J. Phys. Chem. 1993,97, 12858-12861
12858
Crystalline Structures of Monolayers on the Water Surface As Determined by Grazing Incidence X-ray Diffraction: N-Eicosanoyl-3-aminopropionicAcid and Its Mixtures with Eicosanamide 1. Weissbuch,*JF. Leveiller,? D. Jacquemainf K. Kjaer,s J. Als-Nielsen,# and L. Leiserowitz'lt Department of Materials & Interfaces. The Weizmann Institute of Science, 76100 Rehovot, Israel, and Physics Department, Rise National Laboratory, DK4000 Roskilde. Denmark Received: June IS, 1993'
The crystalline structures of monolayers of the N-eicosanoyl-3-aminopropionicacid C H r ( C H 2 ) 18-CONH(CH2)2-C02H and its mixtures with eicosanamide C H ~ - ( C H ~ ) I ~ - C O N onHa ~water subphase weredetermined, at near-atomic resolution, by grazing incidence X-ray diffraction (GID), using synchrotron radiation. The N-eicosanoyl-3-aminopropionicacid molecule must adopt a very particular conformation in order to ensure satisfactory packing requirements. The packing characteristics were tested by fitting the measured GID data using atomic coordinate models for various conformers, complemented by lattice energy calculations. The structure of the mixed monolayer in 1:l and 1:2 molar ratios is different from that of each pure component. We propose that the two components do not form a random solid solution but rather the N-eicosanoyl-3aminopropionic acid forms domains within the sea of eicosanamide molecules, stabilized by the interactions between the - ( C H 2 ) K 0 2 H moieties.
Introduction Grazing incidence X-ray diffraction (GID) has proved to be a powerful tool for the study of the crystallinepacking arrangement of monolayers' of insolubleamphiphilesspread on various aqueous subphases. Two-dimensional (2-D) crystal structures of several long-chain a-amino acids,2J alcohols,4~5acids,- and amidesg spread on water orland ionic solutionslOJ1 have been reported, and the steps involved in structure determination at near-atomic resolution have been desribed in a previous study.12 All the 2-D crystal structures reported to date describe the packing arrangements of simple planar molecules with minor conformational freedom. The aim of this study was to determine the 2-D crystal structure of a monolayer composed of the N-eicosanoyl-3-aminopropionic acid amphiphile, in which various functional groups along the molecule must adopt particular conformations in order to fulfill satisfactory packing requirements. The packing characteristics of the various conformers, using atomic coordinate models, were tested by structure refinement of the GID data, complemented by lattice energy calculations. Mixing the N-eicosanoyl-3-aminopropionicacid amphiphile with eicosanamide causes a dramatic change in the 2-D crystal structure. Such mixtures were studied in connection with a program involving the use of mixed monolayers for induced nucleation of 3-D crystals at the monolayer-solution interface.13 Mixed monolayers of molar ratios 1:l and 1:2 spread on either pure water or ionic solutions yielded very stable 2-D crystal structures different from those of the pure components. Here we present the 2-D crystal structure of the uncompressed 1:2 mixed monolayers spread on water.
