J . Phys. Chem. 1989, 93, 1532-1536
1532
30
25
P 1
lo[. 5
“0
’
2
4
6
8
10
&=/E K Figure 9. Parson-Zobel plot at (I = 0 for ai = 0.97 A, ai = 1.33 A: (-) = 1.8; (- - -) e = 78.5; A, full linear; B, linear dipole; ( 0 )Grahame’s” experimental points for Hg-aqueous NaF.
aj = 1.33 A. Usually the diffuse layer capacitance is calculated from the Gouy-Chapman theory. However, in our work no clear distinction can be made between the inner and diffuse layer capacitance, so from the equal ion size results and the work done with the MSA theory16 we take cd-’= h / t K . The Parson-Zobel graphs in Figure 9 have the same structure for both the linear and linear dipole theories. The plots are linear except at high
(16) Blum, L.; Henderson, D.; Parsons, R. J . Elecrroanal. Chem. 1984, 161, 389.
electrolyte concentration where the deviations are in the same direction as those observed experimentally. Agreement with Grahame’s’’ Hg-aqueous NaF experimental results can be made arising by adding a concentration-independent term to CD-’, possibly from the electrode, such as suggested in the MSA approach.I6 A difficulty with comparing the experimental and theoretical results, apart from the inherent approximations in the model and analysis, is that cd cannot be consistently specified. Conclusion A limiting feature of the mean field analysis is the dilute gas value of the permittivity of the pure solvent. This low dielectric constant restricts any serious application of the theory to essentially nonaqueous solutions. Irrespective of this restriction some interesting features occur in the theory. The different ionic and dipolar radii lead to asymmetric capacitances and nonzero pzc’s, while the nonlinear results for the equal ion size case indicate the importance of dielectric saturation effects when the solvent molecules are smaller than the ions. For the unequal ion size situation the linear dipole theory gives a good representation of the nonlinear theory when the counterions are smaller than the dipoles. The linear theory is inadequate in the unequal ion size case and is only accurate at the pzc for equal ion size. In spite of its shortcomings the linear theory gives the correct qualitative behavior of the Parson-Zobel plot as also seen in the MSA theory. The poor treatment of solvent effects leads to a lack of structure in the singlet distribution functions. Improvements can be made to the model by adding in higher order solvent moments, adsorption, etc. The main improvements though to the theory is in the application of a second-order closure rather than the first-order mean field closure. Acknowledgment. This work was partially supported by U S . Army Research Office, Grant No. DAAG29-85-G-0083. (17) G r a h a m , D. C. J . Am. Chem. SOC.1954, 76,4819; Ibid. 1957, 79, 2093.
Reduction of Rh3+ on Supported Rhodium Surfaces by CO C. P. Booker and J. T. Keiser* Chemistry Department, University of Richmond, Richmond, Virginia 231 73 (Received: June 3, 1988;
In Final Form: July 19, 1988)
-
The reduction of Rh3+supported on alumina by CO at room temperature has been observed by Fourier transform infrared spectroscopy. The reaction is proposed to be Rh3’ + 3CO + 20H- Rh’(C0)2 + C 0 2 + H20. A subsequent reaction of C 0 2 with alumina to form bicarbonate has also been observed. The surface OH species on alumina are shown to play an important role in both of the above reactions.
Introduction Numerous investigations of the CO/Rh/alumina system have been conducted.’-’* Most of these have been concerned with C.; Garland, C. W. J . Phys. Chem. 1957.61, 1504. (2) Basu, P.; Panayotov, D.; Yates, J. T., Jr. J . Phys. Chem. 1987, 91,
characterizing the CO-rhodium species that form under various conditions. Particular attention has been paid to the rhodium dicarbonyl, Rh(C0)2. This species is known to form on isolated
( 1 ) Yang, A.
