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A comparison of the behavior of vitamin K1 and K2 monolayers at the air–water interface
Colloids and Surfaces A: Physicochemical and Engineering Aspects 138 (1998) 91–95
A comparison of the behavior of vitamin K and K monolayers 1 2 at the air–water interface Alexa Barnoski Serfis *, Rebecca Katzenberger Department of Chemistry, Saint Louis University, 221 N. Grand Blvd., St. Louis, MO 63103, USA Received 14 July 1997; received in revised form 28 October 1997; accepted 30 October 1997
Keywords: Monolayers; Vitamin K; Surface potential
1. Introduction In an effort to understand the functions of surfactants in naturally occurring systems, many research efforts have involved the study of biological surfactant molecules as insoluble monolayers at the air–water interface. In particular, lipid soluble vitamins including A, E, and K have been 1 studied as monolayers on water [1]. The pressure–area isotherms of vitamin K and 6-O-stearo1 ylascorbic acid mixed monolayers have also been reported [2]. Although the surface activity of vitamin K has been reported, the properties of 1 monolayers of vitamin K have not. 2 Vitamins K (phylloquinone) and K (menaqui1 2 none 4) are co-enzymes involved in the production of blood coagulation factors, and are important for normal functioning of the blood clotting cascade. Vitamin K dependent carboxylases are also integral membrane proteins [3]. Vitamin K and 1 vitamin K also function as electron acceptors in 2 chloroplast and bacterial photosynthesis [4]. While their biological importance is well known, the mechanisms of interaction in each process are not completely understood. Studies of their behavior
* Corresponding author. E-mail: [email protected] 0927-7757/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0 9 2 7 -7 7 5 7 ( 9 7 ) 0 0 36 7 - 1
as pure and mixed monolayers on water may eventually lead to a better understanding of their functions in such systems. The structures of the K vitamins studied are shown in Fig. 1. The K vitamins are 2methyl-1,4-naphthoquinones which contain a hydrophilic naphthoquinone head group and a hydrophobic carbon chain tail. The primary difference between the structures of these two vitamins exists in the structure of the carbon chain. Vitamin K contains a number of double bonds 2 along its chain, while vitamin K contains only one. 1 The K vitamins are surfactant materials and form stable liquid phase monolayers at the air–water interface. Studies of the surfactant nature of vitamin K have already been reported. There have 1 been studies of surface pressure and surface potential [1,5], and we have previously reported the nonlinear optical properties [6,7] of vitamin K 1 monolayers at the air–water interface. We have recently discovered that vitamin K also exhibits 2 some interesting behavior as a monolayer on water, and report surface pressure and surface potential measurements on this vitamin monolayer for the first time. The vitamin K monolayer exhibits 2 surface activity different from its K counterpart 1 due primarily to the nature of its hydrocarbon tail.
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Fig. 1. Structures of vitamins K and K . 1 2
2. Materials and methods The K vitamins were purchased from ICN Pharmaceuticals and used as received. The purity of each sample was given by the manufacturer as >99% for vitamin K , and >98% for vitamin 1 K . The purity was confirmed by thin layer chro2 matography. Vitamin K is a viscous yellow oil 1 with a melting point of −20°C. Vitamin K is a 2 yellow powder and melts at 42°C. Surfactant solutions in chloroform were spread dropwise onto an acidic water subphase. The subphase water was distilled once and passed through a Barnstead ion exchange system which yielded surfactant free water having a specific resistivity of 18 MV cm. Concentrated hydrochloric acid was added to the subphase to give a pH of 2.00 (0.01 N ). The pH of the subphase does not affect the behavior of the monolayer on the water, but the acidic subphase was used so that a direct comparison to previously published results on vitamin K could be made. 1 The subphase was contained in a NIMA model 611D Langmuir–Blodgett trough, and all measurements were performed at 20°C. In separate studies, 48 ml of a 1.292 mg/ml solution of vitamin K and 1 92 ml of a 0.959 mg/ml solution of vitamin K were 2 spread at the air–water interface to form a monolayer. The solvent was allowed 15 min to evaporate before the films were compressed. The monolayers ˚ 2/ were then compressed at a rate of 12 A molecule/min. Both vitamin K and K mono1 2
layers were found to be stable. Repeated compression and expansion of each monolayer yielded reproducible isotherms. Surface potential measurements were performed with a home-made apparatus which employed an ionizing air-gap electrode. It measured the difference in Volta potential between the bare water surface and a monolayer covered surface during compression and expansion cycles. The surface potential is given as DV=4pnm cos H
(1)
where DV is the difference in Volta potential between the bare water surface and a surfactant covered one, n is the number of molecules, m is the surface dipole moment, and H is the angle of inclination of the dipoles to the surface normal. Generally, the surface potential increases as a monolayer is compressed, because the molecules adapt a more vertical configuration. The contribution to the surface potential due to rearrangement of subphase water molecules is possibly significant, but unknown. Typically, the surface dipole moment is divided into components, with the vertical component expressed as m=ADV/12p
(2)
where A is the area occupied per molecule on the water surface. The vertical component of the surface dipole moment is commonly plotted against molecular area to show how it changes during
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compression. The value may decrease during compression due to the small surface areas the molecules are forced to occupy.
