Aba A L E C Priev | The Hebrew University of Jerusalem (original) (raw)

Papers by Aba A L E C Priev

Journal of Controlled Release, 2000

The present study was conducted in order to probe the microstructure, microviscosity, and hydrati... more The present study was conducted in order to probe the microstructure, microviscosity, and hydration properties of matrices containing two model drugs, naproxen sodium (NS) and naproxen (N), and egg albumin (EA) as matrix carrier. The results suggested that N release from EA matrix was controlled by a bulk erosion mechanism in combination with additional processes (crystal dissolution/ crystallization rate) compared with NS matrix, which behaved as a non-erodible matrix and drug release occurred by diffusion through the gel. Using EPR technique it has been shown that incorporating NS into EA matrix strongly influences the microstructure of the protein gel, and hence the transport of the penetrant within the matrix, compared with matrices containing N. The presence of NS increased the protein chain mobility and hydration which supports our previous results showing that NS cause unfolding of EA. In contrast, N caused only marginal effect on EA chain mobility. The gel formed in EA/NS matrices was more porous compared with EA/N matrices as revealed by the lower rotational correlation time of PCA (lower microviscosity) in EA/ NS matrices compared with EA/N. However, EA/N gelled matrices were more heterogeneous, i.e., containing a higher number of components having different mobility. The T 1 and T 2 relaxation studies by NMR provided an additional support for the higher chain hydration in EA/NS matrices compared with EA/N as indicated by the higher relaxation rates in the gelled matrices. Internal pH measurements by EPR revealed that the micro-pH inside 100% EA and 50/50 EA/N matrices were lower than 50/50 EA/NS matrices and in all cases lower than the penetrating buffer pH. The lower pH compartment formed in N matrices affected N solubility and crystal dissolution rate, which can explain its lower release rate compared with EA, from the same formulation. The EPR and NMR data supports our findings that NS caused unfolding of the protein, affected matrix structure, and converted it to a hydrophobic non-erodible matrix compared with EA/N matrix in which the native properties of EA were mainly retained.

Targeting of drugs 6: …, 1998

... Thermodynamic properties of solutes, such as heat capacity, partial volume, and compressibili... more ... Thermodynamic properties of solutes, such as heat capacity, partial volume, and compressibility, are known ... hydration and have been widely used to estimate the hydration of proteins (Almagor et ... al., 1989, Buckin et al., 1994, Chalikian et al., 1994), and liposomes (Barenholz et ...

Biochimica Et Biophysica Acta-protein Structure and Molecular Enzymology, 1998

Ž. Ž. The partial specific volume V and adiabatic compressibility b of myoglobin have been shown ... more Ž. Ž. The partial specific volume V and adiabatic compressibility b of myoglobin have been shown to be reduced by Ž Ž. . small cosolvents such as glycerol A.

Langmuir, 2009

Phospholipids (PL) form the matrix of biological membranes and of the lipoprotein envelope monola... more Phospholipids (PL) form the matrix of biological membranes and of the lipoprotein envelope monolayer, and are responsible for many of the unique physicochemical, biochemical, and biological properties of these supermolecular bioassemblies. It was suggested that phospholipids present in the synovial fluid (SF) and on the surface of articular cartilage have major involvement in the low friction of cartilage, which is essential for proper mobility of synovial joints. In pathologies, such as impaired biolubrication (leading to common joint disorders such as osteoarthritis), the level of phospholipids in the SF is reduced. Using a human-sourced cartilage-on-cartilage setup, we studied to what extent and how phospholipids act as highly effective cartilage biolubricants. We found that large multilamellar vesicles (MLV), >800 nm in diameter, composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) or of a mixture of DMPC and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) are superior lubricants in comparison to MLV composed of other phosphatidylcholines. Introducing cholesterol into liposomes resulted in less effective lubricants. DMPC-MLV was also superior to small unilamellar vesicles (SUV), <100 nm in diameter, composed of DMPC. MLV are superior to SUV due to MLV retention at and near (<200 microm below) the cartilage surface, while SUV penetrate deeper into the cartilage (450-730 microm). Superiority of specific PL compositions is explained by the thermotropic behavior (including compressibility) of the lipid bilayer. Correlating physicochemical properties of the MLV with the friction results suggests that MLV having lipid bilayers in the liquid-disordered phase and having a solid-ordered to liquid-disordered phase transition temperature slightly below physiological temperature are optimal for lubrication. High phospholipid headgroup hydration, high compressibility, and softness are the common denominators of all efficient PL compositions. The high efficiency of DMPC-MLV and DMPC/DPPC-MLV as cartilage lubricants combined with their resistance to degradation at 37 degrees C supports further evaluation of these MLV for treatment of joint impairments related to poor lubrication. This work also demonstrates the relevance of basic physicochemical properties of phospholipids to their activities in biological systems.