3 133. ( 3 ) Rice, C. A,; Worley, S . D.; Curtis, C. W.; Guin, J. A,; Tarrer, A. R. J . Chem. Phys. 1981, 74, 6487. (4) Wang, H. P.; Yates, J. T., Jr. J . Catal. 1984, 89, 19. (5) Robbins, J. L. J . Phys. Chem. 1985, 90, 3381. (6) Yates, J. T., Jr.; Duncan, T. M.; Worley, S . D.; Vaughan, R. W. J . Chem. Phys. 1979, 70, 1219. (7) Yates, J. T., Jr.; Duncan, T. M.; Vaughan, R. W. J . Chem. Phys. 1979, 7 1 , 3809. (8) Cavanagh, R. R.; Yates, J. T., Jr. J . Chem. Phys. 1981, 74, 4150. (9) Yates, J. T., Jr.; Kolasinski, K. J . Chem. Phys. 1983, 79, 1026. (10) Yates, J. T., Jr.; Williams, E. D.; Weinberg, W. H. Surf. Sci. 1980, 91, 562.
(11) Solymosi, R.; Erdohelyi, A. Surf. Sci. Lett 1981, 110, L630. (12) Bilhou, J. L.; Bilhou-Bougnol, V. B.; Graydon, W. R.; Basset, J.; Smith, A. K.; Zanderigi, G. M.;Ugo, R. J. Organomet. Chem. 1978, 153,73. (13) Solymosi, F.;Pasztor, M. J . Phys. Chem. 1985, 89, 4789. (14) Solymosi, F.;Pasztor, M. J . Phys. Chem. 1986, 90, 5312. (15) Smith, A. K.; Hugues, F.; Theolier, A.; Basset, J. M.; Ugo, R.; Zanderighi, G. M.; Bilhou, J. L.; Bilhou-Borgnol, V.; Graydon, W. R. Inorg. Chem. 1979, 18, 3104. (16) Van’t Blik, H. F. J.; Van Zon, J. B. A. D.; Huizinga, T.; Vis, J. C.; Koningsberger, D. C.; Prins, R. J . Phys. Chem. 1983, 87, 2264. (17) Van’t Blik, F. J.; Van Zon, J. B. A. D.; Koningsberger, D. C.; Prins, R. J . Mol. Catal. 1984, 25, 379. (18) Van’t Blik, H. F. J.; Van Zon, J. B. A. D.; Huizinga, T.; Vis, J. C.; Koningsberger, D. C.; Prins, R. J . Am. Chem. SOC.1985, 107, 3139.
0022-3654/89/2Q93-1532$01 .5Q/Q 0 1989 American Chemical Society
The Journal of Physical Chemistry, Vol. 93, No. 4, 1989 1533
Reduction of Rh3+ on Rhodium Surfaces by C O Rh atoms for which the addition of two CO molecules is not sterically hindered. It is typically produced by dispersing a small amount of a Rh3+ salt on an alumina support, reducing the Rh to the zero oxidation state by baking the surface in hydrogen, and then exposing the reduced surface to CO(g). Although the rhodium is in the zero oxidation state before exposure to CO, it is reported to be in the +1 oxidation state in the Rh(C0)2 s p e c i e ~ . ~ ~ ~Several * ~ ~ l groups ~ ' * have proposed mechanisms by which metallic rhodium is oxidized by C O and surface hydroxyl group^.'^-'^ Recently, Yates et al. reported direct spectroscopic evidence for the consumption of surface O H groups on alumina to the formation of Rh+(C0)2in the presence of CO(g).2 This was interpreted as being due to an oxidation/reduction reaction between surface O H and a RhO(C0). This observation documented the importance of the support OH groups on the surface chemistry of rhodium. Although much work has been done regarding the reactions of C O with a reduced Rh/alumina surface, comparatively little work has been done concerning the reactions of C O with an unreduced Rh3+ alumina surface. In the original work on the investigation of the CO/Rh/alumina system,' RhC13/alumina slurries were pretreated in three different ways: (a) Hz at 150 OC; (b) CO at 150 OC; (c) baking in vacuo at 450 OC. Samples were subsequently exposed to room-temperature CO, and the Rh(C0)2 species described above was observed. Since the starting material was