Dental restorative composites fabricated from a novel organic matrix without an additional diluent

Commercial organic matrixes of dental composites generally include diluents such as triethylene glycol dimethacrylate (TEGDMA) to reduce viscosity. However, the diluent exhibits adverse effects such as curing shrinkage and diminished mechanical properties of the dental composites. To overcome these adverse effects, organic monomers that can be used as an organic matrix may be developed. In this study, various novel organic monomers were developed by substituting alkoxy for hydroxyl groups in 2,2-bis[4-(2-hydroxy-3-methacryloyloxy propoxy)phenyl]propane (bis-GMA). Viscosities of the alkoxy-substituted monomers were decreased by increasing substituent size. The viscosity of 2,2-bis[4-(2-ethoxy-3-methacryloyloxy propoxy)phenyl]propane (bis-E-GMA) was higher than the control organic matrix (70 wt % bis-GMA and 30 wt % TEGDMA). However, those of 2,2-bis[4-(2-propoxy-3-methacryloyloxy propoxy)phenyl]propane (bis-Pr-GMA), 2,2-bis[4-(2-butoxy-3-methacryloyloxy propoxy)phenyl]propane (bis-B-GMA), and 2,2-bis[4-(2-pentoxy-3-methacryloyloxy propoxy)phenyl]propane (bis-P-GMA) were lower than the control organic matrix. To this end, these monomers could be used as organic matrixes of dental composites without an additional diluent. Among these monomers, bis-B-GMA exhibited the lowest curing shrinkage. In comparison to the control organic matrix, the curing shrinkage of the bis-B-GMA dental composite was approximately 40%. Additionally, dental composites prepared from bis-B-GMA exhibited excellent mechanical properties.     

Characteristics of novel dental composites containing 2,2-bis[4-(2-methoxy-3-methacryloyloxy propoxy) phenyl] propane as a base resin

Many dental restorative dental composites still utilize 2,2-bis[4-(2-hydroxy-3-methacryloyloxy propoxy) phenyl] propane (Bis-GMA) as base resin. The high viscosity of Bis-GMA necessitates dilution with dimethacrylate ethers of low viscosity such as triethylene glycol dimethacrylate (TEGDMA). However, increased amounts of the TEGDMA have adverse effects on properties such as water uptake and curing shrinkage. The viscosity of the base resin should be as low as possible to enable the preparation of dental composites with a minimum content of diluent. To overcome the disadvantage of Bis-GMA, i.e., its high viscosity caused by hydrogen bonding between hydroxyl groups, 2,2-bis[4-(2-methoxy-3-methacryloyloxy propoxy) phenyl propane (Bis-M-GMA) was prepared by substituting methoxy groups for hydroxyl groups in Bis-GMA. The viscosity of Bis-GMA was dramatically decreased from 574 (Pa.s) to 3.7 (Pa.s) by substitution of methoxy group. Consequently, the amount of TEGDMA included in the resin matrix could be minimized. Dental composites were prepared from Bis-M-GMA (or Bis-GMA) mixtures with TEGDMA filled with 75 wt % filler. Comparing the curing shrinkage of dental composite containing Bis-M-GMA with that prepared from Bis-GMA, the reduction in curing shrinkage was about 47%. Dental composites prepared from new resin matrixes also exhibited low water uptake and better properties in mechanical strength.     

The effects of radioprotectant and potential antioxidant agent amifostine on the structure and dynamics of DPPC and DPPG liposomes