Oyo Butsurigaku Kankei …, 2003

Ceramides mediate antiproliferative responses, and it has been proposed that increasing the level... more Ceramides mediate antiproliferative responses, and it has been proposed that increasing the level of ceramides in cancer cells may have a therapeutic antitumor effect. However, ceramides, because of their high ″packing parameter″ (PP), do not form lipid assemblies that can be dispersed in a form suitable for intravenous administration. We found that nanoliposomes containing short-or medium-chain ceramides are unstable because of their very high (>1.3) PP. To overcome this major obstacle, we included the lipopolymer 2k PEG-DSPE, which reduces the additive PP. The presence of PEG-DSPE allows the formation of highly stable (>1 year) ceramide (Cer)-containing nanoliposomes suitable for systemic administration. Using tumor cell lines, we found that the ceramide cytotoxicity was not impaired by their inclusion in nanoliposomes. The use of 14 C-labeled ceramides shows that the C 6 Cer, but not C 16 Cer, was transferred from the nanoliposomes to the cells and metabolized efficiently. The difference between the two ceramides is related to the large difference between their critical aggregation concentration and was correlated with the much higher cytotoxity of liposomal C 6 Cer. The activity of 2k PEG-DSPE as a steric stabilizer (as previously shown for Doxil) was also confirmed for C 6 Cer-containing nanoliposomes. The 2k PEG-DSPE lipopolymer significantly reduced the desorption rate of the ceramide from the liposome bilayer, thereby allowing liposomes containing C 6 Cer to reach the tumor site and to demonstrate therapeutic efficacy.

An innovative technology based on the use of ultrasonic cylindrical standing waves for continuous... more An innovative technology based on the use of ultrasonic cylindrical standing waves for continuous monitoring of quality of various liquid food products, such as milk, juice, beer, wine, and drinking water, is described. A proprietary unique feature of the developed ultrasonic analyzer is that it employs a combined mode of operation using both highintensity (5 W/cm 2 ) waves for separation and concentration of the large particles (fat globules, bacteria, or yeasts) and low-intensity (0.5 W/cm 2 ) waves for compositional analysis. High accuracy for sound velocity (> 0.001%) and sound absorption (~0.5%) measurements and rapid testing time (5-20s) have been achieved. Comparative analyses of the ultrasonic method with standard reference techniques have produced linear calibration curves for major components, with correlation coefficients higher than 0.95. It is thus possible to monitor total protein and fat content, and somatic cell count in raw milk in cowsheds, or salinity, turbidity, specific gravity, and particles (bacteria) in drinking water directly. Advantages of the proposed technology include its reagent-free nature, no need for sample pretreatment, ease-of-use, and low cost.

