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Langmuir 1997, 13, 6040-6046
Immobilization of Retinoic Acid by Cationic Polyelectrolytes Andreas Thu¨nemann Max Planck Institut fu¨ r Kolloid- & Grenzfla¨ chenforschung, Kantstrasse 55, D-14513 Teltow-Seehof, Germany Received July 8, 1997. In Final Form: August 28, 1997X
Retinoic acid was immobilized by precipitating its complexes with cationic polyelectrolytes from aqueous solution. Polyelectrolytes with different architectures, such as poly(ionene-6,3 bromide), poly(dimethyldiallylammonium chloride), and poly(N-methyl-4-vinylpyridinium chloride), form self-assembling complexes containing retinoic acid (70% (w/w)). All these complexes are thermodynamically stable and can be processed into mesomorphously ordered films with interesting physical properties. In contrast to the brittle crystalline retinoic acid the complexes with polyelectrolytes are highly deformable viscoelastic materials. All materials show lamellar mesophase structures; their Tg value strongly depends on the polyelectrolyte. It is suggested that these materials have great potential as pharmaceutical agents as well as models for the investigation and the mimicking of chromophores in visual pigments and photosynthetic bacteria. The properties of the complexes are examined by X-ray diffraction, differential scanning calorimetry, polarization optical microscopy, UV-vis spectroscopy, and stress-strain measurements.
1. Introduction Lipophilic hormones, such as retinoic acid, steroids, thyroid hormone, and vitamin D3, function by interacting with ligand-activated transcription factors comprising the steroid/nuclear receptor superfamily.1 Currently a lot of interest is focused on understanding the role of retinoic acid in cell differentiation by investigating the binding properties of retinoids to specific proteins.2,3 In addition to its important role of transducing pleiotropic effects on morphogenesis, differentiation, and homeostasis during embryonal and postnatal life, retinoic acid has great potential as a pharmacological agent. Currently it is used in the external therapy of serious cases of acne, and it is discussed for use in skin rejuvenation therapy.4 Further, there have been reports on malignant-tumor inhibition by retinoids.5,6 All retinoids possess the same characteristic highly UV active chromophor and are hardly soluble as well as chemically unstable in aqueous media. Therefore, natural retinoids need to be bound to specific retinoic-binding proteins in order to allow protection, solubilization, and transport in body fluids. With respect to administrating retinoic acid as a pharmacological agent, its immobilization is a major problem. One way to achieve such immobilization and protection of retinoic acid is binding it to a protein as demonstrated by nature. A successful example for mimicking this natural strategy has been shown by Zanotti et al.,5 who cocrystallized transthyretin and retinoic acid. This kind of formulation, however, is a difficult and costly procedure. In this paper we pursue a different and inexpensive strategy for the immobilization of retinoic acid: the complexation of retinoic acid with cationic polyelectrolytes. This approach is based on the discovery that the formation of ordered structures in X
Abstract published in Advance ACS Abstracts, October 15, 1997.
(1) Evans, R. M. Science 1988, 240, 889. (2) Bourguet, W.; Ruff, M.; Chambon, P.; Gonemeyer, H.; Moras, D. Nature 1995, 375, 377. (3) Renaud, J.-P.; Rochel, N.; Ruff, M.; Vivat, V.; Chambon, P.; Gronemeyer, H.; Moras, D. Nature 1995, 378, 681. (4) Lewin, A. H.; Bos, M. E.; Zusi, F. C.; Nair, X.; Whiting, G.;Bouquin, G.; Tetrault, Carroll, F. I. Pharm. Res. 1994, 11, 192. (5) Zanotti, G.; D’Acunto, M. R.; Malpeli, G.; Folli, C.; Berni, R. Eur. J. Biochem. 1995, 234 (2), 563. (6) Jaeger, E. P.; Jurs, P. C.; Stouch, T. R. Eur. J. Med. Chem. 1993, 28 (4), 275.