Results and Discussion
Experimental Section Materiels. N-Eicosanoyl-3-aminopropionicacid CHr(CH2)lr CONH-CHrCHz-COzH and eicosanoamide CH3-(CH2) ISCONH2 were prepared as described in the accompanying paper.13 Spreading solutions of the amphiphiles were prepared in chloroform (Merck, analytical grade) with concentrations close to 0.5 mM. Millipore purified water was used. The monolayer solution was spread at room temperature before cooling the + The Weizmann
8
subphase for the GID experiments. The measurements were started 1 h after spreading of the monolayer over the subphase, and the film remained stable for many hours. GID experiments were performed using the surface liquid diffractometer on the synchrotronX-ray beam line D4 at Hasylab, DESY, Hamburg. Thediffractometer wasequippedwith a sealed and thermostat4 Langmuir trough, which had a Wilhelmy balance for controlling the surface pressure. The synchrotron X-ray beam was monochromated to X = 1.39 A, and the angle of incidence (equal to the exit angle) was 0.85 a0 where a, = 0.138O is the critical angle for total external reflection. The footprint of the X-rays on the liquid surface was 50 X 5 mm. Detection was made by a linear position sensitive detector (PSD) mounted vertically behind the collimating Soller slits (resolution 6(20) = 1.5 mrad). Collection of data was made in two ways: the scattered intensity was integrated over the whole qr window of the PSD yielding Bragg peaks and the intensity was recorded in channels along the PSD but integrated over qxy-the scattering vector in the horizontal plane-yielding the Bragg rod profiles. The GID measurements were performed for the N-eicosanoyl3-aminopropionicacid monolayer on water and for its 1:1 and 1:2 mixtures with eicosanamide on both water and 1 mM CdCl2 solution at pH = 8.4. The GID patterns and the Bragg rod intensity profiles were very similar for the 1:l and 1:2 mixtures on both subphases. Therefore, we present here the data for the 1:2 mixture spread on water. The analysis consisted of calculating the Bragg rod intensity profiles using atomic coordinate models to obtain the molecular structure factor amplitude, as previously reported.12 An overall molecular thermal parameter U = 0.5 A2 was used.
Institute of Science. Riso National Laboratory. Abstract published in Aduance ACS Absrrclcrs, November 1, 1993. 0022-365419312097-12858$04.00/0
KEicosanoyl-3-aminopropionicAcid. The GID pattern of the N-eicosanoyl-3-aminopropionicacid (abbreviated as C19 CONH C2 C02H) monolayer was recorded over a pure water subphase (pH = 5.8) at 11 OC and at a surface pressure If = 1 mN/m with a surface coverage of 93%. The powder pattern was measured for the low-order reflections in the qxyrange 1.15 Iqxy I 1.73 A-1 and consists of two peaks at qxy = 1.39 and 1.45 A-l, corresponding to d spacings of 4.53 and 4.32 A, respectively (Figure la). Thus the unit cell is rectangular. Because the peak at qxu = 1.45 A-1 is 2.5 times more intense than that at qxv = 1.39 A-1, it was attributed to the degenerate 0 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 49, 1993 12859
Crystalline Structures of Monolayers
a
1.25
1.15
1.35
145
1.55
1.65
1.75
qxY
-b
b 150
Figure 2. Packing arrangement of the N-eicosanoyl-3-aminopropionic acid monolayer on a water subphase, assuming the all-trans molecular model and plane groupplgl with a molecular tilt of 32O along the b axis, as viewed along (a) the u axis, (b) the b axis, and (c) the molecular axis,
C ,
10.21
200
CHART I
.soo~
x
I
~OZ IINVERSE ! i ~ ~ ~ ! ~ ~.50!, ~ ' ' ~O0.2 I ! 0.4 ~I ~' '0.6I~. ,'0.8 I .~' , , , i ' * AWSTROHSI L IINVERSE A f f i S l R M I S I 1
.
4
'
Figwe 1. Grazing incidence X-ray diffraction (GID) pattern of the N-eicosanoyl-3-aminopropionic acid monolayer on a water subphase at 1I O C , at a surface pressure of II = 1 mN/m: (a) Bragg peaks of the two reflections assigned to (0,2) and (1,l) + (1,i). (b and c) Measured (points) and calculated (full line) Bragg rod intensity profiles assumicg theall-tra~molecularmodelforthe[0,2)reflectionand the(l,l)+ (1,l) reflection.
