20 The Growth of Molecular Assemblies in Mild Surfactant Solutions 1
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Keishiro Shirahama , Koji Takashima , Noboru Takisawa , Keiichi Kameyama, and Toshio Takagi Downloaded by PENNSYLVANIA STATE UNIV on February 16, 2013 | http://pubs.acs.org Publication Date: June 5, 1986 | doi: 10.1021/bk-1986-0311.ch020
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Department of Chemistry, Faculty of Science and Engineering, Saga University, Saga 840, Japan Institute for Protein Research, Osaka University, Suita 565, Japan
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The size of molecular assembly of six synthetic dialkyl amphiphiles as determined by a quasi-elastic light scattering is varied in the presence of nonionic MEGA-n surfactants (N-D-gluco-N-methylalkanamideC = 7-9). At high MEGA-n concentration, the size is as small as a MEGA-n micelle itself suggesting that dialkyl amphiphile is solubilized in the nonionic micelle. At low concentration a dialkyl amphiphile vesicle keeps its size relatively constant and takes up MEGA-n molecules. In between, the molecular assembly size increases with MEGA-n concentration. It is only this concentration region where the size shows even more increase on dialytic removal of MEGA-n surfactant. These phenomena are closely related with the "detergent-removal method" often employed in phospholipid-mild surfactant systems. n
Phospholipids are a major component of living cell membranes. Physical and chemical properties of bilayer structure composed of phospholipids have been well studied, (ij.2) One of the intriguing properties of Phospholipids is that they form a closed structure hereafter referred to as vesicles. Vesicles have attracted much attention since they are considered to mimic bi ocelIs. Preparation methods of vesicles have been known and employed for various purposes: the ultrasonic irradiation method, and the injection method, for example. In addition to them. there has appeared a novel method, which consists of solubilizing phospholipids in surfactant solution. and subsequently removing the surfactant by 0097-6156/ 86/ 0311 -0270S06.00/ 0 © 1986 American Chemical Society
In Phenomena in Mixed Surfactant Systems; Scamehorn, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
Downloaded by PENNSYLVANIA STATE UNIV on February 16, 2013 | http://pubs.acs.org Publication Date: June 5, 1986 | doi: 10.1021/bk-1986-0311.ch020
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SHIRAHAMA ET AL.
Molecular Assemblies in Mild Surfactant Solutions
dialysis or gel chromatography at a temperature higher than the transition point, Τ of the phospholipid, thus called the "detergent-removal method"· where the surfactant such as octylglucoside and b i l e s a l t s , or "mild" surfactant has relatively low surface-activity for easy removal. It is shown that the new method creates a large homogeneous unilamellar vesicle (3z9>· It is also claimed that uptake of various substances including very labile biomaterials is feasible without drastic exposures to ultrasonication or organic solvents(iQ). A growth of molecular assembly was recognized with mixtures of phospholipid and surfactant even before d i a l y t i c removal. Gofli et. al (ilj.12) have shown that sonication of phospholipid suspension at T>T may be required before addition of surfactant in order to obtain the growth of molecular assembly. They also found that the growth is induced with not only mild surfactants but also "hard" surfactants such as SDS and Triton X-100. More recently, Ueno et a l . also reported that dodecylocta(ethylene oxide) would produce a large homogeneous vesicle(13>. There are several works that have studied more or less the relevant phenomena in a systematic manner• but l i t t l e is understood yet. So we thought that i t would be informative to change the chemical species that forms the vesicles to some other species. This corre sponds to changing "chemical variables" just as one changes physical variables such as temperature and pres sure. Thus, i t is expected that not only phospholipids but also dialkyl amphiphiles (Figure 1) can form vesicles with these favorable characteristics by the "detergentremoval method". So we chose synthetic dialkyl amphi philes