Agents capable of scavenging ROS have attracted attention recently because of their potential use as antioxidative agents. Amifostine, a ROS scavenger, has the potential to be used as an antioxidant in therapeutic applications. In this study, the effect of amifostine on neutral zwitterionic dipalmitoylphosphatidylcholine (DPPC) and anionic dipalmitoylphosphatidylglycerol (DPPG) model membranes' structure and dynamics is aimed to be examined by Fourier transform infrared (FTIR) spectroscopy and differential scanning calorimetry (DSC). Our results revealed that amifostine at concentrations used (1–24 mol%) does not induce any important alteration in the shape of phase transition curve and phase transition temperature in the DPPC and DPPG membranes. High concentrations of amifostine slightly increased the acyl chain flexibility of DPPC membranes in the liquid crystalline phase and DPPG membranes in the gel phase. A lessening in the dynamics of DPPC liposomes was observed for all concentrations of amifostine in both phases but slight dual effect was observed only in the gel phase as a decrease in dynamics at low concentrations and an increase at higher concentrations of amifostine in DPPG liposomes. Additionally, strong hydrogen bonding was observed for both CO and PO₂⁻ groups in case of DPPC and for PO₂⁻ groups in case of DPPG. Dehydration around the CO regions occurred in case of DPPG. Accordingly, amifostine is proposed to be interacting strongly with zwitterionic and negatively charged membrane head groups and glycerol backbone in all concentrations and because of this interaction it causes some changes in lipid order and dynamics especially at high concentrations.     

Effects of phospholipid hydrolysis on the aggregate structure in DPPC/DSPE-PEG2000 liposome preparations after gel to liquid crystalline phase transition

Upon storage of phospholipid liposome samples, lysolipids, fatty acids, and glycerol-3-phosphatidylcholine are generated as a result of acid- or base-catalyzed hydrolysis. Accumulation of hydrolysis products in the liposome membrane can induce fusion, leakage, and structural transformations of the liposomes, which may be detrimental or beneficial to their performance depending on their applications as, e.g., drug delivery devices. We investigated in the present study the influence of phospholipid hydrolysis on the aggregate morphology of DPPC/DSPE-PEG2000 liposomes after transition of the phospholipid membrane from the gel phase to liquid crystalline phase using high performance liquid chromatography (HPLC) in combination with static light scattering, dynamic light scattering, and cryo-transmission electron microscopy (cryo-TEM). The rates of DPPC hydrolysis in DPPC/DSPE-PEG2000 liposomes were investigated at a pH of 2, 4, or 6.5 and temperatures of 22 degrees C or 4 degrees C. Results indicate that following phase transition, severe structural reorganizations occurred in liposome samples that were partially hydrolyzed in the gel phase. The most prominent effect was an increasing tendency of liposomes to disintegrate into membrane discs in accordance with an increasing degree of phospholipid hydrolysis. Complete disintegration occurred when DPPC concentrations had decreased by, in some cases, as little as 3.6%. After extensive phospholipid hydrolysis, liposomes and discs fused to form large bilayer sheets as well as other more complex bilayer structures apparently due to a decreased ratio of lysolipid to palmitic acid levels in the liposome membrane.     

Effect of 2,4-dichlorophenol on DPPC/water liposomes studied by X-ray and freeze-fracture electron microscopy

The effect of 2,4-dichlorophenol (DCP) was studied on the fully hydrated 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC)--water liposomes. The structure and the thermotropic phase behaviour of the liposomes was examined in the presence of DCP (DCP/DPPC molar ratio, varied from 2x10(-2) up to 1) using small- and wide-angle X-ray scattering (SAXS, WAXS) and freeze-fracture electron microscopy. The structural behaviour of the DPPC/DCP/water system was strongly dependent on the concentration of the DCP. In the pretransition range the DCP molecules (at 2x10(-2) DCP/DPPC molar ratio) induced the interdigitated phase beside the parent (gel and rippled gel) phases, locally which can be form at higher DCP concentration. When the DCP/DPPC molar ratio was increased the pretransition disappeared and the main transition was shifted to lower temperatures. In the molar ratio range from 2x10(-1) up to 5x10(-1), a coexistence of different phases was observed in the wide temperature range from 20 up to 40 degrees C. With a further increase of the DCP/DPPC molar ratio (6x10(-1) to 1) only the interdigitated gel phase occurred below 25 degrees C. A schematic phase diagram of DPPC/DCP/water system was constructed to summarise the results.     

