systems to other heterogeneous reactions, significantly advancing UV-vis spectroscopic techniques toward practical integration into automated experimental platforms for probing aqueous chemical processes and beyond.

The presence of uncoated gold nanoparticle aggregates may alter the phase of phosphatidylcholine lipid as evidenced by vibrational spectroscopies

Spherical structures built from uni- and multilamellar lipid bilayers (LUV and MLV) are nowadays considered not just as nanocarriers of various kinds of therapeutics, but also as the vehicles that, when coupled with gold (Au) nanoparticles (NPs), can also serve as a tool for imaging and discriminating healthy and diseased tissues. Since the presence of Au NPs or their aggregates may affect the properties of the drug delivery vehicle, we investigated how the shape and position of Au NP aggregates adsorbed on the surface of MLV affect the arrangement and conformation of lipid molecules. By preparing MLVs constituted from 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) in the presence of uncoated Au NP aggregates found i) both within liposome core and on the surface of the outer lipid bilayer, or ii) adsorbed on the outer lipid bilayer surface only, we demonstrated the maintenance of lipid bilayer integrity by microscopic techniques (cryo-TEM, and AFM). The employment of SERS and FTIR-ATR techniques enabled us not only to elucidate the lipid interaction pattern and their orientation in regards to Au NP aggregates but also unequivocally confirmed the impact of Au NP aggregates on the persistence/breaking of van der Waals interactions between hydrocarbon chains of DPPC.

The rise of FTIR spectroscopy in the characterization of asymmetric lipid membranes

In contrast to symmetric unilamellar liposomes (sLUVs) prepared from a mixture of different lipids, asymmetric ones (aLUVs) with different lipid composition in the inner and outer membrane leaflets are more suitable model systems of eukaryotic plasma membranes. However, apart from the challenging preparation of asymmetric liposomes and small amounts of obtained asymmetric unilamellar liposomes (aLUVs), a major drawback is the qualitative characterization of asymmetry, as each of the techniques used so far has certain limitations. In this regard, we prepared aLUVs composed dominantly of DPPC(out)/DPPS(in) lipids and, along with 1H NMR and DSC characterization, we showed for the first time how FTIR spectroscopy can be used in the presence of (a)symmetry between DPPC/DPPS lipid bilayers. Using second derivative FTIR spectra we demonstrated not only that the hydration of lipids glycerol backbone and choline moiety of DPPC differs in s/aLUVs, but in addition that the lateral interactions between hydrocarbon chains during the phase change display different trend in s/aLUVs. Molecular dynamics simulations confirmed different chain ordering and packing between s/a bilayers, with a significant influence of temperature, i.e. membrane phase.

Phase-Dependent Adsorption of Myelin Basic Protein to Phosphatidylcholine Lipid Bilayers

The dense packing of opposite cytoplasmic surfaces of the lipid-enriched myelin membrane, responsible for the proper saltatory conduction of nerve impulses through axons, is ensured by the adhesive properties of myelin basic protein (MBP). Although preferentially interacting with negatively charged phosphatidylserine (PS) lipids, as an intrinsically disordered protein, it can easily adapt its shape to its immediate environment and thus adsorb to domains made of zwitterionic phosphatidylcholine (PC) lipids. As the molecular-level interaction pattern between MBP and PC lipid membranes suffers from scarce characterization, an experimental and computational study of multilamellar liposomes (MLVs) composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) in the presence of bovine MBP is presented here. Calorimetric and temperature-dependent UV-Vis measurements identified DPPC pretransition temperature (Tp) and calorimetric enthalpy (ΔHcal) as the physicochemical parameters most responsive to the presence of MBP. Besides suggesting an increase in β-sheet fractions of structured MBP segments as DPPC lipids undergo from the gel (20 °C) to the fluid (50 °C) phase, FTIR spectra unraveled the significant contribution of lysine (Lys) residues in the adsorption pattern, especially when DPPC is in the fluid (50 °C) phase. In addition to highlighting the importance of Lys residues in the MBP adsorption on DPPC lipid bilayer, employing salt bridges (SBs) and hydrogen bonds (HBs), MD data suggest the crucial importance of the orientation of MBP with respect to the surface of the DPPC lipid bilayer.

Adsorption/Desorption of Cationic-Hydrophobic Peptides on Zwitterionic Lipid Bilayer Is Associated with the Possibility of Proton Transfer

