3434
J . Phys. Chem. 1989, 93, 3434-3436
Optical Rotatory Dispersion of Saccharides: Testing a Theory Eugene S. Stevens,*,+Bangalore K. Sathyanarayana,+.sand Edwin R. Morris* Department of Chemistry, State University of New York. Binghamton, New York 13901, and Department of Food Research and Technology, Cranfeld Institute of Technology, Silsoe College, Silsoe, Bedford, MK45 4DT, U.K. (Received: June 20, 1988; In Final Form: September 29, 1988)
We have previously tested a novel semiempirical theory of saccharide optical activity by its ability to reproduce molar rotations at the single wavelength of the NaD line. The present work is a more extensive comparison of theory and experiment which includes the dispersion of optical rotation.
Introduction The chemistry of sugars and the phenomenon of optical activity have been intimately connected historically,’,* and there has been a systematic improvement in empirical methods of interpreting optical rotation data, beginning with the early rules of Hudson3 and continuing with the methods of Whiffen: B r e ~ s t e r ,Lemieux ~,~ and Brewer,’ and others. There has been a parallel development of optical activity theory, but its applications have mainly been to molecules containing P-electrons. For example, sector rules and calculational models are available for ketones, amides, purines, and pyrimidine^,^^^ but saturated compounds, with their higher energy electronic transitions, have received less attention: The development of photoacoustic modulators now allows easy production of vacuum-UV circularly polarized radiation, and there is a growing amount of experimental vacuum-UV circular dichroism (CD) data.”I2 It is therefore timely to direct renewed attention to the origin of optical activity in saccharides. Saccharide C D measured in solution to approximately 160 nm13,14is useful for conformational studies of polysaccharide^,'^ but it is too weak to account for the observed optical rotatory dispersion (ORD) and is usually of the wrong sign to do so. There is evidently a strong background contribution to O R D from electronic transitions at even higher energy. We have recently described a semiempirical theory of saccharide optical activity which includes explicit reference to these highenergy electronic transitions.I6 It is a Kirkwood model” which has been modified so that the bond-localized transitions, rather than being included through the polarizability, are allowed first to interact among themselves via perturbation theory. The local symmetry provided by tetrahedral carbon atoms in a puckered ring is sufficient to generate states of well-defined symmetry with a characteristic energy spectrum. In particular, a low-energy component consisently appears near 145-165 nm, which is well-separated from other higher energy components and whose rotational strength consistently matches the sign of the observed optical rotation.16 A Kronig-Kramers transform of the CD generates an ORD, the value of which at 589 nm is the NaD molar rotation [MI. The theory has been applied to monosaccharides,18 pyrano~ides,’~ small cyclic ethers,I9 cyclohexane-polyols,20 and the maltose and cellobiose disaccharides.2’ In comparison of theory with experiment, the primary criterion has been the agreement between calculated and observed NaD molar rotations. In its present form the theory yields almost quantitative accuracy (see below). Direct comparison of the calculated high-energy CD with experiment is still limited because the intense CD bands predicted by theory lie outside the range of solution studies. A few measurements have been made on saccharide films to approximately 135 nm, and these reveal a strong CD band near 150 nm which consistently has the same sign as the NaD molar r ~ t a t i o n . ’ ~ , ‘State Universitv of New York. ‘Silsoe College. . Present address: Frederick Cancer Research Facility, NCI, Frederick, M D 21701.
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TABLE I: Calculated and Observedu NaD Molar Rotations for Methylpyranosides (Unitsof deg cm2 dmol-I) [MIobd hexopyranosides
methyl-8-D-mannopyranoside methyl-P-o-glucopyranoside methyl-8-o-galactopyranoside methyl-a-o-mannopyranoside methyl-a-o-glucopyranoside methyl-a-o-galactopyranoside pen topyranosides methyl-8-o-arabinopyranoside met hvl-B-o-xvloovranoside , , methyl-a-o-xylopyranoside I
-89 -32 53 203 28 1 368
-177 -113 -36 183 309 385
.,
-136 -66 4 232 344 417
-120 -60 22 194 296 392
-136 -66 0 154 309 380
-266 -57 +324
-403 -108 +253
But no CD measurements on saccharides have been made in the region below 135 nm where the sum rule requires, and the theory predicts, other CD transitions. ORD data, however, are available for some saccharide^,^^ and we therefore make here a direct comparison with calculated ORD.