S0743-7463(97)00756-7 CCC: $14.00
solution or in the solid state often occurs by attaching a surfactant to a polyelectrolyte via self-assembly.7 This process is driven by electrostatic and hydrophobic interactions in aqueous solution. Recently Tirrell et al.8,9 reported a detailed investigation of self-assembled complexes of synthetic polypeptides with oppositely charged low molecular weight surfactants. Further, it was shown, that complexation of surfactants with polyelectrolytes yields a large number of stable mesophases with a great structural variety.10-12 The preparation of free-standing films by complexation of soybean lecithin with poly(dimethyldiallylammonium chloride) (PDADMAC) was reported to result in the formation of lamellar mesophases with high undulations of the lamellae, so-called “plastic membranes”. It became clear that not only synthetic surfactants but also natural amphiphiles are useful in this field of research. Retinoic acid may be considered as a type of amphiphile, too: The carboxylic acid function is the polar, hydrophilic head group and the elongated hydrocarbon part of the molecule acts as the hydrophobic tail (Figure 1). In this work three different polyelectrolytes were used for the complexation of retinoic acid. The first was PDADMAC because it is known from the complexation of natural lipids that it forms stable, soluble complexes12,13 and nanoscale supramolecular ordered gels with sodium dodecyl sulfate (SDS).14 In addition, two differently structured cationic polyelectrolytes were used for complexation of retinoic acid. One of these is poly(N-methyl4-vinylpyridinium chloride) (PM4VP), a polyelectrolyte with its charges located on side groups, a pendant-type (7) Antonietti, Ml; Conrad, J.; Thu¨nemann, A. Macromolecules 1994, 27, 6007. (8) Ponomarenko, E. A.; Waddon, A. J.; Tirrell, D. A.; MacKnight, W. J. Langmuir 1996, 12, 2169. (9) Ponomarenko, A.; Waddon, A. J.; Bakeev, K. N.; Tirrell, D. A.; MacKnight, W. J. Macromolecules 1996, 29, 4340. (10) Antonietti, M.; Conrad, J.; Thu¨nemann, A. Macromolecules 1994, 27, 6007. (11) Antonietti, M.; Henke, S.; Thu¨nemann, A. Advanced Materials 1996, 8, 41. (12) Antonietti, M.; Kaul, A.; Thu¨nemann, A. Langmuir 1995, 11, 2633. (13) Antonietti, M.; Wenzel, A.; Thu¨nemann, A. Langmuir 1996, 12, 2111. (14) Yeh, F.; Sokolov, E. L.; Khokhlov, A. R.; Chu, B. J. Am. Chem. Soc. 1996, 118.
© 1997 American Chemical Society
Immobilization of Retinoic Acid
Figure 1. Conformations of retinoic acid: (1) all trans; (1′) 11-cis; (1′′) 13-cis.
Figure 2. Polyelectrolytes used for complexation integral type: (2) poly(ionene-6,3), pendant type; (3) poly(N-methylene4-vinylene) intermediate type; (4) poly(diallyldimethylammonium).
of polyelectrolyte.15 The other is poly(ionene-6,3) with its positive charges directly within the polymer main chain (Figure 2), so-called integral type of polyelectrolyte. With respect to the position of charges PDADMAC adopts a position between PM4VP and poly(ionene-6,3). Therefore PDADMAC is referred as an intermediate type of polyelectrolyte. It was expected that complexes of retinoic acid with these three polyelectrolytes are able to demonstrate the variety of structure and properties possible for complexes of retinoic acid with cationic polyelectrolytes in general. 2. Experimental Section 2.1. Materials. Crystalline all-trans retinoic acid (tretionin, vitamin A acid) powder, high molecular weight poly(diallyldim(15) Philipp, B.; Dawydoff, W.; Linow, K.-J. Z. Chem. 1982, 22, 1.