+
(1,l) (1,T) reflections and the other peak was indexed as the (0,2) reflection. With this assignment, the rectangular unit cell has dimensions a = 4.92 A, b = 9.05 A, y = 90' with an area per molecule (ab/2) of 22.3 A2. Assuming that the aliphatic tails are uniformly and rigidly tilted in the monolayer, the direction and magnitude of the tilt between the molecular chain axis and the surface normal may be deduced from the positions of the maxima in qr along the two Bragg rods (Figure 1, part band c) (qrmax = 0.86 and 0.44 A-1) and are given by:12 tan t = qr max/(qhk cos $hk), where +hk is the angle between the reciprocal lattice vector qhk and the projection of the molecular axis on the basal plane. This results in a tilt angle t of 32' in the direction of the long b axis. The tilt angle t may also be derived from a comparison of the molecular area, 22.3 A2, with the cross-sectional area, 18.6 A2, of hydrocarbon chains,14using the formula cos t = 18.6/22.3. This yields a molecular tilt angle t of 33', in good agreement with the determination from the positions of the Bragg rod maxima. The alternative possibility of Bragg peak assignment (Le,, { 1,l) (i,i] at qxr = 1.39 A-1 and (0,2]at q = 1.45 A-I) yields a cell of dimensions a = 5.31 A, b = 8.65 y = 90' and area per molecule of 22.9 A2. This assignmentcould be discarded because it cannot yield a consistent solution of the molecular tilt and direction calculated from the two Bragg rod maxima. On the basis of the first assignment and the molecular tilt of 32', the dimensions of the unit cell projected onto a plane perpendicular to the molecular axis are up = 4.92 A, b, = 7.67 A. These values are generally indicative of the orthogonal 01 motifI4 in which all chains are parallel and tilted in a direction parallel to the glide plane. In this motif, the molecular chains are related by glide symmetry making herringbone contacts. Furthermore, the length of 4.9 A of the a axis is fingerprint evidence that the amide groups must be interlinked by a N-H.4-C hydrogen bond along the a axis.15 Therefore the plane group of the 2-D crystal structure must beplgl, in which the molecular chain axes, tilted at an angle of 32' along the b dirction, are all parallel and oriented so as to form the orthogonal 01 packing type. In order to determine the structure of the monolayer at nearatomic resolution, we constructed, using atomiccoordinate models,
+
1,
possible packing arrangements and tested them by comparing their calculated Bragg rod intensity profiles with the observed data, complemented by lattice energy calculations.12 One model was constructed assuming a planar C19-CONHC r C 0 2 H molecule in an all-trans conformation. This model yielded Bragg rod intensity profiles which compared relatively well with the measured data parts (Figure 1, b parts and c). But, such a molecular model does not allow the formation of hydrogen bonds along the a axis as observed from the packing arrangement shown in Figure 2. Thus the conformation of the C,-CO-NHC H A H S O z H moiety cannot be all-trans planar. To construct a plausible model, we made use of the molecular conformation of N-(2-hydroxyethyl)octadecanamide (C, CONH-CrOH) in its three-dimensional (3-D) crystal structure,16 this molecule being very similar to the monolayer amphiphile but for the hydroxyl group. The 3-D packing arrangementcontains hydrogen-bonded layers of axial dimensions (4.9 A by 9.0 A)17 very similar to those of the monolayer (4.92 A by 9.05 A). The molecular chains in the 3-D crystal are tilted by 34.5' along the 9.0 A axis and pack in the herringbone arrangement, generated by a glide along this axis. The molecular conformation is such that the amide groups form linear N - H 4 - C hydrogen bonds by translation along the short 4.9-A axis. On the basis of this 3-D crystal structure, a molecular model of the C1&ONH-C&02H amphiphile was constructed,shown in Chart I, with the following torsion angles: (03)C4-Cs-C6-C7, 7 1.1'; (Ha)NI-C4(03)-C5-C6,-1 28.6'; and C & T ( H ~ ) N ~ - C ~ , 84.8'. Note that the requirement for a planar amide group (40NH-) is maintained but that there are essentially two possible conformations of the carboxyl (C02H) groups. In one conformation, the 4 4 4 0 2 H system is assumed planar (Chart I), yielding the arrangement, shown in Figure 3, in which the molecules pack in the plane groupplgl with a chain tilt of 32O along b. The ratio of the integrated Bragg peak intensities I , ~ , ~ I + ~ ~= J I2.3 / I ,compares O , ~ ~ well with the observed value of 2.5. Moreover, the calculated integrated intensity of the (1.0) reflection, I,l,ol, is only 8% of that of the measured weak {0,2)peak(TableI),accountingfor theunobserved(1,O)reflection. The intermolecular N-H-O==C(amide) distance between hydrogen-bonded molecules related by translation along the u axis is 2.87 A. The assumed conformation of the carboxyl group
,-
Weissbuch et al.