Calculational Methods The model of optical activity we use has been described in detail previously.16 We here apply it to eight methylpyranosides. The (1) Fischer, E. Ber. 1895, 28, 1145-1167.
Lowry, T. M. Optical Rotatory Power; Dover: New York, 1964. Hudson, C. S. J . Am. Chem. SOC.1925, 47, 268-280. Whiffen, D. H. Chem. Ind. (London) 1956, 964-968. Brewster, J. H. J . Am. Chem. Soc. 1959,81, 5483-5493. Brewster, J. H . In Topics in Sfereochemistry; Allinger, N . L., E M , E. L., Eds.; Interscience: New York, 1967; Vol. 2, pp 1-72. (7) Lemieux, R. U.; Brewer, J. T. Adu. Chem. Ser. 1973, No. 117, 121-1 46. (8) Bouman, T. D.; Lightner, D. A. J . Am. Chem. SOC. 1976, 98, 3145-3154. (9) Woody, R. W. In ORD and CD: Theory, Chemical Practices and Biochemical Applications; Ciardelli, F., Mason, S. F., Salvadori, P., Snatzke, G., Eds.; Wiley: New York, 1983. (10) Johnson, W. C., Jr. Annu. Rev. Phys. Chem. 1978, 29, 93-114. (1 1) Stevens. E. S. In Aoolications of Circularlv Polarized Radiation Using Synchrotron and Orkiary Sources; Allen, F.: Bustamante, C., Eds.; Plenum: New York, 1985; pp 173-189. (12) Stevens, E. S. Photochem. Photobiol. 1986, 44, 287-293. (13) Nelson, R. G.; Johnson, W. C., Jr. J . Am. Chem. SOC.1976, 98, 4290-4295. (14) Nelson, R. G.; Johnson, W. C., Jr. J . Am. Chem. SOC.1976, 98, 4296-4301. ( 1 5) Stevens, E. S . In Industrial Polysaccharides;Stivala, S . S . , Crescenzi, V., Dea, I. C. M., Eds.; Gordon and Breach: New York, 1987; pp 255-265. (16) Stevens, E. S.; Sathyanarayana, B. K. Carbohydr. Res. 1987, 166, 181-193. (17) Kirkwood, J. G. J . Chem. Phys. 1937, 5, 479-491. (18) Stevens, E. S.; Sathyanarayana, B. K. Biopolymers 1988, 27, 41 5-421. (19) Sathyanarayana, B. K.; Stevens, E. S . Carbohydr. Res. 1988, 181, 223-228. (20) Sathyanarayana, B. K.; Stevens, E. S. J . Org. Chem. 1987, 52, ~3170-3171. ~(21) ~ ~Stevens, ~ ~ ~ E. ~ S.; Sathyanarayana, B. K. J . Am. Chem. Soc., in press. (22) Buffington, L. A,; Stevens, E. S.; Morris, E. R.; Rees, D. A. Inr. J. Biol. Macromol. 1980, 2 , 199-203. (23) Morris, E. R.; Stevens, E. S.;Frangou, S.A,; Rees, D. A. Biopolymers 1986, 25, 959-973. (24) Listowsky, I.; Avigad, G.; Englard, S . J . Am. Chem. Soc. 1965, 87, 1765-1771. (2) (3) (4) (5) (6)
0 1989 American Chemical Society
The Journal of Physical Chemistry, Vol. 93, No. 9, 1989 3435
i
200
300
400
500
600
A, nm Figure 1. Observed2‘ (-) and calculated (---) ORD of a- and fianomers of methyl-D-glucopyranoside and methyl-D-galactopyranoside.