Langmuir, Vol. 13, No. 23, 1997 6041 ethylammonium chloride) (20% (w/w) aqueous solution) and highmolecular weight poly(ionene-6,3 bromide) were purchased from Aldrich Chemical Co. and used as received. The molecular weight of poly(diallyldimethylammonium chloride) determined by aqueous gel permeation chromatography (GPC) was Mw ) 623 000 g/mol, Mn ) 187 000 g/mol (0.5 mol/L NaNO3, Progel-TSK-PW column by Tosohaas, refraction index and light scattering detector). Static light scattering in 0.5 M NaCl gave a value of Mw ) 525 000 g/mol. Poly(4-vinylpyridine) was prepared by radical solution polymerization reaction.16 Its molecular weight was determined by DMA-GPC to be Mw ) 228 000 g/mol, Mn ) 69 000 g/mol. Poly(N-methyl-4-vinylpyridinium chloride) was prepared from poly(4-vinylpyridine) via a polymer-analogous reaction with 3 equiv of methyl iodide in nitromethane. Iodide was exchanged by chloride by ultrafiltration using a 0.5 M sodium chloride solution. The yield of methylation determined by 1 H-NMR spectroscopy was 100%. For the poly(ionene-6,3 chloride) used in our experiments, the molecular weight determined by static light scattering in 0.5 M NaCl is in the order of Mw ) 5000 g/mol. The solvent for film casting was HPLC grade methanol and ethanol (Aldrich). 2.2. Complex Formation. Retinoic acid (100 mg) was dissolved in aqueous sodium hydroxide solution, and a 0.5% aqueous solution of the polyelectrolyte was added dropwise while stirring until no further precipitation was observed. The resulting crude complexes were separated and redissolved in methanol. Removal of excess retinoic acid and salt was achieved by ultrafiltration. Elemental analysis of all complexes showed the sodium and chloride (respectively sodium and bromide) content to be lower than 0.01%. Free-standing films of all three complexes were cast by pouring their solutions in methanol or ethanol onto glass plates. The two-dimensional geometry of the films was controlled by glass frames of variable size which were mounted on top of the glass plate. After evaporation of the solvent at 20 °C, remaining traces of solvent were removed in vacuo at room temperature for 24 h. 2.3. Methods. Wide angle X-ray scattering (WAXS) measurements were carried out with a Nonius PDS120 powder diffractometer in transmission geometry. A FR590 generator was used as the source for Cu KR radiation, monochromatization of the primary beam was achieved by means of a curved Ge crystal, and the scattered radiation was measured with a Nonius CPS120 position sensitive detector (Germany). The resolution of this detector is better than 0.018°. Small angle X-ray scattering (SAXS) measurements were recorded with a X-ray vacuum camera with pinhole collimation (Anton Paar, Austria, Model A-8054) equipped with image plates (type BAS III, Fuji, Japan). The image plates were read with a MAC Science Dip-Scanner IPR-420 and IP reader DIPR-420 (Japan). Differential scanning calorimetry (DSC) measurements were performed on a Netzsch DSC 200 (Germany). The samples were examined at a scan rate of 10 K/min in two heating and one cooling scan. First and second heating traces were essentially identical. Stress-strain measurements were carried out with a Zwick Material Tester Z010 (Germany). Polarized light optical microscopic observations of the films were performed with a Zeiss DMRB microscope (Germany). UV-vis spectra were recorded on a UVICON spectrophotometer, Model 931 (Kontron Instruments). Molecular modeling simulations of the complexes were performed using Insight & Discover (BIOSYM Technologies, USA).
3. Results and Discussion All complexes (PDADMAC retinoate, poly(ionene-6,3) retinoate, PM4VP retinoate) are soluble in a variety of polar organic solvents, such as methanol, ethanol, 2-butanol, 2-propanol, and chloroform. In polar solvents, polyelectrolyte-surfactant complexes are likely to be partially dissociated,17 while in solvents of low polarity such complexes can be expected to remain associated.18 The solubility of the three complexes is in agreement (16) Fouss, R. M.; Wanatabe, M.; Coleman, B. D. J. Polym. Sci 1960, 48, 5. (17) Antonietti, M.; Fo¨rster, S.; Zisenis, M.; Conrad, J. Macromolecules 1995, 28, 2270. (18) Bakeev, K.; Chugunov, S. A.; Teraoka, I.; MacKnight, W. J.; Zezin, A. B.; Kabanov, V. A. Macromolecules 1994, 27, 3926.
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Figure 3. DSC traces of PM4VP retinoate (dashed line), poly(ionene-6,3) retinoate (solid line), and PDADMAC retinoate (dotted line).
Figure 5. Polarization micrograph of PDADMAC retinoate formulated as film (magnification 50 and 150).