Effects of phospholipid hydrolysis on the aggregate structure in DPPC/DSPE-PEG2000 liposome preparations after gel to liquid crystalline phase transition

Upon storage of phospholipid liposome samples, lysolipids, fatty acids, and glycerol-3-phosphatidylcholine are generated as a result of acid- or base-catalyzed hydrolysis. Accumulation of hydrolysis products in the liposome membrane can induce fusion, leakage, and structural transformations of the liposomes, which may be detrimental or beneficial to their performance depending on their applications as, e.g., drug delivery devices. We investigated in the present study the influence of phospholipid hydrolysis on the aggregate morphology of DPPC/DSPE-PEG₂₀₀₀ liposomes after transition of the phospholipid membrane from the gel phase to liquid crystalline phase using high performance liquid chromatography (HPLC) in combination with static light scattering, dynamic light scattering, and cryo-transmission electron microscopy (cryo-TEM). The rates of DPPC hydrolysis in DPPC/DSPE-PEG₂₀₀₀ liposomes were investigated at a pH of 2, 4, or 6.5 and temperatures of 22 °C or 4 °C. Results indicate that following phase transition, severe structural reorganizations occurred in liposome samples that were partially hydrolyzed in the gel phase. The most prominent effect was an increasing tendency of liposomes to disintegrate into membrane discs in accordance with an increasing degree of phospholipid hydrolysis. Complete disintegration occurred when DPPC concentrations had decreased by, in some cases, as little as 3.6%. After extensive phospholipid hydrolysis, liposomes and discs fused to form large bilayer sheets as well as other more complex bilayer structures apparently due to a decreased ratio of lysolipid to palmitic acid levels in the liposome membrane.     

Effects of molecular structure of the resins on the volumetric shrinkage and the mechanical strength of dental restorative composites

To prepare a dental composite that has a low amount of curing shrinkage and excellent mechanical strength, various 2,2-bis[4-(2-hydroxy-3-methacryloyloxy propoxy) phenyl] propane (Bis-GMA) derivatives were synthesized via molecular structure design, and afterward, properties of their mixtures were explored. Bis-GMA derivatives, which were obtained by substituting methyl groups for hydrogen on the phenyl ring in the Bis-GMA, exhibited lower curing shrinkage than Bis-GMA, whereas their viscosities were higher than that of Bis-GMA. Other Bis-GMA derivatives, which contained a glycidyl methacrylate as a molecular end group exhibited reduced curing shrinkage and viscosity. Methoxy substitution for hydroxyl groups on the Bis-GMA derivatives was performed for the further reduction of the viscosity and curing shrinkage. Various resin mixtures, which had the same viscosity as the commercial one, were prepared, and their curing shrinkage was examined. A resin mixture containing 2,2-bis[3,5-dimethyl, 4-(2-methoxy-3-methacryloyloxy propoxy) phenyl] propane] (TMBis-M-GMA) as a base resin and 4-tert-butylphenoxy-2-methyoxypropyl methacrylate (t-BP-M-GMA) as a diluent exhibited the lowest curing shrinkage among them. The composite prepared from this resin mixture also exhibited the lowest curing shrinkage along with enhanced mechanical properties.     

The role of surface charge in the activation of the classical and alternative pathways of complement by liposomes

We have studied the complement-activating properties of liposomes. We show that surface charge is a key determinant of complement-activating liposomes. The nature of the charge, whether negative or positive, appears to dictate which pathway of the complement system is activated. Phosphatidylcholine:cholesterol (PC:CHOL, 55:45 mol/mol) liposomes were made to exhibit a positive or negative surface charge by the addition of cationic or anionic lipids, respectively. Normal human or guinea pig serum was incubated with liposomes, followed by determining the residual hemolytic activity of the serum as a measure of complement activation. Negatively charged liposomes containing phosphatidyl-glycerol, phosphatidic acid, cardiolipin, phosphatidylinositol, or phosphatidylserine activated complement in a Ca(2+)-dependent manner suggesting activation occurred via the classical pathway. Positively charged liposomes containing stearylamine or 1,2-bis(oleoyloxy)-3-(trimethylammonio)propane activated complement via the alternative pathway. Neutral liposomes, PC:CHOL (55:45) and PC:CHOL:dipalmitoylphosphatidylethanolamine (35:45:20), failed to activate complement as measured by the hemolytic assays. We show that unsaturated liposomes are more potent complement activators than saturated liposomes and that 45 mol% cholesterol promotes complement protein-liposome interactions. Immunoblot analysis of phosphatidylglycerol-containing liposomes showed that C3b and C9 were associated with these liposomes. Thus, the complement consumption measured in the hemolytic assays represents active cleavage of the complement components and not passive adsorption to the liposome surface. These studies suggest that membranes composed of net charged phospholipids can activate the complement system. This observation underlines the importance in biologic membranes of complement regulatory proteins that protect normal cells from complement attack.     