An innovative technology based on the use of ultrasonic cylindrical standing waves for continuous... more An innovative technology based on the use of ultrasonic cylindrical standing waves for continuous monitoring of quality of various liquid food products, such as milk, juice, beer, wine, and drinking water, is described. A proprietary unique feature of the developed ultrasonic analyzer is that it employs a combined mode of operation using both highintensity (5 W/cm 2 ) waves for separation and concentration of the large particles (fat globules, bacteria, or yeasts) and low-intensity (0.5 W/cm 2 ) waves for compositional analysis. High accuracy for sound velocity (> 0.001%) and sound absorption (~0.5%) measurements and rapid testing time (5-20s) have been achieved. Comparative analyses of the ultrasonic method with standard reference techniques have produced linear calibration curves for major components, with correlation coefficients higher than 0.95. It is thus possible to monitor total protein and fat content, and somatic cell count in raw milk in cowsheds, or salinity, turbidity, specific gravity, and particles (bacteria) in drinking water directly. Advantages of the proposed technology include its reagent-free nature, no need for sample pretreatment, ease-of-use, and low cost.

Chemistry and physics of lipids, Jan 1, 2005

The aim of this study was to elucidate the effect of various mole percentages (0-25 mol%) of 2000... more The aim of this study was to elucidate the effect of various mole percentages (0-25 mol%) of 2000 Da polyethylene glycol-disteroylphosphoethanolamine (PEG-DSPE) in the presence or absence of 40 mol% cholesterol and the effect of degree of saturation of phosphatidylcholine (PC) on the size and the lipid bilayer packing of large unilamellar vesicles (LUV). Egg PC (EPC, unsaturated) LUV and fully hydrogenated soy PC (HSPC, saturated) LUV partial specific volume, specific compressibility, size, and packing parameter (PP) of lipids were characterized by measurements of density, ultrasonic velocity, specific turbidity, and dynamic light scattering. Liposome size and specific turbidity decreased with increase in temperature and PEG-DSPE mol%, except at 7 ± 2 mol%. At this PEG-DSPE mol%, an anomalous peak in liposome size of 15 ± 5 nm was observed. We attribute this effect mainly to the change in the spatial structure of the PEG-DSPE molecule, depending on whether the grafted PEG is in the mushroom or brush configuration. In the mushroom regime, i.e., when the grafted PEG is up to 4 mol% in LUV, the PEG moiety did not affect the additive PP of the lipids in the bilayer, and the PP value of PEG-DSPE is 1.044; while in the brush regime, i.e., when the grafted PEG is higher than 4 mol%, the PP of PEG-DSPE decreases exponentially, reaching the value of 0.487 at 30 mol% of grafted lipopolymer. The specific compressibility and additive PP values for the mixture of matrix lipid (EPC or HSPC), cholesterol, and PEG-DSPE for all liposome compositions investigated reached their maximum at 7 ± 2 mol% PEG-DSPE, the concentration of PEG-DSPE at which the highest biological stability of the LUV is achieved.

Biochemistry, Jan 1, 1996

The addition of hydrogen-bonded cosolvents to aqueous solutions of proteins is known to modify bo... more The addition of hydrogen-bonded cosolvents to aqueous solutions of proteins is known to modify both thermodynamic and dynamic properties of the proteins in a variety of ways. Previous studies suggest that glycerol reduces the free volume and compressibility of proteins. However, there is no directly measured evidence for that. We have measured the apparent specific volume (V) and adiabatic compressibility (K) of a number of proteins, sugars, and amino acids in water and in 30% glycerol at pH 7.4 and 30°C. The values of V and K in water and their changes induced by glycerol were extrapolated to the limit of infinite solute size. The main results were the following: (a) glycerol decreases V and K of proteins, but increases it for amino acids; (b) the V and K values of the protein interior in water were found to be 0.784 ( 0.026 mL/g and (12.8 ( 2.5) × 10 -6 mL/g‚atm, where the glycerol reduces these values by 8 and 32%, respectively; (c) the coefficient of adiabatic compressibility of the structural component of proteins affected by the glycerol is estimated to be (50 ( 10) × 10 -6 atm -1 , which is comparable to that of water. We propose that the glycerol induces a release of the so-called "lubricant" water, which maintains conformational flexibility by keeping apart neighboring segments of the polypeptide chain. This is expected to lead to the collapsing of the voids containing this water as well as to increase intramolecular bonding, which explains the observed effect.