Figure 4. Stress-strain diagram of PDADMAC retinoate formulated as film.
with results published recently for complexes consisting of conventional synthetic polyelectrolytes and oppositely charged surfactants.7 All films are flexible, highly deformable, transparent, and of yellow to brownish color. 3.1. DSC. DSC measurements on the films revealed that the glass transition temperature of the materials strongly depends on the nature of the polyelectrolyte. The glass transition increases in the order PM4VP retinoate (Tg ) -19 °C), poly(ionene-6,3) retinoate (Tg ) -1 °C), and PDADMAC retinoate (Tg ) 28 °C) (Figure 3). First and second heating curves are essentially identical, which means that no memory effect regarding the glass transition can be observed by the film preparation procedure. From the increase in the glass transition temperature we conclude an increasing binding strength of the retinoate anion onto the polyelectrolyte in the same order. In no case was an exothermic or endothermic peak found. Consequently the films can be assumed to be noncrystalline. 3.2. Stress-Strain Measurements. Because of the highest glass transition temperature, films cast from PDADMAC retinoate are mechanically the most stable in the series. It was therefore possible to carry out stressstrain measurements. A typical stress-strain curve of a PDADMAC retinoate film is shown in Figure 4. The stress-strain behavior is similar to that typically observed for a rubbery material. At a strain of 1% the tensile modulus of PDADMAC retinoate was determined to be 4 MPa. In the range between 30 and 150%, elongation under constant stress is observed. During further elongation the stress increases up to a maximum value of 0.125 MPa. The material rips at an elongation of 200%. The formation of such flexible films consisting predominantly of rigid
Figure 6. Wide-angle X-ray diffraction of a PDADMAC retinoate (upper curve) and retinoic acid powder (lower curve).
rodlike molecules is remarkable. Interactions on a molecular level, above all, Coulomb forces between retinoate moieties and the polyelectrolyte, are responsible for the transformation of brittle crystals into a viscoelastic polymer. 3.3. Optical Microscopy. The films of all complexes are optically anisotropic, as found during examination between crossed polarizers. An example of the optical textures is shown in Figure 5. Obviously the complexes are mesomorphous materials, but an unambiguous identification of the mesophase on the basis of the optical textures is not possible. 3.4. X-ray Scattering. The lack of sharp reflections in the wide-angle regime proves that the three retinoate complexes are indeed amorphous on an atomic length scale. The diagrams of the three complexes are essentially identical. As an example the WAXS curve of PDADMAC is shown in Figure 6. The scattering curve exhibits a characteristic broad amorphous halo corresponding to a Bragg spacing of about 0.52 nm. This value is considerably higher than those observed for the amorphous packing of saturated alkyl chains in complexes of low molecular weight surfactants with synthetic polypeptides (0.45 nm)9
Immobilization of Retinoic Acid
or that observed for polystyrenesulfonate-alkyltrimethylammonium surfactant complexes (0.43 nm).7 The lower maximum position of the amorphous halo in retinoate complexes compared to that observed in complexes with saturated alkyl chains indicates that the average atomic distance in the former is significantly larger. This is expected: due to the bulky hexene ring and the rigid conjugated alkylene moiety of the retinoate, the molecules cannot pack amorphously with the same density as flexible chains. Free retinoic acid has a strong tendency to crystallize (Figure 6) and two similar crystalline modifications of retinoic acid are known (a triclinic and a monoclinic one).19 As shown in Figure 6, the ability of retinoic acid to crystallize is strongly suppressed by complexation with a polyelectrolyte. The complexes remain amorphous for several months, and therefore it is concluded that they are very likely to be thermodynamically stable. This conclusion is confirmed by the identical first and second DSC heating cycles. The lack of any crystallinity is surprising because the weight percentage of the crystallizable retinoic molecules in all complexes is about 70%. This is also a remarkably high content for a noncovalently bonded chromophore in a material above or near the glass transition. The suppression of crystallinity can be explained as follows: In order to maintain electrostatic neutrality, any diffusion of retinoic moieties must be accompanied by a correlated movement of polyelectrolyte chains, so that no phase separation, and consequently no crystallization, can occur. Parts a-c of Figure 7 present the results of small-angle scattering measurements of the three different complexes. In the diagram of PM4VP retinoate three peaks with spacing ratios 1:2:3 are found. The diagram of poly(ionene6,3) retinoate exhibits two sharp reflections with spacing ratios 1:2, and that of PDADMAC retinoate has one sharp and two weak, broad reflections with the rations 1:2:3. In all diagrams the