3. Results The pressure–area isotherms for both vitamin K and K monolayers are shown in Fig. 2. The 1 2 isotherm for a vitamin K monolayer has been 1 reported before [1], and is in good agreement with our results. It forms a stable liquid phase monolayer on water, and exhibits an increase in film ˚ 2/molecule. The pressure at approximately 59 A monolayer collapses near 11 dynes/cm at an area ˚ 2/molecule. The isotherm for the vitamin of 49 A K monolayer indicates a first film pressure at 2 ˚ 2/ nearly the same molecular area of 59 A molecule, similar to the vitamin K monolayer. 1 However, the monolayer collapse is observed at a much smaller film pressure, near 7 dynes/cm. The surface potential plots for both monolayers are shown in Fig. 3. Upon spreading of the vitamin K film, the surface potential increases above that 1 of the bare water by 160 mV. Upon compression of the film, the potential steadily increases, reaching 200 mV upon collapse. This agrees well with
Fig. 3. Surface potential–area and surface dipole–area isotherms of vitamin K monolayers (——— vitamin K ; - - - vita1 min K ). 2
Fig. 2. Pressure–area isotherms of vitamin K monolayers at the air–water interface (——— vitamin K ; - - - vitamin K ). 1 2
the data already published for this molecule [1]. The vitamin K monolayer, in contrast, exhibits 2 an increase of 275 mV upon spreading on the water surface. The surface potential increases through compression, reaching nearly 320 mV near collapse. The surface dipole changes (in milliDebyes) for each of the monolayers are also shown in Fig. 3. The vitamin K monolayer exhibits a surface 1 dipole of 300 mD upon spreading, then decreases upon film compression, reaching 230 mD upon monolayer collapse. The vitamin K monolayer 2 shows a value of 520 mD upon spreading, then
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decreases with film compression, to a value of 400 mD at collapse.