Biophysical journal, Jan 1, 1998

This study aimed to characterize the effect of polyethylene glycol of 2000 molecular weight (PEG ... more This study aimed to characterize the effect of polyethylene glycol of 2000 molecular weight (PEG 2000 ) attached to a dialkylphosphatidic acid (dihexadecylphosphatidyl (DHP)-PEG 2000 ) on the hydration and thermodynamic stability of lipid assemblies. Differential scanning calorimetry, densitometry, and ultrasound velocity and absorption measurements were used for thermodynamic and hydrational characterization. Using a differential scanning calorimetry technique we showed that each molecule of PEG 2000 binds 136 Ϯ 4 molecules of water. For PEG 2000 covalently attached to the lipid molecules organized in micelles, the water binding increases to 210 Ϯ 6 water molecules. This demonstrates that the two different structural configurations of the PEG 2000 , a random coil in the case of the free PEG and a brush in the case of DHP-PEG 2000 micelles, differ in their hydration level. Ultrasound absorption changes in liposomes reflect mainly the heterophase fluctuations and packing defects in the lipid bilayer. The PEG-induced excess ultrasound absorption of the lipid bilayer at 7.7 MHz for PEG-lipid concentrations over 5 mol % indicates the increase in the relaxation time of the headgroup rotation due to PEG-PEG interactions. The adiabatic compressibility (calculated from ultrasound velocity and density) of the lipid bilayer of the liposome increases monotonically with PEG-lipid concentration up to ϳ7 mol %, reflecting release of water from the lipid headgroup region. Elimination of this water, induced by grafted PEG, leads to a decrease in bilayer defects and enhanced lateral packing of the phospholipid acyl chains. We assume that the dehydration of the lipid headgroup region in conjunction with the increase of the hydration of the outer layer by grafting PEG in brush configuration are responsible for increasing thermodynamic stability of the liposomes at 5-7 mol % of PEG-lipid. At higher PEG-lipid concentrations, compressibility and partial volume of the lipid phase of the samples decrease. This reflects the increase in hydration of the lipid headgroup region (up to five additional water molecules per lipid molecule for 12 mol % PEG-lipid) and the weakening of the bilayer packing due to the lateral repulsion of PEG chains.

Journal of Controlled Release, 2000

The present study was conducted in order to probe the microstructure, microviscosity, and hydrati... more The present study was conducted in order to probe the microstructure, microviscosity, and hydration properties of matrices containing two model drugs, naproxen sodium (NS) and naproxen (N), and egg albumin (EA) as matrix carrier. The results suggested that N release from EA matrix was controlled by a bulk erosion mechanism in combination with additional processes (crystal dissolution/ crystallization rate) compared with NS matrix, which behaved as a non-erodible matrix and drug release occurred by diffusion through the gel. Using EPR technique it has been shown that incorporating NS into EA matrix strongly influences the microstructure of the protein gel, and hence the transport of the penetrant within the matrix, compared with matrices containing N. The presence of NS increased the protein chain mobility and hydration which supports our previous results showing that NS cause unfolding of EA. In contrast, N caused only marginal effect on EA chain mobility. The gel formed in EA/NS matrices was more porous compared with EA/N matrices as revealed by the lower rotational correlation time of PCA (lower microviscosity) in EA/ NS matrices compared with EA/N. However, EA/N gelled matrices were more heterogeneous, i.e., containing a higher number of components having different mobility. The T 1 and T 2 relaxation studies by NMR provided an additional support for the higher chain hydration in EA/NS matrices compared with EA/N as indicated by the higher relaxation rates in the gelled matrices. Internal pH measurements by EPR revealed that the micro-pH inside 100% EA and 50/50 EA/N matrices were lower than 50/50 EA/NS matrices and in all cases lower than the penetrating buffer pH. The lower pH compartment formed in N matrices affected N solubility and crystal dissolution rate, which can explain its lower release rate compared with EA, from the same formulation. The EPR and NMR data supports our findings that NS caused unfolding of the protein, affected matrix structure, and converted it to a hydrophobic non-erodible matrix compared with EA/N matrix in which the native properties of EA were mainly retained.