Rh3+, and since R h ( C 0 ) 2 is reported to have an oxidation state of + 1, it must be assumed that under the conditions of (b) rhodium is reduced by reaction with CO. Smith et al. have also investigated the adsorption of C O onto RhC13 supported on alumina.Is They observed that exposing a Rh3+/alumina sample to 20 Torr of CO at room temperature produced a spectrum that exhibited a strong band at 2135 cm-I. This band was attributed to C O coordinated to a Rh3+ species. Smaller bands were observed at 2088 and 2016 cm-I, which the authors attributed to the Rh+(C0)2 species. Upon treatment of the same sample with 200 Torr of C O at 60 OC for 6 h, bands due to the Rh(C0)2 species greatly increased and the bands due to the Rh3+/C0 species decreased. Further reduction of the Rh+ species with hydrogen was not observed at 60 OC, but after treatment with hydrogen at 100 OC, bands due to C O adsorbed onto metallic rhodium were observed. If the completely reduced surface was treated with oxygen followed by CO, the Rh+(CO)z species was regenerated. Similar results were obtained by Rice et al.3 They treated an evacuated, unreduced RhC13.H20/alumina catalyst with 50 Torr of C O and observed only the band at 2135 cm-I previously observed by Smith et al. After evacuation and reexposure of the sample to 50 Torr of C O at 50 OC, the band at 2135 cm-l was still the dominant peak but small bands at 2098 and 2029 cm-' were observed due to the formation of some Rh+(C0)2. Further reduction in C O at higher temperatures led to an increase in the 2098- and 2029-cm-' bands and a decrease in the intensity of the 2135-cm-' band, similar to what Smith et al. observed. Finally, the sample was reduced in hydrogen at 150 OC and exposed to 50 Torr of C O at 25 OC. In contrast to the work of Smith et al., reduction in hydrogen at 150 OC did not completely remove the band at 2135 cm-'. In conclusion, the literature clearly indicates that a reduction reaction between Rh3+adsorbed on alumina and CO(g) does occur to produce Rh+(CO)> However, every few details of this reaction are documented, including the identification of all of the reactants and products. In the present work, we report direct spectroscopic evidence for the following reaction: Rh3+ + 3CO
+ 20H-
-
Rh+(C0)2 + C 0 2
+ HzO
Similar reactions have been observed previously in aqueous solution,19 as well as for rhodium supported on titaniaz0 and on zeolite,21but to our knowledge this is the first time this reaction (19) James, B. R.; Rempel, G. L.; Ng, F. T. T. J . Chem. SOC.A 1969, 2454. (20) Conesa, J. C.; Sainz, M. T.; Soria, J.; Munuera, G.; River-Arnau, V.; Munoz, A. J. Mol. Carol. 1982, 17, 231.
YI
V
z
d
I I 4000
a3.D
2770
1770 1470 1104 WAVENUMBERS CCM-I>
2104
0%
400
Figure 1. IR spectrum of a Rh'+/alumina surface: (a) after overnight bake at 180 OC;(b) 20 s, (c) 4 h, and (d) 2 weeks after exposure to 50 Torr of CO.
has been documented to occur on an alumina surface. Also, this is the first time that direct experimental evidence for the formation of the products and the consumption of the surface OH groups has been given. Additionally, we report evidence for a reaction intermediate consisting of Rh3+ bound to a surface oxygen on alumina.