4. Discussion Our previous optical studies on vitamin K 1 monolayers [6,7] focused on the behavior of the naphthoquinone head group during compression and expansion cycles. It was found that there was a sharp reorientation of the head group upon compression, resulting in a more vertical orientation of head groups. This reorientation occurred at the first observed film pressure increase. Since both vitamins K and K have the same naph1 2 thoquinone head group, it is expected that differences in the macroscopic film properties can be attributed to the additional double bonds in the hydrocarbon chain on vitamin K . We can attri2 bute the difference in surface potential and surface pressure results reported in this paper solely to differences in hydrocarbon chain structure of the surfactants. In comparing the monolayer properties of these two surfactants with equivalent head groups, we expect the vitamin K head group to 2 behave similarly during compression. The changes in surface pressure and potential during compression are affected primarily by the nature of the tail groups. Although the existence of double bonds in a surfactant hydrocarbon chain may lead to a more expanded film if compared to a similarly saturated compound, the existence of trans double bonds has much less of an impact on the surface properties than cis double bonds. All of the double bonds in the vitamin K chain are trans. Upon film 2 compression, the chains must rearrange above the water surface and pack together as the molecules are pushed together. It is easier for the molecules to pack if the chain is totally saturated, but the packing is less dense for the saturated compound due to the restricted rotation about the additional double bonds. The packing cannot be as tight as the saturated chain, and this results in a collapse pressure which is much lower than the vitamin K monolayer. The absolute minimum area that 1 either molecule can pack into is found at the point ˚ 2/molecule. If the area is of collapse near 49 A
extrapolated to zero pressure, the area is near ˚ 2/molecule. These isotherms indicate that the 59 A packing of chains above the water surface is different for the two monolayers, as indicated by the different collapse pressures. However, the head groups seem to pack similarly below the water surface. The surface potential measurements and the dipole moment plots indicate that there is more vertical alignment of the vitamin K (compared to 2 vitamin K ) chains upon spreading. The trans 1 double bonds introduce an amount of rigidity into the chains which forces them to be more vertical on the water surface upon spreading. It is expected that the vitamin K chains can rotate more freely, 1 and become more entangled with one another, even upon spreading of the film. The more vertical alignment of the vitamin K chains leads to a 2 higher surface potential due to the smaller angle the chain makes with the surface normal. Therefore, this leads to a greater value for the surface dipole moment. Through film compression, the chains in both monolayers are pushed together and forced to take on more vertical alignment. In both cases, the surface potential increases through film compression as the chains take on a more vertical orientation. It is interesting to note that the overall change in surface potential is nearly 40 mV for both monolayers. This suggests that, although the molecular packing is different for the two monolayers above the water surface, the extent of reorientation during compression is similar. It has been reported that unsaturation introduced into the middle of a chain leads to a reduction in surface potential, particularly if there is a cis double bond [8]. A double bond may also have an affinity for the water surface, which leads to a folding of the chain such that more of the chain lies on the water surface [9]. However, if the double bond is trans, and there are a number of these along the chain, the chain does not fall onto the water surface. Instead, the chain is more rigid, imparting more vertical alignment to the molecules at the surface. Hence, the vitamin K monolayer 2 exhibits higher surface potential upon spreading and a lower collapse pressure compared to the vitamin K monolayer due to the presence of trans 1 double bonds along the hydrocarbon chain.
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5. Summary
References
The monolayer properties for the two K vitamins studied indicate differences in collapse pressures and surface potential changes. Both vitamins contain the same naphthoquinone head group, and differences in macroscopic film properties are attributed to differing chain configurations.
[1] G. Weitzel, A.M. Fretzdorff, S. Heller, Hoppe-Seylers Z. Physiol. Chem. 303 (1956) 14. [2] G. Capuzzi, P. Lo Nostro, K. Kulkarni, J. Fernandez, F.F. Vincieri, Langmuir 12 (1996) 5413. [3] D.A. Bender, Nutritional Biochemistry of the Vitamins, Cambridge University Press, New York, 1992. [4] D. Voet, J. Voet, Biochemistry, Wiley, New York, 1990. [5] G.L. Gaines Jr., J. Colloid Interface Sci. 28 (1968) 334. [6 ] A.A. Barnoski, G.S. Frysinger, G.L. Gaines, Jr., G.M. Korenowski, Colloids Surf. A 88 (1994) 123. [7] A.A. Barnoski, G.L. Gaines, Jr., G.M. Korenowski, Colloids Surf. A 94 (1995) 59. [8] G.L. Gaines, Jr., Insoluble Monolayers at Liquid–gas Interfaces, Wiley Interscience, New York, 1966. [9] A.H. Hughes, J. Chem. Soc. (1933) 338.
Acknowledgment The authors thank Dr. Gerald Korenowski from Rensselaer Polytechnic Institute for the use of the surface potential probe. This research was funded by the Beaumont Faculty Development Fund of Saint Louis University.
