1200
Langmuir 1989,5 , 1200-1205
Chiral Discrimination in the Monolayer Packing of Hexadecylthiophospho-2-phenylglycinolwith Two Chiral Centers in the Polar Head Group Maya Dvolaitzky* and Marie-Alice Guedeau-Boudeville Laboratoire de Physique de la Matisre Condensde, College de France, 1 1 , Place M. Berthelot, 75231 Paris, Cedex 05, France Received February 3, 1989 The four possible stereomers of hexadecylthiophospho-2-phenylglycinol, a chiral surfactant with two asymmetric centers within the polar head group (a phosphorus and a carbon atom), have been synthesized, and their absolute confiiation has been determined by X-ray diffraction. Of the two pairs of diastereomers, R,,R, SS , and Rp,S, Spa, only the Rp,R, SS , pair exhibits a chiral discrimination when spread to the air-water interface; the racemic monolayer film (1:l R,,R + S,,S) undergoes a phase transition from a liquid-expanded to a liquid-condensedphase upon compression, while the pure enantiomersRp,R or Sp,S only have a liquid-expandedphase, as revealed by pressure-area isotherms. The transition pressure versus composition diagram seems to indicate that heterochiral interactions are favored. In addition, comparison of the monolayer diastereomericinteractions,Rp,R/S ,R and S,;S/S,,R, suggests that the chiral discrimination is principally due to the asymmetric carbon atom. %he transition pressure in the racemic film rises above the equilibrium spreading pressure. Therefore, the validity of the phase transition treatment is discussed on the basis of the dependence of the II-A curves on compression rate and on temperature as of the variation with time of pressures at constant area. Finally, the orientation of the bipolar surfactant molecule is questioned. In recent years, the effect of chirality on molecular interactions in membranes has received increasing attention.'T2 Although chiral monolayers a t the air-water interface are used to mimic biological membranes where most of the components are chiral, there are only a few studies which show that the behavior of monolayers may be sensitive to ~tereochemistry.~Since the first observation of chiral discrimination in a monolayer by Zeelen4 in 1956,few examples have been reported in the literature; these were reviewed in 1982 by Stewart and Arnett.' More recently, Bouloussa and Dupeyrat have described a very large chiral discrimination in monolayers of N-tetradecan~ylalanine.~Chiral discrimination in monolayers can be investigated by comparing the surface-active properties of the enantiomerically pure surfactants to those of the corresponding racemic mixtures, for instance, by using a Langmuir balance to get the pressure-area isotherms. As reported by Arnett et al.? "the surface properties of all the surfactants studied to date are strongly stereoselective in that they show chiral discrimination between the properties of pure enantiomers and their racemic mixture, with the notable exception of the phosphatidylcholines".6 In this latter case, the asymmetric carbon atom, in the glycerol backbone, is probably "hidden" from its neighbors in the film. In most cases which exhibit a chiral discrimination, the racemic film has less tendency to condense than the enantiomeric one. The purpose of our work was to improve the understanding of molecular interactions in chiral monolayers by introducing a second chiral center in the polar head, with the hope of obtaining a better anchorage at the interface (1) (a) Stewart,M. V.; Amett, E. M. Topics in Stereochemistry;Allinger, N. L., Eliel, E. L., Wilen, S.H., Eds.; Wiley New York, 1982; Vol. 13, pp 195-262. (b)Harvey, N. G.;Mirajovsky, D.; Rose, P. L.; Verbiar, R.; Arnett, E. M.J . Am. Chem. SOC.1989,111,1115-1122. (21 Wisner, D. A.; Rosario-Jansen, T.; Tsai, M.-D. J. Am, Chem. SOC. 1986,108,8064-8068. (3) Harvey, N.; Rose, P.; Porter, N. A.; Huff, J. B.; Arnett, E. M. J. Am. Chem. SOC.1988,110,4395-4399. (4) Zeelen, F.J. Doctoral Thesis. 'Stearoyl-Aminozuren: Synthese, Spreiding, en Photochemie"; Leiden, Netherlands, 1956. (5) Bouloussa, 0.;Dupeyrat, M. Biochim. Biophys. Acta 1988, 938, 396402. (6) Arnett, E. M.; Gold, J. M. J. Am. Chem. SOC.1982,104,636-639.