Targeting of drugs 6: …, 1998

... Thermodynamic properties of solutes, such as heat capacity, partial volume, and compressibili... more ... Thermodynamic properties of solutes, such as heat capacity, partial volume, and compressibility, are known ... hydration and have been widely used to estimate the hydration of proteins (Almagor et ... al., 1989, Buckin et al., 1994, Chalikian et al., 1994), and liposomes (Barenholz et ...

Biochimica Et Biophysica Acta-protein Structure and Molecular Enzymology, 1998

Ž. Ž. The partial specific volume V and adiabatic compressibility b of myoglobin have been shown ... more Ž. Ž. The partial specific volume V and adiabatic compressibility b of myoglobin have been shown to be reduced by Ž Ž. . small cosolvents such as glycerol A.

Langmuir, 2009

Phospholipids (PL) form the matrix of biological membranes and of the lipoprotein envelope monola... more Phospholipids (PL) form the matrix of biological membranes and of the lipoprotein envelope monolayer, and are responsible for many of the unique physicochemical, biochemical, and biological properties of these supermolecular bioassemblies. It was suggested that phospholipids present in the synovial fluid (SF) and on the surface of articular cartilage have major involvement in the low friction of cartilage, which is essential for proper mobility of synovial joints. In pathologies, such as impaired biolubrication (leading to common joint disorders such as osteoarthritis), the level of phospholipids in the SF is reduced. Using a human-sourced cartilage-on-cartilage setup, we studied to what extent and how phospholipids act as highly effective cartilage biolubricants. We found that large multilamellar vesicles (MLV), >800 nm in diameter, composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) or of a mixture of DMPC and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) are superior lubricants in comparison to MLV composed of other phosphatidylcholines. Introducing cholesterol into liposomes resulted in less effective lubricants. DMPC-MLV was also superior to small unilamellar vesicles (SUV), <100 nm in diameter, composed of DMPC. MLV are superior to SUV due to MLV retention at and near (<200 microm below) the cartilage surface, while SUV penetrate deeper into the cartilage (450-730 microm). Superiority of specific PL compositions is explained by the thermotropic behavior (including compressibility) of the lipid bilayer. Correlating physicochemical properties of the MLV with the friction results suggests that MLV having lipid bilayers in the liquid-disordered phase and having a solid-ordered to liquid-disordered phase transition temperature slightly below physiological temperature are optimal for lubrication. High phospholipid headgroup hydration, high compressibility, and softness are the common denominators of all efficient PL compositions. The high efficiency of DMPC-MLV and DMPC/DPPC-MLV as cartilage lubricants combined with their resistance to degradation at 37 degrees C supports further evaluation of these MLV for treatment of joint impairments related to poor lubrication. This work also demonstrates the relevance of basic physicochemical properties of phospholipids to their activities in biological systems.

Oyo Butsurigaku Kankei …, 2003

Ceramides mediate antiproliferative responses, and it has been proposed that increasing the level... more Ceramides mediate antiproliferative responses, and it has been proposed that increasing the level of ceramides in cancer cells may have a therapeutic antitumor effect. However, ceramides, because of their high ″packing parameter″ (PP), do not form lipid assemblies that can be dispersed in a form suitable for intravenous administration. We found that nanoliposomes containing short-or medium-chain ceramides are unstable because of their very high (>1.3) PP. To overcome this major obstacle, we included the lipopolymer 2k PEG-DSPE, which reduces the additive PP. The presence of PEG-DSPE allows the formation of highly stable (>1 year) ceramide (Cer)-containing nanoliposomes suitable for systemic administration. Using tumor cell lines, we found that the ceramide cytotoxicity was not impaired by their inclusion in nanoliposomes. The use of 14 C-labeled ceramides shows that the C 6 Cer, but not C 16 Cer, was transferred from the nanoliposomes to the cells and metabolized efficiently. The difference between the two ceramides is related to the large difference between their critical aggregation concentration and was correlated with the much higher cytotoxity of liposomal C 6 Cer. The activity of 2k PEG-DSPE as a steric stabilizer (as previously shown for Doxil) was also confirmed for C 6 Cer-containing nanoliposomes. The 2k PEG-DSPE lipopolymer significantly reduced the desorption rate of the ceramide from the liposome bilayer, thereby allowing liposomes containing C 6 Cer to reach the tumor site and to demonstrate therapeutic efficacy.