A comparison of the behavior of vitamin K and K monolayers 1 2 at the air–water interface Alexa Barnoski Serfis *, Rebecca Katzenberger Department of Chemistry, Saint Louis University, 221 N. Grand Blvd., St. Louis, MO 63103, USA Received 14 July 1997; received in revised form 28 October 1997; accepted 30 October 1997
Keywords: Monolayers; Vitamin K; Surface potential
1. Introduction In an effort to understand the functions of surfactants in naturally occurring systems, many research efforts have involved the study of biological surfactant molecules as insoluble monolayers at the air–water interface. In particular, lipid soluble vitamins including A, E, and K have been 1 studied as monolayers on water [1]. The pressure–area isotherms of vitamin K and 6-O-stearo1 ylascorbic acid mixed monolayers have also been reported [2]. Although the surface activity of vitamin K has been reported, the properties of 1 monolayers of vitamin K have not. 2 Vitamins K (phylloquinone) and K (menaqui1 2 none 4) are co-enzymes involved in the production of blood coagulation factors, and are important for normal functioning of the blood clotting cascade. Vitamin K dependent carboxylases are also integral membrane proteins [3]. Vitamin K and 1 vitamin K also function as electron acceptors in 2 chloroplast and bacterial photosynthesis [4]. While their biological importance is well known, the mechanisms of interaction in each process are not completely understood. Studies of their behavior
* Corresponding author. E-mail: [email protected] 0927-7757/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0 9 2 7 -7 7 5 7 ( 9 7 ) 0 0 36 7 - 1
as pure and mixed monolayers on water may eventually lead to a better understanding of their functions in such systems. The structures of the K vitamins studied are shown in Fig. 1. The K vitamins are 2methyl-1,4-naphthoquinones which contain a hydrophilic naphthoquinone head group and a hydrophobic carbon chain tail. The primary difference between the structures of these two vitamins exists in the structure of the carbon chain. Vitamin K contains a number of double bonds 2 along its chain, while vitamin K contains only one. 1 The K vitamins are surfactant materials and form stable liquid phase monolayers at the air–water interface. Studies of the surfactant nature of vitamin K have already been reported. There have 1 been studies of surface pressure and surface potential [1,5], and we have previously reported the nonlinear optical properties [6,7] of vitamin K 1 monolayers at the air–water interface. We have recently discovered that vitamin K also exhibits 2 some interesting behavior as a monolayer on water, and report surface pressure and surface potential measurements on this vitamin monolayer for the first time. The vitamin K monolayer exhibits 2 surface activity different from its K counterpart 1 due primarily to the nature of its hydrocarbon tail.
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A.B. Serfis, R. Katzenberger / Colloids Surfaces A: Physicochem. Eng. Aspects 138 (1998) 91–95
Fig. 1. Structures of vitamins K and K . 1 2
2. Materials and methods The K vitamins were purchased from ICN Pharmaceuticals and used as received. The purity of each sample was given by the manufacturer as >99% for vitamin K , and >98% for vitamin 1 K . The purity was confirmed by thin layer chro2 matography. Vitamin K is a viscous yellow oil 1 with a melting point of −20°C. Vitamin K is a 2 yellow powder and melts at 42°C. Surfactant solutions in chloroform were spread dropwise onto an acidic water subphase. The subphase water was distilled once and passed through a Barnstead ion exchange system which yielded surfactant free water having a specific resistivity of 18 MV cm. Concentrated hydrochloric acid was added to the subphase to give a pH of 2.00 (0.01 N ). The pH of the subphase does not affect the behavior of the monolayer on the water, but the acidic subphase was used so that a direct comparison to previously published results on vitamin K could be made. 1 The subphase was contained in a NIMA model 611D Langmuir–Blodgett trough, and all measurements were performed at 20°C. In separate studies, 48 ml of a 1.292 mg/ml solution of vitamin K and 1 92 ml of a 0.959 mg/ml solution of vitamin K were 2 spread at the air–water interface to form a monolayer. The solvent was allowed 15 min to evaporate before the films were compressed. The monolayers ˚ 2/ were then compressed at a rate of 12 A molecule/min. Both vitamin K and K mono1 2
layers were found to be stable. Repeated compression and expansion of each monolayer yielded reproducible isotherms. Surface potential measurements were performed with a home-made apparatus which employed an ionizing air-gap electrode. It measured the difference in Volta potential between the bare water surface and a monolayer covered surface during compression and expansion cycles. The surface potential is given as DV=4pnm cos H
(1)
where DV is the difference in Volta potential between the bare water surface and a surfactant covered one, n is the number of molecules, m is the surface dipole moment, and H is the angle of inclination of the dipoles to the surface normal. Generally, the surface potential increases as a monolayer is compressed, because the molecules adapt a more vertical configuration. The contribution to the surface potential due to rearrangement of subphase water molecules is possibly significant, but unknown. Typically, the surface dipole moment is divided into components, with the vertical component expressed as m=ADV/12p
(2)
where A is the area occupied per molecule on the water surface. The vertical component of the surface dipole moment is commonly plotted against molecular area to show how it changes during
A.B. Serfis, R. Katzenberger / Colloids Surfaces A: Physicochem. Eng. Aspects 138 (1998) 91–95
93
compression. The value may decrease during compression due to the small surface areas the molecules are forced to occupy.