128860 The Journal of Physical Chemistry, Vol. 97, No.49, 1993
1
I mF
a-
-b
Figure 3. Packing arrangement of the N-eiccwanoyl-3-aminopropionic acid monolayer on a water subphase, assuming the molecular model of Chart I (note the planar C-C-COzH moiety) and the plane group plgl with a molecular tilt of 32' along the b axis, as viewed along (a) the u axis,(b) the baxis, and (c) the molecular axis. Note the favorableCH-O(carboxyl) interactions shown in part a.
0.2
QZ
IINVERSE A N O S r R W I
0.4
QZ IINVERSE
04 AYISTRUISI
0.1
Figure4. Measured (points) and calculated (full line) Bragg rod intensity profiles assumingthe molecular model of Chart 11for (a) the {0,2)reflection and (b) the (1,l) + (1.1)reflection.
CHART 111
TABLE k Calculated and Observed X-rs Powder q,-Integrated Intensities (0 5 qr 5 0.9 A-lr Normalized to That of the (1,l) + (1,l) Reflection for the Molecular Packing Models Shown in Figures 3 and 5 qhk
0.96 1.28 1.39 1.45 2.62 2.89
(h94 (0,11 (190)
lo4
+
(1,l) (I,!) (12) + (192) (093)
rots
40 100
Ialc
(I)@ 0 8
44 100 3
0
Ialc
(IIY 0
0.2 43 100 2 0
(I) model with the -C-C-COzH planar system. b (11) model with the C02H plane parallel to the -CONH- plane. @
CHART I1
(C-C-COzH coplanar) leads to a CH-O(carboxy1) interaction, with a C.-0 distance of 3.1 A (see Figure 3). Atom-atom potential energy calculationslz performed using this modeP packing yielded a latticeenergy of-53.2 kcal/mol with a Coulomb contribution of -1 0 kcal/mol from the hydrogen-bonding system. An alternative conformation of the carboxyl groups which can lead to an O-H-O(carbOxy1) hydrogen bond akin to that adopted by theamidegroup would require that the-C02H plane be parallel to that of the -CONH- group as shown in Chart 11. Naturally, an antiplanar OEC-0-H conformation was assumed, as a requirement for the intermolecular O-He-0 hydrogen bond formation. The measured and calculated Bragg rod intensity profiles, Figure 4, show better agreement than that for the allIra'II)Lpmolecule, and the calculated intensity of the (1,O)reflection is negligible (Table I). Note that for the (1.1) + (I,!) reflection, the fit for Vineyard peak19 at qr close to the horizon and the overall shapeof the Bragg rod are very satisfactory. The molecular packing arrangement, Figure 5, displays the double array of hydrogen bonds and the lattice energy calculated for this model of -58.4 kcal/mol with a Coulomb contribution of -1 1.3 kcal/ mol. There can be no doubt that this model is by and large correct. 1:2 N-Eicosrmoyl-3-aminopropionic Acid/Eicosanamide Mixture. The GID pattern of the 1:2 N-eicosanoyl-3-aminopropionic acid/eicosanamide (CI&ONH-CZ-C~~H/CI+ZONH~)mixture was measured over pure water, at 11 OC, at a surface pressure ll = 0 mN/m with a surface coverage of 84%. Two low-order diffraction peaks were observed in the qxyrange 1.15 5 qxyI1.73
H i
A-1 at qxy = 1.52 and 1.67 A-I, corresponding to d spacings of 4.14 and 3.76 A, respectively (Figure 6a). The first reflection at qxy= 1.52 A-1 was indexed as the (1,l) {I,]) reflection and the second peak as the (0,2)reflection yielding dimensions for the rectangular unit cell of a = 4.97 A and b = 7.51 A with an area per molecule (ab/2) of 18.7 A2. As already pointed out, these dimensionsindicate a chain packing in the orthogonal 01 motif with molecules vertically aligned, keeping with the Bragg rod intensity profdes. Since the hydrocarbon chains are vertical, the plane group describingthe orthogonal 01motif may be achieved eitherbyplgl orpl lgsymmetry. Thisambiguity may beresolved by consideringthe packing of the C O N H - C H ~ C H Z - C O ~head H groups. To avoid interpenetration between amide groups along the b direction, the glide must be parallel to b, as in plane group plgl. In this way the planes of hydrogen-bonded chains related by glide symmetry are parallel to each other (Figure 7). Bragg rod intensity profile calculations were performed using the molecular model given in Chart I11 with a torsion angle (&)NI-C~(O+C~-C~ of -128.3'. Given the 1:2 molar ratio of the two components, we considered here for the -CH&HrC02H moiety of the minor component an occupancy of 33%. A good agreement between the calculated and observed Bragg rod intensity profiles is obtained (Figure 6, part b and c) with a calculated ratio of integrated intensities~l,l)+(l,il/~~~l= 2.5, close to the experimental value of 2.8. In this packing arrangement (Figure 7) the molecules related by translation along the u axis form an amide (N-H-*O-C) hydrogen-bonded layer with an intermolecular N.-O distance of 3.0 A and a O-H-.O(carboxyl) hydrogen bond of length 3.1 A. Finally, we propose that in the mixture, the two monolayer components form, at the amide level, a homogeneous array of hydrogen-bonded chains, within which there exist domains of the minor monolayer component. The formation of such domains may be favored by the interactions interlinking their CH2CH2CO2H moieties. They involve van der Waals contacts between CHzCHz groups, stacking, and hydrogen bond interactions between thecarboxyl groups. Indeed, thecalculatedlatticeenergy for a 2-D crystalline domain of a C M - C O N H - C ~ C O ~ H amphiphile is more stable by about 18 kcal/mol than that of Clp-CONHz molecules (calculated lattice energies of -64.9 and 4 6 . 8 kcal/mol, respectively).
+
Conclusions The 2-D structures of Neiccwanoyl-3-aminopropionicacid and of the eicosanamide/N-eicosanoyl-3-aminopropionic acid mixed monolayers spread on water subphases have been determined to near-atomic resolution. The structures of eicosanamideg and
The Journal of Physical Chemistry, Vol. 97, No. 49, 1993 12861
Crystalline Structures of Monolayers
b
a
-b
C
a-
Figure 5. Packing arrangement of the N-eicosanoyl-3-aminopropionicacid monolayer on a water subphase, assuming the molecular model of Chart I1 (note that the planes of the CO2H and C O N H groups are parallel) and the plane groupplgl with a molecular tilt of 32O along the b axis, as viewed along (a) the a axis, (b) the b axis, and (c) the molecular axis. Note the double array of intermolecular hydrogen bonds running along the a direction.
a
1.40
1.50
qxy
b
1.60
1.70 -b
C 10,21
I
7YO
Y 500
a-
Figure 7. Packing arrangement of the 1:2 mixed monolayer of N-eicosanoyl-3-aminopropionicacid/eicosanamide on a water subphasc, assuming the molecular model of Chart 111 and the plane group p l g l with vertical chains, as viewed along (a) the a axis, (b) the b axis, and (c) the molecular axis. Note the double array of intermolecular hydrogen bonds running along the a direction. For convenience, only the packing arrangement of the minor component domains are drawn.
5 210
References and Notes 0 0
0.2 0.4 0.0 02 IINVERSE A N G S T R U S I
0.8
Figure 6. GID pattern of the 1:2 mixed monolayer of N-eicosanoyl-3aminopropionic acid/eicosanamide on a water subphase a t 11 OC, at a surface pressure of II = 0 mN/m, and at 84% surface-coverage: (a) Bragg peaks of the two reflections assigned to (1,l) + (1,l) and (0,2).(b and c) Measured (points) and calculated (full line) Bragg rod intensiv profiles assuming the molecular model of Chart 111, for the (1,l) + (1,l) reflection and the (0,2)reflection.