Effect of the nature of the 3beta-substitution in manoyl oxides on the thermotropic behavior of DPPC lipid bilayer and on DPPC liposomes

Functionalized manoyl oxide derivatives have been proved over the years to evoke several biological responses. Among them, 3beta-hydroxy-manoyl oxide (1) and 3beta-acetoxy-manoyl oxide (2) have been shown to exhibit in vitro antimicrobial and cytotoxic activity, while N-imidazole-3 beta-thiocarbonyl ester of manoyl oxide (3) was found to exhibit potent cytotoxic effect. Their partitioning into phospholipid bilayers may lead to membrane structure modifications that are crucial in liposome development as they may influence their maintenance and integrity. DSC was used to study the modifications induced in DPPC bilayers by incorporating increasing concentrations of the three manoyl oxide derivatives. All derivatives were found to strongly affect the bilayer structural organization in terms of a decrease of the cooperativity, the fluidization and partially destabilization of the gel phase and the induction of a lateral phase separation in clustering domains. Derivatives 1 and 3 were incorporated into DPPC liposomes and their physicochemical stability was monitored at 4 degrees C. The stability of liposomes was strongly influenced by the presence of 1 and 3 at any molar ratio studied. DPPC/1 liposomes were found to retain its stability for 48 h at low concentration of 10% mol, while at higher concentrations up to 30% mol they collapsed into aggregated material. In all cases DPPC/3 liposomes were found unstable and sticky aggregated structures precipitated from the bulk suspension.     

Preparation of DPPC liposomes using probe-tip sonication: Investigating intrinsic factors affecting temperature phase transitions

Liposomes are an important tool and have gained much attention for their promise as an effective means of delivering small therapeutic compounds to targeted sites. In an effort to establish an effective method to produce liposomes from the lipid, dipalmitoyl-phosphatidylcholine or DPPC, we have found important aspects that must be taken into consideration. Here, we used probe-tip sonication to prepare liposomes on a batch scale. During this process we uncovered interesting steps in their preparation that altered the thermodynamic properties and phase transitions of the resulting liposome mixtures. Using differential scanning calorimetry to assess this we found that increasing the sonication time had the most dramatic effect on our sample, producing almost an entirely separate phase transition relative to the main phase transition. This result is consistent with reports from the current literature. We also highlight a smaller transition, which we attribute to traces of unincorporated lipid that seems to gradually disappear as the total lipid concentration decreases. Overall, sonication is an effective means of producing liposomes, but we cannot assert this method is optimal in producing them with precise physical properties. Here we highlight the physical effects at play during this process.     

Effects of cholesterol on phospholipid membranes: inhibition of the interdigitated gel phase of F-DPPC and F-DPPC/DPPC