Experimental Section RhCl3.3H20 purchased from Alfa mixed with alumina (aluminum oxide C, Degussa), distilled water, and acetone to produce a slurry that was 2.0 wt % of Rh. This mixture was sprayed onto a CaF2 plate that was heated to approximately 50 OC. This "flash evaporation" technique produced a thin self-supporting film weighing around 30 mg per plate. In some cases, only one-half of the plate was sprayed with the rhodium/alumina mixture and the other half of the plate was sprayed with a similar weight of alumina, which was used as an internal reference. The plate was placed in a stainless steel vacuum cell with CaF, windows. The design of this cell has been described previ~usly.~ The vacuum system used was an all welded bakable stainless system pumped by a turbomolecular pump backed by a mechanical pump. The base pressure of the system was around 1 X lo-' Torr. The sample cell was baked overnight at 180 OC while pumping to remove water. After being allowed to cool, the cell was lowered into the sample compartment of the infrared spectrophotometer. The surface was probed by transmission FTIR with a Nicolet 5DX system with 4-wavenumber resolution. Results and Discussion The infrared spectrum taken of the Rh3+/alumina surface after the overnight bake is shown in Figure la. The broad band observed at 3560 cm-' is caused by the vibrations of the OH species in the alumina support. Also evident are small bands at around 2080 (Rh-CO, due to a small amount of C O contamination), 1588, and 1469 cm-' (due to the alumina support). At this point, the sample cell was filled with 50 Torr of C O and then closed off. Initially, spectra were taken as rapidly as possible and then roughly once a day for several weeks. The spectra taken after 20 s, 4 h, and 2 weeks are shown in Figure 1b-d. To accentuate the changes produced by the reaction, the t = 0 spectrum was subtracted from the spectrum taken at t = 2 weeks. This is shown in Figure 2. The most intense features of this spectrum are the bands at 2101 and 2031 cm-I, which are due to the symmetric and asymmetric stretches of Rh+(C0)2. The shoulder at 2131 cm-' is most likely due to C O coordinated to a Rh3+species as reported previo~sly.'~The two bands between 2380 and 2300 cm-' dissappeared when the cell was evacuated and are assigned to gas-phase COz. The leak rate was measured at the end of the experiment and found to be negligible. In (21) Primet, M.; Vedrine, J. C.; Naccache, C. J. Mol. Caral. 1978,4,411.
The Journal of Physical Chemistry, Vol. 93, No. 4, 1989
1534
Booker and Keiser
r
Rh+l(C0)2 VS Time
- 1.2 1
I
0.1 Abs.
Ly
U '
z
U
m
8' 2
Gas Phase CO:,
v) v)
Adsorbed H20 AdsorbedH20
,
1
1
a,
n Surface OH
4050
3s74
a002
zeio
a12e
1823
WAVENUMBERS
1582
1941
980
0.4
CCM-1)
4L
Figure 2. Subtraction of the spectrum shown in Figure la from the spectrum shown in Figure Id. Y . l
addition, the intensity of the COz peak was found to level off with time, and the final intensity was found to be roughly proportional to the rhodium loading to the catalyst. Therefore we feel certain that the C 0 2 was produced by a reaction with the catalyst surface. The three bands at 1637, 1462, and 1233 cm-I grew in at approximately the same rate. These bands are similar to those reported for surface b i c a r b ~ n a t e s . ~ ~ParkynsZ2 -~~ has reported that bicarbonate can be formed with the reaction of COz with alumina, which was identified by characteristic bands at 1236 (C-0-H bend), 1480 (C-0, symmetric stretch), 1642 (C-0 assymmetric search), and 3605 cm-' (0-H stretch). Although we do not observe a band at 3605 cm-', this is the smallest of the bicarbonate bands, and in our case it is probably obscured by the broad band due to the 0 - H