N-eicosanoyl-3-aminopropionicacid are different from each other and from that of the mixed monolayer in 1:I and 1:2 ratios. There isnodoubt that the twocomponents form a homogeneousstructure from the amide group upward. We propose that the two components do not form a random solid solution but rather that the N-eicosanoyl-3-aminopropionicacid forms domains within the sea of eicosanamide molecules, stabilized by interactions between the CH2CH2C02Hmoieties. There is also evidence of segregated domain formation from the 3-D crystallization of silver propionate at the monolayer-solution interface.I3 Acknowledgment. We thank the Israel Academy of Basic Science and Humanity and the Danish Foundation for Natural Sciences for financial support and Hasylab, DESY, Hamburg, Germany for beam time. Supplementary Material Available: Two tables listing the cell constants and fractional atomic coordinates for N-eicosanoyl3-aminopropionic acid on water final model (11) and in the 1:2 mixture on water (4 pages). Ordering information is given on any current masthead page.
(1) Jaquemain, D.; Grayer Wolf, S.;Leveiller, F.; Deutsch, M.;Kjaer, K.; Als-Nielsen, J.; Lahav, M.; Leiserowitz, L. Angew. Chem. Inr. Ed. Engf. 1992, 31, 130. (2) Grayer Wolf, S.;Leiserowitz, L.; Lahav, M.;Deutch, M.;Kjaer, K.; Als-Nielsen, J. Nature 1987,328, 63-66. (3) Jaquemain, D.;Grayer W0lf.S.; Leveiller, F.; Lahav, M.;Leiserowitz, L.; Deutsch, M.;Kjaer, K.;Als-Nielsen, J. J. Am. Chem. Soc. 1990, 112, 7724. (4) Barton, S.W.; Thomas, B. N.; Flom, E. B.; Rice, S.A.; Lin, B.; Peng, J. B.; Ketterson, J. B.; Dutta, P. J. Chem. Phys. 1988, 89, 2257. (.5 .) Wana. J.-L. Manuscriut in ureDaration. (6) Kjaei,. K.; Als-Nielsen, J.. H'elm, C. A,; Tippman-Krayer, P.; MBhwald, H.J. Phys. Chem. 1989, 93, 3200. (7) Kenn, R. M.; Bbhm, C.; Bibo, A. M.; Peterson, I. R.;MBhwald, H.; Als-Nielsen, J.; Kjaer, K. J. Phys. Chem. 1991, 95, 2092. (8) Barton, S. W.; Goudot, A.; Bouloussa, 0.;Rondelez, F.; Lin, B.; Novak, F.;Acero, A.; Rice, S.A. J. Chem. Phys. 1992, 96, 1343. (9) Jaquemain, D.; Leveiller, F.;Weinbach, S.;Lahav, M.;Leiserowitz, L.; Kjaer, K.; Als-Nielsen, J. J. Am. Chem. Soc. 1991, 113, 7684-7691. (10) Lin, B.; Shih, M. C.; Bohanon, T. M.;Ice, G. E.; Dutta, P. Phys. Rev. k r r . 1990, 65, 191. (1 1) Leveiller, F.; Jaquemain, D.; Lahav, M.;Leiserowitz, L.; Dcutsch, M.; Kjaer, K.; Als-Nielsen, J. Science 1991, 252, 1532. ( 12) Leveiller, F.; Jaquemain, D.; Leiserowitz,L.; Kjaer, K.; Als-Nielscn, J. J . Phys. Chem. 1992,96, 10380. (13) Weissbuch, I.; Majewski, J.; Kjaer, K.;Ah-Nielsen, J.; Lahav, M.; Leiserowiz. L. J . Phys. Chem., preceding paper in this issue. (14) Small, D. M.The Physical Chemisrry ofLipids; Plenum Prm: New York, 1986. (15) Berkovitch-Yellin,Z.; Leiserowitz, L. J . Am. Chem. Soc. 1980,102, 7677. (16) Dahlen, B.; Pascher, I.; Sundell, S . Acra Chem. S c a d . 1977,A31, 3 13-320. (17) N-(2-hydroxyethyl)octanamidecrystaldata: II = 47.588A,b = 4.886 A. c = 8.999 A,B = 93.71°,space group Pc, Z = 4. (18) Electrostatic parameters for the amide (CONH) group were taken from ref 15. (19) Vineyard, G. Phys. Rev. B 1982, 26,41464159.