Unlike the parent phospholipid, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), the monofluorinated analog, 1-palmitoyl-2-(16-fluoropalmitoyl)sn-glycero-3-phosphocholine (F-DPPC), spontaneously forms an interdigitated gel phase (L(β)I) below the main transition temperature (T(m)). We have examined the effects of introducing cholesterol to F-DPPC and 1:1 F-DPPC/DPPC membranes using a combination of DSC, optical density, fluorescence intensity and polarization, (31)P NMR, and X-ray diffraction techniques. Cholesterol increases the fluidity of the gel phase, broadens the main transition, and decreases the main transition enthalpy. However, these results also reveal that there is an unusually large degree of phase coexistence between the L(β)I and non-interdigitated gel phases when cholesterol is added. Cholesterol encourages this phase segregation by partitioning into the thicker non-interdigitated domains. At higher cholesterol concentrations, the majority or all of the L(β)I phase of F-DPPC and 1:1 F-DPPC/DPPC is eliminated and is replaced by a non-interdigitated liquid-ordered (l(o)) phase with properties similar to DPPC/cholesterol. Consequently, cholesterol mitigates the influence the CF moiety has on the thermodynamic phase behavior of F-DPPC. Our findings demonstrate that there are multiple characteristics of cholesterol-rich membranes that disfavor interdigitation.     

The influence of selected steroid hormones on the physicochemical behaviour of DPPC liposomes

The physicochemical properties of DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) liposomes used for topical application are pharmaceutically important. Therefore the aim of our study was to establish rapid and efficient methods for the exact characterisation of the physicochemical properties of extruded DPPC liposomes containing low concentration (0.5%, w/w) of different, therapeutically interesting steroid hormones, named 17-beta-estradiol, cpa (cyproterone acetate) and finasteride. In a first step it could be shown that all drugs influenced the liposome size and changed the zeta potential compared to the placebo formulations. Our further analytical strategy was to use micro-calorimetry and ATR-FTIR (Fourier transformed infrared spectroscopy), two powerful and non-destructive methods to confirm the drug incorporation into the liposomes by proving interactions between the phospholipids and the steroids. Thereby it was even possible to localize the location of interaction. The characteristic phase transition temperatures of the phospholipid were decreased by the hormones which was detected by micro-DSC (differential scanning calorimetry). The results of the ATR-FTIR measurements indicated shifts of the specific lipid peaks, the C=O stretching bands and PO(2)(-) antisymmetric double stretching band, in the gel and liquid crystalline phase. A polar as well as a non-polar interaction could be proven. It could be shown that the investigated steroid hormones changed the physical properties of the phospholipid bilayers.     

Effect of the Nature of the 3β-Substitution in Manoyl Oxides on the Thermotropic Behavior of DPPC Lipid Bilayer and on DPPC Liposomes

Functionalized manoyl oxide derivatives have been proved over the years to evoke several biological responses. Among them, 3β-hydroxy-manoyl oxide (1) and 3β-acetoxy-manoyl oxide (2) have been shown to exhibit in vitro antimicrobial and cytotoxic activity, while N-imidazole-3 β-thiocarbonyl ester of manoyl oxide (3) was found to exhibit potent cytotoxic effect. Their partitioning into phospholipid bilayers may lead to membrane structure modifications that are crucial in liposome development as they may influence their maintenance and integrity. DSC was used to study the modifications induced in DPPC bilayers by incorporating increasing concentrations of the three manoyl oxide derivatives. All derivatives were found to strongly affect the bilayer structural organization in terms of a decrease of the cooperativity, the fluidization and partially destabilization of the gel phase and the induction of a lateral phase separation in clustering domains. Derivatives 1 and 3 were incorporated into DPPC liposomes and their physicochemical stability was monitored at 4°C. The stability of liposomes was strongly influenced by the presence of 1 and 3 at any molar ratio studied. DPPC/1 liposomes were found to retain its stability for 48 h at low concentration of 10% mol, while at higher concentrations up to 30% mol they collapsed into aggregated material. In all cases DPPC/3 liposomes were found unstable and sticky aggregated structures precipitated from the bulk suspension.     

Interaction of the local anaesthetic heptacaine with dipalmitoylphosphatidylcholine liposomes: a densimetric study.