stretch of adsorbed water. In addition to the features mentioned above, there are changes in the O H region caused by the exposure of the sample of CO. The negative-going bands at 3741 and 3682 cm-' have been reported to be due to "surface OH" species on alumina.2-26 The fact that they are negative indicates that they were consumed by the reaction. The broad band around 3450 cm-' is very similar to the most intense band observed in our water-saturated (unbaked) alumina samples. Therefore, we have assigned this band to adsorbed water. Although the band due to the scissors mode of water at approximately 1624 cm-' is not observed, it most likely is obscured by the bicarbonate band at 1637 cm-I. In some experiments we have observed a shoulder on the bicarbonate band at around 1624 cm-' which is presumed to be due to the scissors band of water. On the basis of these observations, we conclude that the following reaction occurred: Rh3+ + 3CO + 2 0 H -
-+
Rh+(C0)2 + COZ + H 2 0 (1)
Some of the COz produced by this initial reaction is subsequently removed by a reaction with the alumina substrate: C 0 2 + OH-
-
HC03-
(2)
Although we have indicated in this second reaction that ionic bicarbonate is formed, it is not possible to tell from the infrared spectra if this species is ionic bicarbonate or if it is covalently bound.22 A plot of the peak area of the Rh+(CO)z envelope (between 221 3 and 1945 cm-l) versus time is given in Figure 3. Although this peak was over 30% developed in 20 s, it took over 2 weeks for it to achieve its full intensity. The relative changes in the intensity of the bands due to surface OH, adsorbed water, and (22) Parkyns, N. D.J . Chem. SOC.A 1969, 410. (23) Parkyns, N. D. J . Phys. Chem. 1971, 75, 526. (24) Rethwisch, D.G.; Dumsic, J. A. Langmuir 1986, 2, 7 3 . (25) Baumgarten, E.; Zachos, A. Specrrochim. Acta 1981, 3 7 4 93. (26) Peri, J. B.; Hannan, R. B. J . Phys. Chem. 1960, 64, 1526.
100
0
200
300
400
Time (hr)
Figure 3. Plot of the normalized peak area of the Rht(C0)2envelope versus the time of exposure to 50 Torr of CO.
3088
3785
8 ~ 8 0
a377
WAVENUMBERS
a174
Ccm-l>
Figure 4. IR spectra of (a) alumina, (b) 2.2% Rh3t/alumina after overnight bake at 180 OC, (c) 2.2%Rh3+/aluminaafter 2 weeks of exposure to 50 Torr of CO.
Rh+(C0)2were similar over the course of the experiment. This implies that the same reaction produced Rh+(CO)zthe entire time. However, such a drastic change in the reaction rate implies that this reaction did not proceed by the same mechanism the entire time. We believe that this was due to a reaction between some of the Rh3+ ions and surface O H sites on the support during the slurry-and-bake phase of the sample workup. This idea was investigated by preparing a CaFz plate coated on one-half with 2.2 wt % Rh3+/alumina and on the other half with pure alumina as a blank. After this sample was baked overnight the spectra shown in Figure 4a,b were obtained. It is obvious that the intensity of the band due to the surface OH species (at around 3680 cm-') is significantly decreased on the Rh3+/alumina side compared to what is observed for the pure alumina side. This was probably due to the following reaction: H
Rh3+C13
+
I
'
Rh
II
' C
A-l-rLT
I -t H C I ( g )
(3)
(A similar reaction in which the Rh3+ forms a dative bond to the oxygen of the surface O H could also be a possibility. This type of species might cause the surface O H band to be broadened to the point of being unobservable.) The resulting Rh3+-O-AI species apparently reacts rapidly with CO at room temperature. As these
The Journal of Physical Chemistry, Vol. 93, No. 4, 1989 1535
Reduction of Rh3+ on Rhodium Surfaces by C O Rh+l(C0)2 VS Time
"-
I
1.0-
I-
m
b
2
.
*.
* m
.
D
.