0743-7463/89/2405-1200$01.50/0
lv
11 Et3N 21chromatography
Major
Minor
and an increase of the chiral di~crimination.~This potentially yields two pairs of diastereomers, RR, SS and RS, SR, thus allowing a more complete study of the relationship between the molecular packing and configuration. The present paper reports a study of four diastereomers of hexadecylthiophospho-2-phenylglycinol(see Scheme I) spread at the air-water interface. Experimental Section Materials Preparation and Purification. Hexadecylthiophosphoric Dichloride (1). Triethylamine (3.03g; 30 "01) dissolved in 20 mL of trichloroethylenewas added to a stirred solution of thiophosphoryl chloride (Aldrich;5.08 g, 30 mmol) in 10 mL of hexane at 5 "C; stirring was continued, and a solution (7) Preliminary results were reported in the 3rd Chemical Congress of North America, Toronto, June, 1988; in press in Proceeding of the International Symposium on New Trends in Physics and Physical Chemistry of Polymers.
0 1989 American Chemical Society
Langmuir, Vol. 5, No. 5, 1989 1201
Chiral Discrimination in Monolayer Packing
n=13: n=l :
&
Table I. 'H NMR Chemical Shifts" of (2R,4R)- and (2S,4R)-2-Alkoxy-4-phenyl-1,3,2-oxazaphospholidine-2-thioneb compd 2a 2b 4a 4b
CH3 0.88 0.88 0.96 0.96
(CHz), 1.26 1.26 1.43 1.45
2'-CHz 1.71 1.72
1.70 1.72
1'-CH2 4.11 4.14 4.13 4.16
NH 3.38 3.52 -3.1 3.48
H4 4.75 4.92 4.75 4.93
H5AC
H6BC
4.60 4.56 4.55 4.56
4.17 4.02 4.15 4.03
O 2 0 0 MHz in CDCIS. bPrepared from D-(-)-phenylglycinol. cThe proton H ~ resonates B at a lower field than HsA;the former is probably in a trans relation with H, and is shielded by the phenyl group.
Table 11. 'H NMR Coupling Constants of (2R,4R)- and (2S,4R)-2-Alkoxy-4-phenyl-1,3,2-oxazaphospholidine-2-thionea compd NH-P NH-HI H ~ - H ~ A H ~ - H ~ B ~H ~ A - H ~ B H l r P H4-P H~A-P H~B-P 1'-2' 7 11 13 6.5 9 10.5 7 7 2a 18.5 22 9.5 6.5 9.75 9-9.5 10.5 3 7 3 2b 17.5 6.75 10.75 13 6.5 8.75 10.5 6.5 6 4% 9.75 22 9 9.5 7 2-2.75 9 10.5 4b 17.5 "Prepared from D-(-)-phenylglycinol. only result from two trans protons.20
the minor compounds 2a and 4b, the vicinal coupling constants H4-HSB(2-3 Hz) < 3.8 Hz can
N03PS) C, H. NMR (200 MHz; CDC13/DMSO-d6)8 = 0.81 (t, 3 H, CH,), 1.19 (s, 26 H, (CH2)13), 1.52 (t, 2 H, CH2 chain), 3.82 (t, 2 H, CH20),4.18 (m, 2 H, CH20),4.51 (1, 1 H, CHI, 7.40 (m, 5 H, C6H5), 8.89 (1, 3 H, NH3'). Minor Isomer 3b. To a solution of 120 mg of 2b in DME was added a 40% aqueous solution of trifluoroacetic acid until the mixture was slightly cloudy. After 20 h, water and ether were added; 3 h later, 93 mg of the crystalline product 3b was filtered by suction: RAMeOH-CHC13 1:9) = 0.52, RAMeOH-CH2C12 1:9) = 0.35; 50 mg was chromatographed on TLC (SO2;EtOH-CH2C12 1:9) to yield 30 mg of 