Interaction of the local anaesthetic heptacaine, monohydrochloride of [2-(heptyloxy)-phenyl]-2-(1-piperidinyl)-ethyl ester of carbamic acid, with multilamellar dipalmitoylphosphatidylcholine (DPPC) liposomes in aqueous solution with high excess of water has been studied by means of density measurements in the scanning regime in the main phase transition region. The anaesthetic decreased the temperature of main phase transition. The molar partition coefficients of heptacaine between aqueous phase, liquid crystal and gel phases of DPPC have been determined from a combination of phase transition data obtained by densimetry with a DPPC/heptacaine phase diagram published in the literature. The saturation of heptacaine concentration in liposomes has been observed at higher total amount of anaesthetic. The partial specific volume of heptacaine located in DPPC bilayers is slightly lower than in the aqueous phase.     

Dependence of the surface expression of the glycolipid cerebroside sulfate on its lipid environment: comparison of sphingomyelin and phosphatidylcholine

The influence of the membrane lipid environment on the reactivity with antibody of the acidic glycolipid cerebroside sulfate (CBS) was examined by using a spin membrane immunoassay. Fewer antibodies in a polyclonal anti-CBS antiserum recognized the antigen in a bovine brain sphingomyelin/cholesterol (SM/CHOL) environment than in dipalmitoylphosphatidylcholine/cholesterol (DPPC/CHOL). Changes in the CBS ceramide group appeared to have less influence on antibody recognition of CBS in SM/CHOL than in DPPC/CHOL [Crook et al. (1986) Biochemistry 25, 7488-7494]. Although the fatty acid chain length of phosphatidylcholine strongly influences CBS recognition, the fatty acid chain length of sphingomyelin had only a moderate effect on CBS recognition and did not account for the decreased recognition in SM compared to in DPPC. Inhibition studies revealed that the antibodies which recognize CBS in SM/CHOL (S antibodies) form a population distinct from those which recognize CBS in DPPC/CHOL (P antibodies). The specificity of the P and S antibodies was examined further by comparing the efficacy of various substances, which share chemical features with the components of CBS in a SM/CHOL or DPPC/CHOL environment, to inhibit lysis of liposomes containing CBS. Intact CBS, cholesterol, and a phosphocholine lipid, at certain antigen densities, were required for optimal recognition of the antigen, especially by the P antibodies, suggesting that a complex of all three lipids in a multivalent array may be recognized by these antibodies. The S antibodies may recognize a smaller complex or monomers of CBS.(ABSTRACT TRUNCATED AT 250 WORDS)     

The phase transition behavior of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) model membrane influenced by 2,4-dichlorophenol--an FT-Raman spectroscopy study

The effect of 2,4-dichlorophenol (DCP) on the structures and phase transitions of fully hydrated 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) liposomes was studied using FT-Raman spectroscopy. Whereas the Raman frequency shifts of the most frequently investigated bands of C-C and C-H stretching regions only indicate the main phase transition (P(beta')-L(alpha)) of the pure DPPC/water system, the Raman shift of C-H scissoring vibration at 1440 cm(-1) was found to be able to reveal the pretransition (L(beta')-P(beta')) as well. Analyzing the spectral parameters of the trans band at 1128 cm(-1), which does not overlap with DCP vibrational modes, a continuous decrease of trans conformations was found with increasing DCP concentration at 26 degrees C accompanying the phase transitions L(beta')-P(beta') and P(beta')-L(alpha). The intensity ratio of the symmetrical and asymmetrical methylene stretching bands (at 2850 cm(-1) and 2880 cm(-1)), defined as the disorder parameter by Levin [Levin, I.W., 1985. Two types of hydrocarbon chain interdigitation in sphingomielin bilayers. Biochemistry 24, 6282-6286], indicated that in the interdigitated phase (L(I)) the order is markedly high and comparable with that of L(beta). Both the phase transition P(beta')-L(alpha) in the DCP/DPPC molar ratio range of 10/100-50/100 and the phase transition L(I)-L(alpha) led to a significant increase of disordered chains and the presence of DCP molecules induced a more disordered chain region than that observed in the L(alpha) phase of DPPC. Nevertheless, it was found that the L(alpha) phase with DCP contains approximately the same amount of trans conformers than that without DCP.