3. Results The pressure–area isotherms for both vitamin K and K monolayers are shown in Fig. 2. The 1 2 isotherm for a vitamin K monolayer has been 1 reported before [1], and is in good agreement with our results. It forms a stable liquid phase monolayer on water, and exhibits an increase in film ˚ 2/molecule. The pressure at approximately 59 A monolayer collapses near 11 dynes/cm at an area ˚ 2/molecule. The isotherm for the vitamin of 49 A K monolayer indicates a first film pressure at 2 ˚ 2/ nearly the same molecular area of 59 A molecule, similar to the vitamin K monolayer. 1 However, the monolayer collapse is observed at a much smaller film pressure, near 7 dynes/cm. The surface potential plots for both monolayers are shown in Fig. 3. Upon spreading of the vitamin K film, the surface potential increases above that 1 of the bare water by 160 mV. Upon compression of the film, the potential steadily increases, reaching 200 mV upon collapse. This agrees well with
Fig. 3. Surface potential–area and surface dipole–area isotherms of vitamin K monolayers (——— vitamin K ; - - - vita1 min K ). 2
Fig. 2. Pressure–area isotherms of vitamin K monolayers at the air–water interface (——— vitamin K ; - - - vitamin K ). 1 2
the data already published for this molecule [1]. The vitamin K monolayer, in contrast, exhibits 2 an increase of 275 mV upon spreading on the water surface. The surface potential increases through compression, reaching nearly 320 mV near collapse. The surface dipole changes (in milliDebyes) for each of the monolayers are also shown in Fig. 3. The vitamin K monolayer exhibits a surface 1 dipole of 300 mD upon spreading, then decreases upon film compression, reaching 230 mD upon monolayer collapse. The vitamin K monolayer 2 shows a value of 520 mD upon spreading, then
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decreases with film compression, to a value of 400 mD at collapse.