increase in intensity to higher scattering vectors is due to the influence of the strong amorphous halo in the wide angle region. Both WAXS and SAXS data of the complexes can be explained by lamellar structures for the three complexes. These smectic A-like mesophases of the complexes are formed by alternating ionic and nonionic layers (see Figure 8). From the reflex positions of PM4VP retinoate a long period of L ) 3.12 nm is calculated. The value for poly(ionene-6,3) retinoate is 2.98 nm, and for PDADMAC retinoate it is 3.75 nm. There is obviously no systematic change in the long period going from integral type to intermediate type and pendant type of polyelectrolyte. The difference in the long period between PM4VP and poly(ionene-6,3) is only 0.14 nm. This small difference, which approximately corresponds to the length of a carbon-carbon double bond can be explained by PM4VP being bulkier compared to poly(ionene-6,3). In contrast the long period of PDADMAC retinoate is considerably larger than those of the other complexes. A molecular modeling simulation of the PDADMAC retinoate complex shows that the carboxylic acid group fits well into a “pocket” formed by every other cationic group of the polyelectrolyte (see scheme picture in Figure 11b). On the basis of this image the structure is proposed to be a noninterdigitated perpendicular arrangement of the retinoate molecules with respect to the lamellar plane. This results in a long period of 3.8 nm (see sketch in Figure 8a), which correlates well with the value found experimentally. For the two other complexes it is not possible to create similar “pockets” by computer modeling, where the carboxylic acid head groups could be accommodated. This conformational peculiarity of the (19) Stam, C. H. Acta Crystallogr. 1972, B28, 2936.
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Figure 7. Small angle X-ray scattering diagrams: (a) PM4VP retinoate; (b) poly(ionene-6,3) retinoate; (c) PDADMAC retinoate.
Figure 8. Possible arrangements of the retinoic molecules with respect to the lamellar plane in the complex.
PDADMAC complex is a possible explanation for the difference in lattice parameter compared with the other two complexes. There are two alternative ways of explaining the much smaller long period of poly(ionene6,3) retinoate and PM4VP retinoate: The retinoate tails bound to the polyelectrolyte chains in adjacent layers must be either interdigitated and perpendicular to the lamellar surface (Figure 8b) or tilted with respect to the layers
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Figure 9. UV-vis spectra of retinoic acid complexes in methanolic solution: retinoic acid (solid line); PM4VP retinoate (dashed line); poly(ionene-6,3) (dotted line); PDADMAC retinoate (dash-dotted line). Table 1. UV-vis Data of Complexes in Methanolic Solution and in the Solid State
retinoic acid methanolic sol. PDADMAC retinoate (solution) PDADMAC retinoate (film) polyionene-6,3 retinoate (solution) polyionene-6,3 retinoate (film) PM4VP solution PM4VP retinoate (solution) PM4VP retinoate (film)
λmax,1 (nm)
λmax,2 (nm)
348 336 319 337 297
252
340 334
253 257 257 262
λmax,3 (nm)
226 226 226
without being interdigitated (Figure 8c). For lamellar mesophases of surfactants with alkyl chains, information about the orientation of the chains with respect to the surface can be obtained easily. For isomorphous lamellar structures the dependence of the long spacing on the number of methylene groups in the side chain has to be studied.7,9 Obviously, in the case of retinoate, this method could be ruled out. Therefore it is not possible to discriminate between the tilted or interdigitated structure on the basis of SAXS data. Supposing a tilt structure and taking 3.8 nm as the maximum spacing, a tilt angle between the lamella normals and the main axis of the retinoic molecules of about 38° is calculated for the polyionene complex and about 35° for the PM4VP complex. However, an interdigitated structure seems as likely. It is possible to discriminate between these refined models on the basis of UV-vis data. 3.5. UV-vis Spectroscopy. The strong influence of complexation on the optical properties is best demonstrated by comparing the UV-vis absorption spectra of the complexes in a film with the spectra of complexes in methanol solution (Table 1). The UV-vis spectrum of pure all-trans retinoic acid in methanol shows only one broad peak with a maximum at 348 nm. The spectra of redissolved complex films are very similar to that of pure all-trans retinoic acid. Only a small hypsochromic shift in the range from ∆λmax ) 8 nm (PM4VP retinoate) to 12 nm (PDADMAC retinoate) is found (Figure 9). From this it is concluded that in solution no significant chromophore interaction can be found, and the retinoate moieties behave like isolated chromophores. Assuming the hypsochromic shift as a qualitative measure for the binding constant, the following order for an increasing binding strength is obtained: PM4VP retinoate < poly(ionene-6,3) retinoate < PDADMAC retinoate. This sequence is in agreement with the DSC results, in which an increasing glass transition was found in the same order.