Cell-penetrating peptides (CPPs) are short peptides built up from dominantly cationic and hydrophobic amino acid residues with a distinguished ability to pass through the cell membrane. Due to the possibility of linking and delivering the appropriate cargo at the desired location, CPPs are considered an economic and less invasive alternative to antibiotics. Besides knowing that their membrane passage mechanism is a complex function of CPP chemical composition, the ionic strength of the solution, and the membrane composition, all other details on how they penetrate cell membranes are rather vague. The aim of this study is to elucidate the ad(de)sorption of arginine-/lysine- and phenylalanine-rich peptides on a lipid membrane composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) lipids. DSC and temperature-dependent UV-Vis measurements confirmed the impact of the adsorbed peptides on thermotropic properties of DPPC, but in an inconclusive way. On the other hand, FTIR spectra acquired at 30 °C and 50 °C (when DPPC lipids are found in the gel and fluid phase, respectively) unambiguously confirmed the proton transfer between particular titratable functional groups of R5F2/K5F2 that highly depend on their immediate surroundings (DPPC or a phosphate buffer). Molecular dynamic simulations showed that both peptides may adsorb onto the bilayer, but K5F2 desorbs more easily and favors the solvent, while R5F2 remains attached. The results obtained in this work highlight the importance of proton transfer in the design of CPPs with their desired cargo, as its charge and composition dictates the possibility of entering the cell.

Lamellarity-Driven Differences in Surface Structural Features of DPPS Lipids: Spectroscopic, Calorimetric and Computational Study

Although single-lipid bilayers are usually considered models of eukaryotic plasma membranes, their research drops drastically when it comes to exclusively anionic lipid membranes. Being a major anionic phospholipid in the inner leaflet of eukaryote membranes, phosphatidylserine-constituted lipid membranes were occasionally explored in the form of multilamellar liposomes (MLV), but their inherent instability caused a serious lack of efforts undertaken on large unilamellar liposomes (LUVs) as more realistic model membrane systems. In order to compensate the existing shortcomings, we performed a comprehensive calorimetric, spectroscopic and MD simulation study of time-varying structural features of LUV made from 1,2-dipalmitoyl-sn-glycero-3-phospho-L-serine (DPPS), whereas the corresponding MLV were examined as a reference. A substantial uncertainty of UV/Vis data of LUV from which only T_m was unambiguously determined (53.9 ± 0.8 °C), along with rather high uncertainty on the high-temperature range of DPPS melting profile obtained from DSC (≈50-59 °C), presumably reflect distinguished surface structural features in LUV. The FTIR signatures of glycerol moiety and those originated from carboxyl group serve as a strong support that in LUV, unlike in MLV, highly curved surfaces occur continuously, whereas the details on the attenuation of surface features in MLV were unraveled by molecular dynamics.

Influence of DPPE surface undulations on melting temperature determination: UV/Vis spectroscopic and MD study

One of the most distinguished quantities that describes lipid main phase transition, i.e. the transition from the gel (L_β(')) to the fluid (L_α) phase, is its melting temperature (T_m). Because melting is accompanied by a large change in enthalpy the, L_β(') → L_α transition can be monitored by various calorimetric, structural and spectroscopic techniques and T_m should be the same regardless of the metric monitored or the technique employed. However, in the case of DPPE multilamellar aggregates there is a small but systematic deviation of T_m values determined by DSC and FTIR spectroscopy. The aim of this paper is to explain this discrepancy by combined UV/Vis spectroscopic and MD computational approach. Multivariate analysis performed on temperature-dependent UV/Vis spectra of DPPE suspensions demonstrated that at 55 ± 1 °C certain phenomenon causes a small but detectable change in suspension turbidity, whereas a dominant change in the latter is registered at 63.2 ± 0.4 °C that coincides with T_m value determined from DSC curve. If this effect should be ignored, the overall data give T_m value the same as FTIR spectra data (61.0 ± 0.4 °C). As the classical MD simulations suggest that about 10° below T_m certain undulations appear at the surface of DPPE bilayers, we concluded that certain discontinuities in curvature fluctuations arise at reported temperature which are to some extent coupled with lipid melting. Ultimately, such events and the associated changes in curvature affect T_m value measured by different techniques.

Deciphering the origin of the melting profile of unilamellar phosphatidylcholine liposomes by measuring the turbidity of its suspensions

The elucidation of the thermal properties of phosphatidylcholine liposomes is often based on the analysis of the thermal capacity profiles of multilamellar liposomes (MLV), which may qualitatively disagree with those of unilamellar liposomes (LUV). Experiments and interpretation of LUV liposomes is further complicated by aggregation and lamellarization of lipid bilayers in a short time period, which makes it almost impossible to distinguish the signatures of the two types of bilayers. To characterize independently MLV and LUV of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), the latter were prepared with the addition of small amounts of 1,2-dipalmitoyl-sn-glycero-3-phosphatidylglycerol (DPPG) which, due to the sterical hindrance and negative charge at a given pH value, cause LUV repellence and contribute to their stability. Differential scanning calorimetry curves and temperature-dependent UV/Vis spectra of the prepared MLV and LUV were measured. Multivariate analysis of spectrophotometric data determined the phase transition temperatures (pretransition at T_p and the main phase transition at T_m), and based on the changes in turbidities, the thickness of the lipid bilayer in LUV was determined. The obtained data suggested that the curvature change is a key distinguishing factor in MLV and LUV heat capacity profiles. By combining the experimental results and those obtained by MD simulations, the interfacial water layer was characterized and its contribution to the thermal properties of LUV was discussed.