An innovative technology based on the use of ultrasonic cylindrical standing waves for continuous... more An innovative technology based on the use of ultrasonic cylindrical standing waves for continuous monitoring of quality of various liquid food products, such as milk, juice, beer, wine, and drinking water, is described. A proprietary unique feature of the developed ultrasonic analyzer is that it employs a combined mode of operation using both highintensity (5 W/cm 2 ) waves for separation and concentration of the large particles (fat globules, bacteria, or yeasts) and low-intensity (0.5 W/cm 2 ) waves for compositional analysis. High accuracy for sound velocity (> 0.001%) and sound absorption (~0.5%) measurements and rapid testing time (5-20s) have been achieved. Comparative analyses of the ultrasonic method with standard reference techniques have produced linear calibration curves for major components, with correlation coefficients higher than 0.95. It is thus possible to monitor total protein and fat content, and somatic cell count in raw milk in cowsheds, or salinity, turbidity, specific gravity, and particles (bacteria) in drinking water directly. Advantages of the proposed technology include its reagent-free nature, no need for sample pretreatment, ease-of-use, and low cost.

An innovative technology based on the use of ultrasonic cylindrical standing waves for continuous... more An innovative technology based on the use of ultrasonic cylindrical standing waves for continuous monitoring of quality of various liquid food products, such as milk, juice, beer, wine, and drinking water, is described. A proprietary unique feature of the developed ultrasonic analyzer is that it employs a combined mode of operation using both highintensity (5 W/cm 2 ) waves for separation and concentration of the large particles (fat globules, bacteria, or yeasts) and low-intensity (0.5 W/cm 2 ) waves for compositional analysis. High accuracy for sound velocity (> 0.001%) and sound absorption (~0.5%) measurements and rapid testing time (5-20s) have been achieved. Comparative analyses of the ultrasonic method with standard reference techniques have produced linear calibration curves for major components, with correlation coefficients higher than 0.95. It is thus possible to monitor total protein and fat content, and somatic cell count in raw milk in cowsheds, or salinity, turbidity, specific gravity, and particles (bacteria) in drinking water directly. Advantages of the proposed technology include its reagent-free nature, no need for sample pretreatment, ease-of-use, and low cost.

Chemistry and physics of lipids, Jan 1, 2005

The aim of this study was to elucidate the effect of various mole percentages (0-25 mol%) of 2000... more The aim of this study was to elucidate the effect of various mole percentages (0-25 mol%) of 2000 Da polyethylene glycol-disteroylphosphoethanolamine (PEG-DSPE) in the presence or absence of 40 mol% cholesterol and the effect of degree of saturation of phosphatidylcholine (PC) on the size and the lipid bilayer packing of large unilamellar vesicles (LUV). Egg PC (EPC, unsaturated) LUV and fully hydrogenated soy PC (HSPC, saturated) LUV partial specific volume, specific compressibility, size, and packing parameter (PP) of lipids were characterized by measurements of density, ultrasonic velocity, specific turbidity, and dynamic light scattering. Liposome size and specific turbidity decreased with increase in temperature and PEG-DSPE mol%, except at 7 ± 2 mol%. At this PEG-DSPE mol%, an anomalous peak in liposome size of 15 ± 5 nm was observed. We attribute this effect mainly to the change in the spatial structure of the PEG-DSPE molecule, depending on whether the grafted PEG is in the mushroom or brush configuration. In the mushroom regime, i.e., when the grafted PEG is up to 4 mol% in LUV, the PEG moiety did not affect the additive PP of the lipids in the bilayer, and the PP value of PEG-DSPE is 1.044; while in the brush regime, i.e., when the grafted PEG is higher than 4 mol%, the PP of PEG-DSPE decreases exponentially, reaching the value of 0.487 at 30 mol% of grafted lipopolymer. The specific compressibility and additive PP values for the mixture of matrix lipid (EPC or HSPC), cholesterol, and PEG-DSPE for all liposome compositions investigated reached their maximum at 7 ± 2 mol% PEG-DSPE, the concentration of PEG-DSPE at which the highest biological stability of the LUV is achieved.