Figure 10. Comparison of UV spectra of methanolic solution and films of retinoic acid complexes; (a) PDADMAC retinoate; (b) polyionene retinoate; (c) PM4VP retinoate.
The absorption behavior of the complexes in films is very much different from that in solution (Figure 10): The spectrum of solid PDADMAC retinoate shows an additional absorption maximum at 252 nm, which is much more intense than the second one at 319 nm. Compared to the solution for the latter an additional hypsochromic shift of ∆λmax ) 17 nm is observed for the film. A very similar spectrum to that of PDADMAC retinoate was found for poly(ionene-6,3) retinoate films (Figure 10b). Characteristic is the absorption maximum at 253 nm. Again this maximum is considerably more intense than the second absorption band at 297 nm. The latter shows an additional hypsochromic shift of ∆λmax ) nm compared to the UV-vis of the complex in solution. The spectrum of PM4VP retinoate films is more structured; three maxima were found (Figure 10c). The additional absorption bands are due to the UV activity of the quaternized vinylpyridinium moiety. In contrast to the spectra of the two other complexes with UV inactive polyelectrolytes, a maximum at higher wavelength (334 nm) is predominate. The
Immobilization of Retinoic Acid
Figure 11. Binding properties of retinoids: (a) retinal in bacteriorhodopsin; (b) pocket model of PDADMAC retinoate complex.
spectrum shows a second maximum at 262 nm with a shoulder toward higher wavelengths and a third one at 226 nm. The low intensity of the higher energetic absorption in the region of 250-270 nm of PM4VP retinoate compared with the short-wavelength absorption of the two other complexes is attributed to a strong influence of the N-methylpyridinium chromophor on the retinoate. The same spectra are obtained after redissolution and subsequent film recasting. The data are summarized in Table 1. It is concluded that the strong difference in the absorption behavior between a complex in solution and in the solid state is caused by two effects: The first is a change in conformation. It is expected from solutions of retinoids that a stretched planar polyene chain is the preferred conformation as it is known for retinal.20,21 Further, the all-trans conformation is also found in the two crystal modifications of retinoic acid.19 For this conformation only one absorption maximum at 348 nm is observed. Higher energetic absorption maxima can occur when significant amounts of cis-conformation are introduced. For example, the absorption spectrum of 15-cisβ-carotene shows an additional shorter-wavelength absorption, which is not found in the spectrum of all-transβ-carotene. This is associated to the second harmonic absorption band which is symmetrically forbidden in the all-trans conformation. The higher-energetic absorption band in the solid state of retinoate complexes may be due to a significant amount of cis-configured retinoate moieties, and a significant amount of 11-cis or 13-cis conformation in the solid state is proposed (see Figure (1′ and 1′′). The latter is found in considerable amounts in bacteriorhodopsin. In rhodopsin as well as in bacteriorhodopsin a retinal moiety is bound to lysin via a protonated Schiff base. This structure has a conspicuous similarity to the complexes introduced here (compare Figure 11). Because of the high intensity of the higher-energetic absorption maximum in the solid state, it is concluded that in addition to a certain amount of cis-configured retinoates, packing effects strongly affect the absorption properties in the solid state. It is known that interaction of chromophores may alter the UV activity in a characteristic way. For example, Kunitake et. al.22 investigated double layers of a homologue series of cationic azobenzene surfactants. They found strong hypsochromic as well as bathochromic effects depending on the arrangement of chromophores in the double layers: For a parallel align(20) Rowan, R.; Warshel, A.; Sykes, B. D.; Karplus, M. Biochemistry 1974, 13, 970. (21) Wilbrandt R.; Jensen, N.-H. J. Am. Chem. Soc. 1981, 103, 1036. (22) Kunitake, T. Angew. Chem. 1992, 104, 692.