3b, which was dried at 50 "C under reduced pressure (yield 40%); mp (DSC-2; 5 "C/min) 83.4,87.8, 102 "C. Anal. (C2,H,NO3PS.l.5H2O) C, H. NMR (200 MHz; CDC13) 6 = 0.88 (t, 3 H, CH,), 1.25 (s, 26 H, (CHJ13), 1.56 (t, 2 H, CH2 chain), 3.87 (m, 2 H, CH20 chain), 4.01 (t, 1 H, CH), 4.59 (m, 2 H, CH20), 7.45 (m, 5 H, C6H5),8.7 (1, 3 H, NH3+). The enantiomers 3c and 3d were obtained in the same manner from 2c and 2d, respectively. Major Isomer 3c: mp (DSC-2) 106.4, 133.7, 136.5,164.5 OC; [a]25D = +12.4", [(YIZ55,s= +13.3", [a]25m= +15.2", [ ( ~ ] ' ~ 1 3 6= = +37" (EtOH; c = 5). Anal. (C24H44N03PS) C, +24.7", H. Minor Isomer 3d. First heating (DSC-2): mp 81.3-84.3 and 94.7-102.1 "C. Second heating: mp 79.8-81.2,84.7, and 100 "C. Anal. (CZ4HuNO3PS,2CH40) C, H. Major Racemate 3a/3c was obtained by hydrolysis of the racemate 2a/2c: yield 85%; mp (DSC-2) 109 and 189 "C dec; 50 mg was recrystallized from 20 mL of EtOH and 10 mL of H20. Minor Racemate 3b/3d was obtained by mixing equal quantities of the two enantiomers: mp (DSC-2) 125.9 "C. Spreading Solutions. Because the major diastereomer is very insoluble in most organic solvents, each enantiomer (about 3-4 mg), weighed on a Perkin-Elmer AD-2Z automicrobalance, was (2S,4R)-2-Butoxy-4-phenyl-1,3,2-oxazaphospholidine-2- dissolved in the ternary mixture dimethylformamide/absolute ethanol/hexane (4:4:2) to a concentration range of 1.1X lo9 to thione (4a) was similarly prepared from D(-)-phenylglycinol and 3.7 X lo-, M (by weighing). The solutions were stored in a 1-butanol. Anal. (C12H18NOPS)C, H, N. NMR data in Tables desiccator under an atmosphere of hexane. The Surfactants were I and 11. delivered dropwise to the air/water interface with a 0.25-mL Agla (Rp,R)- a n d (Sp,R)-Hexadecylt hiophospho-2-phenylor a 0.50-mL Hamilton syringe. The racemic surfactants were glycinol (3a and 3b): 87 mg of 2a (0.2 mmol) was dissolved in obtained by mixing equal volumes of enantiomer solutions. The 2 mL of dimethoxyethane (DME), and 0.1 mmol of trifluoroacetic amount of each product to be delivered to the surface was (2.5-4) anhydride in 0.5 mL of H 2 0was added. After 1h, crystals began x 10'~ molecules to a typical area of 5 x 10'~ A2. to precipitate, 85 mg of analytical 3a (93%) was filtered after 48 h at room temperature and dried: mp (DSC-2; 5 "C min) 127.7, Subphase. The subphase used was either triply distilled water or acid water obtained by addition of H2S04 or HCl (Prolabo, 13a.i,i63.1 oc; = -12.50, = -13.20, I[. 2 Lb16 = -15.30, Rectapur). = -23.8*, [a]253e5= -34" (EtOH; c = 5). The product is very insoluble but may be recrystallized by dissolution in warm absolute ethanol followed by addition of water. Anal. (CZ4Hu(8)Brusik, K.;Tsai, M.-D. J. Am. Chem. SOC.1982, 104, 863-865.