4 y
0.8.
m
a U
.--8 m E
06.:
2.0% 1
1.0%
=
04 1
0.5%
u
4000
same
2770
PI04
1770
1470
WAVENUMBERS
1104
ese
400
CM-1)
Figure 6. IR spectra of (a) alumina and (b) 2.2% Rh3+/alumina,after 15 days of exposure to 50 Torr of CO. 02 100
Time (hr)
Figure 5. Plot of the normalized peak area of the Rh+(C0)2envelope as a function of the time of exposure to 50 Torr of CO for 0.5% Rh'+/alumina, 1% Rh3+/alumina,and 2% Rh3+/alumina.
species are depleted, the remaining Rh3+ probably reacts with surface OH by a slow diffusion limited process. This idea is supported by the fact that surface OH is consumed both in the sample preparation stage and during the reaction phase as shown in Figure 4. If the spectrum of the OH'S consumed during the bake phase (inset, Figure 4) are compared with the surface OH'S consumed after the sample was exposed to CO (Figure 2), they appear to be very similar (Le.? the same "kind" of surface OH species reacted in both stages). If the total decrease of the decrease of the area of the surface OH peaks is quantitatively measured (and corrected for the slightly different loadings) it is found that approximately 75% of the peak area is lost during the bake phase and about 25% is lost after the sample is exposed to CO. These percentages are in agreement with the observation that the reaction dramatically slowed down after about 60-75% of the Rh+(C0)2 product was formed as shown in Figure 3. Further evidence for the presence of a R h 3 + U A l intermediates is the effect of loading on the rate of the reaction. If the reaction rate was controlled by the Rh3+ concentration, then decreasing the loading should decrease the rate of the reaction. However, exactly the opposite was observed as is shown in Figure 5 . The time for the reaction to reach 80% completion was about 5 min if a sample of 0.5% loading was used, whereas for a sample with 2% loading it was 44 h! Another possible explanation for the increase in the reaction rate with a decrease in the Rh3+ loading might be the decrease in the concentration of the surface OH groups caused by reaction with Rh3+. However, if the concentration of the surface OH was critical to the reaction rate, then an unbaked sample (which should have the highest concentration of surface OH) should react faster than baked samples. Yet, just the opposite has been observed, and the reduction of Rh3+ by CO reportedly proceeds very slowly if at all at room temperature for unbaked catalyst^.^^'^ Therefore, our interpretation of the increased rate of the reaction for lower loadings is based on the effect that the loading has on reaction 3. In other words, in the case of the 0.5% loading, most of the Rh3+ ions were able to react with a surface OH group during the sample preparation "bake". As a result, when this sample was exposed to CO, the reaction proceeded rapidly to completion at room temperature. In contrast, for the cases of the higher loadings, some of the Rh3+ species reacted with a surface O H group during the sample "bake", but a significant percentage did not. When these samples were exposed to CO(g) at room temperature, this resulted in an initial rapid reaction followed by a slow diffusion limited reaction. Our experiments indicate that the Rh3+-O-AI species may require elevated temperatures to form (in our case the bake is
performed at 180 "C), but once formed it will react rapidly with C O at room temperature. This interpretation is consistent with the literature. For example, both Smith et aI.l5 and Rice et aL3 using samples that were not baked before exposure to C O obtained spectra similar to our Figure Id only if the experiments were conducted at elevated temperatures (60-100 "C). If their samples were prepared by evacuation at room temperature (Le., without baking the sample n the preparation phase), then they observed that essentially no reduction of Rh3+ by C O occurred. Yet, if Rice et al., prebaked their samples at 150 "C they obtained spectra similar to ours at room temperature. This is because the Rh3+-O-Al species was formed during their prebake at 150 "C, and that species reacts rapidly with C O at room temperature as we have shown. These experiments demonstrate that the presence of this Rh3+-O-Al species can be indicated by its room-temperature reaction with CO. It should be noted, however, that the formation of this species does not require CO, only the heating of Rh3+C13 and alumina. It seems likely, therefore, that this species may play a significant role in many other reactions involving Rh3+C13and alumina.