Biochemistry, Jan 1, 1996

The addition of hydrogen-bonded cosolvents to aqueous solutions of proteins is known to modify bo... more The addition of hydrogen-bonded cosolvents to aqueous solutions of proteins is known to modify both thermodynamic and dynamic properties of the proteins in a variety of ways. Previous studies suggest that glycerol reduces the free volume and compressibility of proteins. However, there is no directly measured evidence for that. We have measured the apparent specific volume (V) and adiabatic compressibility (K) of a number of proteins, sugars, and amino acids in water and in 30% glycerol at pH 7.4 and 30°C. The values of V and K in water and their changes induced by glycerol were extrapolated to the limit of infinite solute size. The main results were the following: (a) glycerol decreases V and K of proteins, but increases it for amino acids; (b) the V and K values of the protein interior in water were found to be 0.784 ( 0.026 mL/g and (12.8 ( 2.5) × 10 -6 mL/g‚atm, where the glycerol reduces these values by 8 and 32%, respectively; (c) the coefficient of adiabatic compressibility of the structural component of proteins affected by the glycerol is estimated to be (50 ( 10) × 10 -6 atm -1 , which is comparable to that of water. We propose that the glycerol induces a release of the so-called "lubricant" water, which maintains conformational flexibility by keeping apart neighboring segments of the polypeptide chain. This is expected to lead to the collapsing of the voids containing this water as well as to increase intramolecular bonding, which explains the observed effect.

Biophysical journal, Jan 1, 1998

This study aimed to characterize the effect of polyethylene glycol of 2000 molecular weight (PEG ... more This study aimed to characterize the effect of polyethylene glycol of 2000 molecular weight (PEG 2000 ) attached to a dialkylphosphatidic acid (dihexadecylphosphatidyl (DHP)-PEG 2000 ) on the hydration and thermodynamic stability of lipid assemblies. Differential scanning calorimetry, densitometry, and ultrasound velocity and absorption measurements were used for thermodynamic and hydrational characterization. Using a differential scanning calorimetry technique we showed that each molecule of PEG 2000 binds 136 Ϯ 4 molecules of water. For PEG 2000 covalently attached to the lipid molecules organized in micelles, the water binding increases to 210 Ϯ 6 water molecules. This demonstrates that the two different structural configurations of the PEG 2000 , a random coil in the case of the free PEG and a brush in the case of DHP-PEG 2000 micelles, differ in their hydration level. Ultrasound absorption changes in liposomes reflect mainly the heterophase fluctuations and packing defects in the lipid bilayer. The PEG-induced excess ultrasound absorption of the lipid bilayer at 7.7 MHz for PEG-lipid concentrations over 5 mol % indicates the increase in the relaxation time of the headgroup rotation due to PEG-PEG interactions. The adiabatic compressibility (calculated from ultrasound velocity and density) of the lipid bilayer of the liposome increases monotonically with PEG-lipid concentration up to ϳ7 mol %, reflecting release of water from the lipid headgroup region. Elimination of this water, induced by grafted PEG, leads to a decrease in bilayer defects and enhanced lateral packing of the phospholipid acyl chains. We assume that the dehydration of the lipid headgroup region in conjunction with the increase of the hydration of the outer layer by grafting PEG in brush configuration are responsible for increasing thermodynamic stability of the liposomes at 5-7 mol % of PEG-lipid. At higher PEG-lipid concentrations, compressibility and partial volume of the lipid phase of the samples decrease. This reflects the increase in hydration of the lipid headgroup region (up to five additional water molecules per lipid molecule for 12 mol % PEG-lipid) and the weakening of the bilayer packing due to the lateral repulsion of PEG chains.