of 4.82 g (20 mmol) of hexadecanol in 80 mL of trichloroethylene was added dropwise in 45 min. The mixture was warmed to room temperature, and stirring was continued for 1h. The precipitated triethylammonium chloride was filtered off by suction. After 20 mL of toluene was added to the filtrate, the solvents were removed under reduced pressure. Toluene was added to the oily residue, the solution filtered, and the filtrate evaporated again to give 1 as an oil: R = 0.61 (hexane; Si02);NMR (200 MHz; CDC13) 6 = 0.88 (t, 3 CH3),1.26 (26 H), 1.80 (m, 2 H, CH2),4.33 (m, 2 H, CH20). The product was dissolved in 50 mL of dry THF to give a 0.4 M solution of 1 (stock solution). (2S,4R )- a n d (2R ,4R )-2-(Hexadecyloxy)-4-phenyl-1,3,2oxazaphospholidine-2-thione(2a and 2b). A solution of 0.822 g of D-(-)-phenylglycinol (Aldrich; 6 mmol) and 4.17 mL of triethylamine in 15 mL of dry THF was added dropwise in 20 min to 17 mL of the stirred stock solution of 1 (6.8 mmol) at 10 "C. The mixture was stirred for 1 h at room temperature and the triethylammonium chloride filtered off by suction; evaporation of the filtrate gave a partially crystallized product. Ether-hexane (1:4) was added, and 0.79 g of a mixture of 2a and 2b was recovered by filtration: Rf = 0.28 and 0.41 (SO,; ether-hexane 1:4). Crystallization from 10 mL of hexane gave 0.459 g of 2a (Rf = 0.43). By chromatography of the filtrate on 150 g of silica gel with 1:4 ether-hexane, 0.641 g of 2a was first eluted (yield 42%) and then 0.958 g of oily 2b (yield 36%). The major diastereomer 2a was crystallized once more from hexane (487 mg in 6 mL) to give an analytical product: mp (DSC-2, Perkin-Elmer) 63.2-63.9 "C; [ a ] 2 5 D = -39.5" (CHCl,; c = 1.005). Enantiomers 2c and 2d were similarly prepared from L(+)-phenylglycinol(Aldrich). 2c: mp (DSC-2)63.2-64.3 "C; [a]=D = +39" (CHCI,; c = 1.025). The major racemate was obtained by crystallization of an equal mixture of 2a and 2c from hexane. NMR data are given in Tables I and 11.
fr,
1202 Langmuir, Vol. 5, No. 5, 1989
Duolaitzky et al.
t
Minor H20
2001
150r 0
(V
0
I
A
50
B
100
%Cp)
Figure 1. Melting phase diagram of the major diastereomer 3a and 3c (R,$ and S,,S). Experimental points were obtained either by DSC or by microscopy between crossed polarizers; due to crystal-crystal transitions, the accuracy is not good. This diagram exhibits partial solid solutions; namely, between A and B there is incorporation in the racemic compound lattice of that enantiomer which is in excess.
Langmuir Film Balance. The f i i balance used was a Lauda apparatus. Each isotherm was obtained between 19 and 23 "C
(A2 /molecule)
A
Figure 2. II-A isotherms of the minor diastereomer 3b and 3d (Spa and R 3)obtained by compressionat the akwater interface at 20
OC:
(--) pure enantiomers; (-)
racemate.
Major UI
and reproduced 2-5 times.
Results Synthesis of the Surfactants and Their Absolute Configurations. After reviewing the work of Tsai et a1.8 on thiophospholipids chiral a t phosphorus, we chose to prepare the hexadecylthiophospho-Zphenylglycinols (3). The synthesis of the two diastereomers 3a and 3b from D-(-)-(R)-phenylglycinolis outlined in Scheme I. The enantiomers 3c and 3d were similarly prepared from the L-(+)-(S)-phenylglycinol. The NMR spectra of the phospholidinethiones 2a and 2b (Tables I and 11) do not give rise to the configuration of the phosphorus chiral center: and no compound has given crystals suitable for X-ray analysis. We were successful in forming these crystals only for 4a, a homologue of 2a with a shorter chain; the crystal structure of the major isomer 2-butoxy-4-phenyl-l,3,2-oxazaphospholidine-2-thione (4a, R = C4H9)shows the relative S,R configuration on positions 2 and 4.'O We assumed that the configuration of 2a (R = C16H33)is the same. Since the acid cleavage is known to proceed in similar substrates, with inversion of the configuration," we can assign the configuration Rp,R to the major surfactant 3a and S,,R to the minor isomer 3b (see Scheme I). Three factors demonstrate that heterochiral interactions are favored in a three-dimensional array: (1) the infrared spectra of the racemates in the solid state are different from those of enantiomers for both diastereomers, (2) differential scanning calorimetry (DSC) shows that the racemates melt at higher temperature than the enantiomers, (3)the melting phase diagram of mixtures of Rp,R and S,,S diastereomers (Figure 1)is characteristic of racemic compounds.12 Chiral Monolayers of Enantiomeric and Racemic Thiophosphophenylglycinols at the Air-Water Interface. The spreading experiments were first performed (9) Cooper, D.B.;Hall, C. R.; Harrison, J. M.; Inch, T. D. J . Chem. Soc., Perkin Trans. I 1977, 1969-1980. (10) X-ray structure determination waa performed by M. Cesario and C. Pascard, Institut des substances Naturelles, Gif sur Yvette, France. (11) Abbot, S.J.; Jones, S. R.; Weinman, S. A.; Knowles, J. R. J. Am. Chem. SOC.1978,100, 2558-2560. (12) Jacques, J.; Collet, A.; Wilen, S. H. Enantiomers, Racemates and Resolutions; Wiley: New York, 1981; (a) pp 18, (b) 126.