Reaction of COz with Alumina A secondary reaction that we observed in these experiments was that of C 0 2 with alumina to form surface bicarbonate. This reaction has been reported p r e v i o ~ s l y . ~In ~ -our ~ ~ experiments, however, this reaction was observed to be dramatically affected if the alumina is mixed with 2.2% Rh and baked at 180 "C as described in the Experimental Section. This can be seen in Figure 6, in which we compare the reaction of CO (and subsequently C02) with a pure alumina surface vs. a 2.2%Rh/alumina surface contained in the same cell. After 15 days of exposure to 50 Torr of C O at room temperature (and to whatever levels of C 0 2that were produced) the bicarbonate bands on the alumina side were approximately 10 times bigger than those seen on the Rh/alumina side. Yet both sides were exposed to equivalent amounts of C02. It is known that loadings of this percentage produce mostly isolated atoms of Rh. Therefore, this suppression of the C02/alumina reaction is not likely to be due to simply coating a large area of the alumina surface with Rh. A significant difference between the alumina side and the Rh3+/alumina side is that virtually all of the surface OH species that give rise to the band at 3682 cm-', as well as a significant amount of the OH species which absorbs at 3741 cm-', have reacted on the Rh3+/alumina side (Figure 4). Therefore, this difference in reactivity with C 0 2 is probably caused by the absence of these surface OH species on the Rh3+/alumina side. Conclusions The reaction Rh3+ + 3CO + 2 0 H - -,Rh+(C0)2 + COz + H,O has been observed to occur at room temperature for Rh3+
1536
J. Phys. Chem. 1989, 93, 1536-1542
supported on alumina. The appearance of all of the products and the disappearance of one of the reactants (OH-) can be simultaneously followed by FTIR. RhC13 has been shown to react with surface O H groups on alumina at 180 “C. The product of this reaction is believed to be Rh3+ bonded to a surface oxygen on alumina. This species is an intermediate in the room-temperature oxidation of C O listed above. Removal of the surface O H groups by reaction with RhC1,
dramatically suppresses the reaction of COz with alumina. Acknowledgment. This work has been funded in part by the Office of Naval Research, and the University of Richmond Faculty Research Committee. We thank Dr. John Yates for his helpful comments and for his assistance in designing the experimental setup and Hampton Rexrode for his work on the apparatus construction. Registry No. Rh3*, 16065-89-7; CO, 630-08-0.
Sodium Glyco- and Taurodeoxycholate: Possible Helical Models for Conjugated Bile Salt Micelles A. R. Campanelli,+S. Candeloro De Sanctis,+ E. Chiessi,t M. D’Alagni,t*tE. GigIio,*,+ and L. Scaramuzzas Dipartimento di Chimica, Universitd di Roma “La Sapienza”, P . le A . Moro 5, 00185 Roma, Italy, Centro di Studio per la Chimica dei Recettori e delle Molecole Biologicamente Attive del C.N.R, c/o Istituto di Chimica. Universitd Cattolica, Largo F. Vito 1, 00168 Roma, Italy, and Istituto di Teoria e Struttura Elettronica e Comportamento Spettrochimico dei Composti di Coordinazione del CNR, Area della Ricerca, CP 10, 00016 Monterotondo Stazione. Roma, Italy (Received: June 6, 1988)
Previously a helical model was satisfactorily verified for sodium (NaDC) and rubidium (RbDC) deoxycholate micelles in aqueous solutions by means of SAXS, EXAFS, NMR, ESR, and CD measurements. Here we report the beginning of an analogous study carried out on sodium glycodeoxycholate(NaGDC) and taurodeoxycholate (NaTDC) following the strategy applied to NaDC and RbDC. The crystal structure of NaGDC sesquihydrate, solved by X-ray analysis, provides again a helical model that is compared with those of NaDC, RbDC, and NaTDC. Since it was previously observed that bilirubin-IXa (BR) exhibits a bisignate CD Cotton effect in NaDC aqueous solutions, and it was suggested that the chiral micelles of NaDC interact preferentially or exclusively with one of the two enantiomeric conformers of BR, we have recorded CD spectra of BR in aqueous micellar solutions of the bile salts in order to check the helical models. The spectra show in all cases two large and proximate bands of opposite sign between 400 and 500 nm, which support both our chiral models and the selection of the BR left-handed enantiomer. Moreover, we have accomplishedvan der Waals energy calculations for the system formed by a NaGDC helix and the left- or right-handed BR molecule to test if the interaction energy is lower for one of the two BR enantiomeric conformers. The results of the calculations seem to indicate that the NaGDC helix binds preferentially the BR molecule with left-handed chirality. Interaction models are proposed.