4. Discussion Our previous optical studies on vitamin K 1 monolayers [6,7] focused on the behavior of the naphthoquinone head group during compression and expansion cycles. It was found that there was a sharp reorientation of the head group upon compression, resulting in a more vertical orientation of head groups. This reorientation occurred at the first observed film pressure increase. Since both vitamins K and K have the same naph1 2 thoquinone head group, it is expected that differences in the macroscopic film properties can be attributed to the additional double bonds in the hydrocarbon chain on vitamin K . We can attri2 bute the difference in surface potential and surface pressure results reported in this paper solely to differences in hydrocarbon chain structure of the surfactants. In comparing the monolayer properties of these two surfactants with equivalent head groups, we expect the vitamin K head group to 2 behave similarly during compression. The changes in surface pressure and potential during compression are affected primarily by the nature of the tail groups. Although the existence of double bonds in a surfactant hydrocarbon chain may lead to a more expanded film if compared to a similarly saturated compound, the existence of trans double bonds has much less of an impact on the surface properties than cis double bonds. All of the double bonds in the vitamin K chain are trans. Upon film 2 compression, the chains must rearrange above the water surface and pack together as the molecules are pushed together. It is easier for the molecules to pack if the chain is totally saturated, but the packing is less dense for the saturated compound due to the restricted rotation about the additional double bonds. The packing cannot be as tight as the saturated chain, and this results in a collapse pressure which is much lower than the vitamin K monolayer. The absolute minimum area that 1 either molecule can pack into is found at the point ˚ 2/molecule. If the area is of collapse near 49 A
extrapolated to zero pressure, the area is near ˚ 2/molecule. These isotherms indicate that the 59 A packing of chains above the water surface is different for the two monolayers, as indicated by the different collapse pressures. However, the head groups seem to pack similarly below the water surface. The surface potential measurements and the dipole moment plots indicate that there is more vertical alignment of the vitamin K (compared to 2 vitamin K ) chains upon spreading. The trans 1 double bonds introduce an amount of rigidity into the chains which forces them to be more vertical on the water surface upon spreading. It is expected that the vitamin K chains can rotate more freely, 1 and become more entangled with one another, even upon spreading of the film. The more vertical alignment of the vitamin K chains leads to a 2 higher surface potential due to the smaller angle the chain makes with the surface normal. Therefore, this leads to a greater value for the surface dipole moment. Through film compression, the chains in both monolayers are pushed together and forced to take on more vertical alignment. In both cases, the surface potential increases through film compression as the chains take on a more vertical orientation. It is interesting to note that the overall change in surface potential is nearly 40 mV for both monolayers. This suggests that, although the molecular packing is different for the two monolayers above the water surface, the extent of reorientation during compression is similar. It has been reported that unsaturation introduced into the middle of a chain leads to a reduction in surface potential, particularly if there is a cis double bond [8]. A double bond may also have an affinity for the water surface, which leads to a folding of the chain such that more of the chain lies on the water surface [9]. However, if the double bond is trans, and there are a number of these along the chain, the chain does not fall onto the water surface. Instead, the chain is more rigid, imparting more vertical alignment to the molecules at the surface. Hence, the vitamin K monolayer 2 exhibits higher surface potential upon spreading and a lower collapse pressure compared to the vitamin K monolayer due to the presence of trans 1 double bonds along the hydrocarbon chain.
A.B. Serfis, R. Katzenberger / Colloids Surfaces A: Physicochem. Eng. Aspects 138 (1998) 91–95
95
5. Summary
References
The monolayer properties for the two K vitamins studied indicate differences in collapse pressures and surface potential changes. Both vitamins contain the same naphthoquinone head group, and differences in macroscopic film properties are attributed to differing chain configurations.
[1] G. Weitzel, A.M. Fretzdorff, S. Heller, Hoppe-Seylers Z. Physiol. Chem. 303 (1956) 14. [2] G. Capuzzi, P. Lo Nostro, K. Kulkarni, J. Fernandez, F.F. Vincieri, Langmuir 12 (1996) 5413. [3] D.A. Bender, Nutritional Biochemistry of the Vitamins, Cambridge University Press, New York, 1992. [4] D. Voet, J. Voet, Biochemistry, Wiley, New York, 1990. [5] G.L. Gaines Jr., J. Colloid Interface Sci. 28 (1968) 334. [6 ] A.A. Barnoski, G.S. Frysinger, G.L. Gaines, Jr., G.M. Korenowski, Colloids Surf. A 88 (1994) 123. [7] A.A. Barnoski, G.L. Gaines, Jr., G.M. Korenowski, Colloids Surf. A 94 (1995) 59. [8] G.L. Gaines, Jr., Insoluble Monolayers at Liquid–gas Interfaces, Wiley Interscience, New York, 1966. [9] A.H. Hughes, J. Chem. Soc. (1933) 338.
Acknowledgment The authors thank Dr. Gerald Korenowski from Rensselaer Polytechnic Institute for the use of the surface potential probe. This research was funded by the Beaumont Faculty Development Fund of Saint Louis University.