- -
- - - - - - - - -- - -- -- - -
I _
1
'
'
1
10
1
'
'
'
1
4NH2S04 H20
t (mn)
50
Figure 3. Dependence of surface pressure on the acid concen-
tration of the subphase: (- - -) crystals of pure enantiomer; (-) racemic crystals for the R,$, S,,S diastereomer.
c
40
E
s
Major 0.3N H2SO4
4Rac.
2ot \
=I-
\
A
(A2/mo~ecu~)
Figure 4. II-A isotherms of the major diastereomer obtained
by compression at the air-0.3 N H2S04interface at 20 O C : (- - -) pure enantiomers; (-) racemate.
on pure water a t room temperature. The racemate and enantiomers of the R,,S, S,,R diastereomer give identical E A isotherms of a liquid-expanded phase and therefore show no chiral discrimination (Figure 2). On the other hand, a chiral discrimination is displayed in the case of the R,,R, S,,S diastereomer: a phase transition is observed in the racemic film but not in the pure enantiomeric fiim. However, the poor reproducibility on pure water subphase led us to measure the equilibrium spreading pressure (ESP) using the Wilhelmy plate method and to dispose crystals of surfactants on the water surface. As shown in Figure 3, no spreading pressure develops on pure water; a surface-active behavior appears only on acid subphase (2 N H2S04),and the ESP increases with the acid concentration. Thus, strong aggregation forces in the crystal of the zwitterionic R ,R, S,,S surfactant prevent spreading. By ionizing the head)group, the acid medium increases the
Langmuir, Vol. 5 , No. 5 , 1989 1203
Chiral Discrimination in Monolayer Packing
i
40-
5
\
\
0 C
P
-
20
5
50
75 1
Figure 5. Influence of compression/expansionrate on ll-A isotherms of the major diastereomer at the 0.3 N H2S04-air interface: ll-A curves of (- - -) pure enantiomer S,S and of (-1 racemate at ( 0 )21.6 and (D) 7.5 A2/molecule per min.
repulsion forces and allows the molecules to escape from the crystal, producing a monolayer. Moreover, as shown by Arnett et al.13 in the case of N-methylbenzylstearamide, the racemate exhibits a higher surface pressure than the enantiomer on 6 N sulfuric acid for the same area per molecule. The pressurearea curves were then again measured on aqueous HzS04. The results are now reproducible, even a t weak acid c~ncentration'~ (Figure 4): up to about 50 A2/molecule,the enantiomers and racemate give the same expanded phase. For smaller areas, the racemic film exhibits a phase transition toward a more condensed phase. The collapse of the racemate occurs at a higher pressure than that of the enantiomer, suggesting that the racemic film is more stable. The E A isotherms measured at two compression rates are shown in Figure 5; the compression curves are identical for compression rates of 7.5 and 21.6 A2/molecule per min: the chiral films have only weak kinetic effects. However, the compression/expansion cycles are not identical (especially in racemic films). After recompression of these films, we return to the same compression isotherms. Figure 6 shows the stepwise compression of the racemic film, which permits determination of its stability limit: it can be seen that up to 25 dyn/cm the loss of surface pressure is small (