Introduction Studies carried out on sodium and rubidium deoxycholate (NaDC and RbDC, respectively) following a strategy previously reported’ showed that a helical model, observed in the crystal structures of NaDC and RbDC,2v3describes very satisfactorily the behavior of their micellar aggregates in aqueous solutions.’*e7 The helical structure of the NaDC and RbDC micelles was verified unambiguously by nuclear magnetic resonan~e,’.~ circular dichroism! electron spin resonance! small-angle X-ray scattering,6 and extended X-ray absorption fine structure7 measurements. Subsequently, the investigation of the structure of the sodium taurodeoxycholate (NaTDC) micellar aggregates was undert a k e ~ ? +and ~ other helical models were identified. However, although there are some evidences in favor of the NaTDC helical models, these models were not yet definitively proved, contrary to the case of NaDC and RbDC. This paper deals with the crystal structure of sodium glycodeoxycholate (NaGDC), which behaves in a similar manner to NaDC and NaTDC, giving rise from its aqueous micellar solutions to gels, macromolecular fibers, and crystals.’v8 The NaGDC crystal structure was judged of great interest for the following reasons: (a) It is the first crystallographic study of a common Dipartimento di Chimica, Universitl di Roma “La Sapienza”.
*Centra di Studio per la Chimica dei Recettori e delle Molecole Biologicamente Attive del CNR c/o Istituto di Chimica, Universitl Cattolica. 8 Istituto di Teoria e Struttura Elettronica e Comportamento Spettrochimico dei Composti di Coordinazione del CNR.
0022-365418912093-1536$01.50/0
glycine conjugated bile salt. (b) NaGDC is a biological compound of paramount importance since the majority of bile salts in man are conjugated with glycine. (c) If the NaGDC structural unit in the crystal and in the micelle is a helix, the idea that the micelles of the bile salts are all helices would be reinforced. (d) It is often assumed that the micelles of bile salts can be represented by a unique model. The knowledge of the NaDC, NaTDC and NaGDC micellar structures could both clarify this point and account for the similar and dissimilar physical-chemical properties of these bile salts. As subsequent step the interaction between bilirubin-IXa (BR) and the bile salt micelles was investigated. BR and its conjugates, (1) Conte, G.; Di Blasi, R.; Giglio, E.; Parretta, A,; Pavel, N. V . J. Phys. Chem. 1984,88, 5720. (2) Campanelli, A. R.;Ferro, D.; Giglio, E.; Imperatori, P.; Piacente, V. Thermochim. Acta 1983, 67, 223. (3) Campanelli, A. R.; Candeloro De Sanctis, S.; Giglio, E.; Petriconi, S. Acta Crystallogr., Sect. C 1984, C40, 631. (4) D’Alagni, M.; Forcellese, M. L.; Giglio, E. Colloid Polym. Sci. 1985, 263, 160. ( 5 ) Esposito, G.; Zanobi, A,; Giglio, E.; Pavel, N. V.; Campbell, I. D. J . Phys. Chem. 1987, 91, 83. (6) Esposito, G.; Giglio, E.; Pavel, N. V.; Zanobi, A. J . Phys. Chem. 1987, 91, 356. ( 7 ) Giglio, E.; Loreti, S.; Pavel, N. V. J . Phys. Chem. 1988, 92, 2858. (8) Campanelli, A. R.; Candeloro De Sanctis, S.;Giglio, E.; Scaramuzza, L. J . Lipid Res. 1987, 28, 483. (9) D’Alagni, M.; Giglio, E.; Petriconi, S. Colloid Polym. Sci. 1987, 265, 517.
0 1989 American Chemical Society