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ment of neighboring transition dipole moments (H aggregates) a hypsochromic effect was observed. A bathochromic effect was found for a skew alignment (J aggregates) of chromophores. Considering the lowerenergetic absorption of the complexes introduced here, a hypsochromic shift for the complex solutions as well as for all complexes in the solid state is found (see Table 1). The problem to discriminate between an interdigitated and skew alignment of the chromophores can be solved considering the molecule exciton model of Kasha23 which Kunitake applied to bilayers.22 According to this theory a hypsochromic shift in a bilayer compared to the isolated chromophore will be observed for a parallel alignment of neighboring transition dipoles. Oppositely, a bathochromic effect will be found for a skew alignment. A hypsochromic shift was observed for all of the three spectra, which is a strong indication of the interdigitated lamellar structure for poly(ionene-6,3) retinoate and PM4VP retinoate. Therefore, for both, the structure of the schematic drawing in Figure 8b is much more likely than that in Figure 8c. For the strong absorption maximum at 252 nm no simple explanation can be found. It can be assumed that in addition to the proposed 10-cis and 13-cis conformation, strong chromophor interactions are responsible for this high absorption intensity. Furthermore, it is interesting that a quenching of the high-energy maximum happens in favor of the lower energetic absorption in the presence of a second chromophor as is the case of PM4VP. 4. Summary and Conclusions The complexation of retinoic acid with structurally different cationic polyelectrolytes (integral, intermediate, and pendant type) results in the formation of new materials with interesting structural and optical properties as well as a possible new pharmaceutical formulation. Their main features are as follows: 1. The new mesomorphous complexes contain up to about 70% (wt/wt) optically active molecules. Due to strong chromophore interactions in the solid state the complexes show an additional strong high-energetic absorption at 252 nm. Furthermore, the solid state UVvis spectrum can be significantly influenced by additional chromophores, such as methyl-4-vinylpyridine, which opens the possibility of tuning the absorption characteristics. 2. The complexes may be simply cast into films with different lamellar structures, morphologically very similar to SA liquid crystals. 3. Depending on the polyelectrolyte structure, the glass transition temperature can be adjusted in the range between -19 and 28 °C and the mechanical properties cover a wide range, accordingly. From a pharmaceutical point of view the complexes can be regarded as a new formulation of a very active drug. It seems to be promising to evaluate whether the complexes show a reduced toxicity and teratogenicity compared to classical formulations. The complexes could be used asmade for the treatment of dermatological disorders such as acne, psoriasis, and hyperkeratosis. The formulation of these complexes as colloidal particles may be a promising way to exploit the whole pharmaceutical potential of retinoic acid, e.g., as a drug for the inhibition of malignanttumor growth. Retinoic acid ionically bound to very different polyelectrolytes is a promising material for biomimetic applications. It can be speculated that the complexes could (23) Kasha, M. In Spectroscopy of the exited state; Plenum Press: New York, 1976.
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be used as part of a photosynthetic system, pumping protons from the inside of a membrane to the outside and thereby forming an electrochemical gradient which is thought to promote the synthesis of ATP. But in any case the optical activity of natural photosensitive pigments is of considerable interest, since it can provide clues to the nature of the protein-chromophore interaction as well as to the conformational changes which occur subsequent to light absorption. It is likely that with the detailed investigation of uniaxially aligned multilamellar complex
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films, a contribution to understanding the molecular basis of the optical activity of complex natural systems will be made. Acknowledgment. The author wish to thank C. Remde for help during the preparation of the samples and C. Go¨ltner and M. Antonietti for helpful discussions. Financial support was provided by the Max Planck Society. LA970756K