which had been reported to form vesicles: dioctadecyldimethylammonium bromide and chloride (DODABr and CI)(14> didodecyldimethylammonium bromide (DDDABr) (15), N.N-di(dodecanoyloxyethyl)amide derivative (DDdeACl) (16>· and 1,3-didodecyl-2-oligoethyleneglycol glycerines with average number of ethyleneglycol, m=13 and 17 (DDGE )(1Z>. As mild surfactants, N-D-gluco-Nmethylalkanaraide(lfi) (Figure 1) were used, since three homologues with reasonable purity were commercially available. It may be possible to see the effect of hydrophile-hydrophobe balance of surfactant by using these compounds, i.e., "another chemical variable". Quasi-elastic light scattering was employed success f u l l y to estimate sizes of molecular assemblies in terms of hydrodynamic radius, R„. In the present paper, we concentrated on a "phase diagrammatic" study of molecular assembly size. EXEEBiyENIÔL tfâifiCiâlSx Dioctadecyldimethylammonium bromide (Eastman Kodak) and chloride (Tokyo Kasei)· didodecyldlmethyl-
In Phenomena in Mixed Surfactant Systems; Scamehorn, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
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PHENOMENA IN MIXED SURFACTANT SYSTEMS
Downloaded by PENNSYLVANIA STATE UNIV on February 16, 2013 | http://pubs.acs.org Publication Date: June 5, 1986 | doi: 10.1021/bk-1986-0311.ch020
ammonium b r o m i d e (Eastman K o d a k ) , and N , N - ( d i d o d e c a n o y l o x e t h y l ) a m i d e d e r i v a t i v e ( d o n a t e d f r o m Sogo Yakuko Co.) were r e c r y s t a l 1 i z e d f r o m d r i e d a c e t o n e . Didodecyloligoethyleneglycol glycerines (kind gifts from Prof. T. Kuwamura, Gunma U n i v . ) were u s e d as r e c e i v e d . MEGA-n s u r f a c t a n t s ( D o j i n Chem.) were r e c r y s t a l 1 i z e d f r o m dried acetone. The c r i t i c a l m i c e l l e c o n c e n t r a t i o n s (cmc) were determined from a b r e a k p o i n t on a 11 I / I r a t i o ( s e e below) v s . s u r f a c t a n t concentration plot. Purified p y r e n e was a d o n a t i o n f r o m Dr. K. N a k a j i m a ( N i s h i k y u s h u Univ.). ÔPPâCâiUS^ A q u a s i - e l a s t i c l i g h t s c a t t e r i n g measurement system was composed o f a l i g h t scattering photometer ( U n i o n G i k e n , LS-601) e q u i p p e d with a single-photon counting unit. L i g h t s o u r c e was a He-Ne l a s e r (5 mW). Autocorrelation function was d e r i v e d by a digital c o r r e l a t o r (KANOMAX, S A I - 4 3 A ) . A l l t h e measurements were carried o u t a t room t e m p e r a t u r e (22°C) and r e c o r d e d on a p l o t t e r t h r o u g h a m i c r o c o m p u t o r ( S o r d , M243). E m i s s i o n s p e c t r a were r e c o r d e d on a f l u o r e s c e n c e spectrophotometer ( H i t a c h i , MPF-2A) w i t h t h e e x c i t a t i o n w a v e l e n g t h = 337 nm. ECQCedUtÊS^ D i a l k y l compound s u s p e n s i o n (0.4-4.Omg/ml) was d i s p e r s e d i n water by an u l t r a s o n i c irradiator (Branson s o n i f i e r c e l l d i s r u p t o r 185) f o r an hour above t r a n s i t i o n temperature. The s u s p e n s i o n was t h e n f i l t e r e d through a N u c l e o p o r e membrane f i l t e r (5 urn) t o remove titanium d u s t o r i g i n a t i n g from t h e s o n i c a t o r t i p . The MEGA-n c o n c e n t r a t i o n was v a r i e d by one o f t h e f o l l o w i n g two methods: ( l ) a d d i t i o n o f s o l i d MEGA-n t o a v e s i c l e s u s p e n s i o n o r ( 2 ) c o m b i n i n g c o n c e n t r a t e d MEGA-n (usually 50 mg/ml) s o l u t i o n c o n t a i n i n g a d i a l k y l a m p h i p h i l e w i t h t h e v e s i c l e s u s p e n s i o n h a v i n g t h e same d i a l k y l a m p h i p h i l e c o n c e n t r a t i o n as above. The r e s u l t i n g m i x t u r e was t r e a t e d above Τ f o r more t h a n 10 m i n u t e s . Experimental r e s u l t s were t h e same i r r e s p e c t i v e o f t h e p r e p a r a t i o n methods. The s a m p l e s o l u t i o n t h u s p r e p a r e d was f i l t e r e d t h r o u g h a Nucleopore membrane f i l t e r w i t h an a p p r o p r i a t e p o r e s i z e (0.5-3 um) p r i o r t o q u a s i - e l a s t i c scattering measure ments .
Results
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