-
I
\ \
I
\
does not justify such an adjustment. We note that our results account for one of Hudson’s early rules of saccharide optical r ~ t a t i o nLe., , ~ that an a-pyranoside rotation is approximately 380 deg cm2 dmol-’ more positive than that of the corresponding @-pyranoside, independent of sugar. The agreement for methyl-a-D-mannopyranoside is not as good as for the other five hexopyranosides and cannot be improved by adjusting the statistical weights of the hydroxymethyl conformers (Table I). Whiffen’s4 and Brewster’ss empirical treatments produce similar results; for all three methods the error for methyl-a-D-mannopyranoside is approximately twice as large as the standard deviation for the other five hexopyranosides. Although this feature may be a coincidentally similar reflection of the general uncertainties in the three analyses, we note that trans-diaxial C-1 and C-2 substituents may permit the C-1 methoxy group to adopt a tg conformation to some small extent, which in the other five structures would be even more strongly disallowed. In terms of the empirical treatments, this feature would invalidate the use of the same parameter for methoxy contribution to rotation for all six sugars. Our method allows a test of the suggestion by direct calculation. We found the tg methoxy conformer to make a strong negative average contribution of -1 17 deg cm2 dmol-I, so that approximately 10% of such a conformer would bring the calculated and observed values into agreement. Our empirical scale factor21 was determined from the results for six hexopyranosides, assuming that only the gt methoxy group conformer was present. In empirical treatments4’ the rotation of pentopyranosides is less accurately reproduced than that of hexopyranosides, with standard deviations even greater than the error for methyl-a-Dmannopyranoside. We find the same result (Table I). In our method a partial explanation comes from the use of hexopyranosides to establish our empirical scale factor (see above); Le., a larger basis would have distributed the error more evenly. A similar reason does not explain why the empirical treatments give the same behavior. The lack of a hydroxymethyl group at C-5 may allow some feature of conformational flexibility not incorported into any of the methods. One possibility is alternative ring forms. The comparison of calculated and observed ORD for four hexopyranosides (Figure 1) is also good. (The ORD of methyl-@-D-mannopyranosidewas not reported in ref 24.) On the scale of that figure the inaccuracies in rotation at the NaD line are too small to be seen. As the wavelength decreases, differences between the calculated and observed ORD become apparent for all four compounds but can be attributed to the known weak C D in the range 165-1 85 nm.I4 For example, methyl-@-D-glucopyranoside shows positive CD in the region 165-175 nmI4 whose contribution is not included in the calculation; the observed ORD is thus slightly more positive than that calculated as the wavelength decreases (Figure 1). Similarly, methyl-a-D-glucopyranoside and methyl-a-D-galactopyranoside display negative CD at approximately 170 nm,14 and the observed ORD is thereby more negative than the calculated ORD. The observed ORD of methyl-@-Dgalactopyranoside is approximately zero over the range 300-600 nm.24 The present work accounts for its unusual ORD as being the sume of a positive background dispersion and a negative contribution from the low-energy CD band observed near 173 nm.I4 The comparison of calculated and observed ORD for the four compounds in Figure 2 is not as good, reflecting the larger error in molar rotation at the NaD line for those compounds (Table I). Using the known weak CD in the 165-185-nm region,14 we subtracted its contribution to the observed ORD to obtain an “adjusted” ORD, for further comparison with the theoretical model, which similarly does not include lower lying electronic transitions. The procedure is subject to uncertainty arising from the fact that, at the 165-nm cutoff of those CD measurement^,'^ only the tail of the shortest wavelength band can be observed. We nevertheless fitted the observed CD to three Gaussian bands at 161, 173, and 182 nm, each with a bandwidth of 7.5 nm. Intensities were assigned to the bands so as to minimize the
s-2L - 3 200
300
400
500
600
A, nm Figure 2. Observed24(-) and calculated (---) ORD of a- and fianomers of methyl-D-xylopyranoside, of methyl-a-D-mannopyranoside, and of methyl-8-D-arabinopyranoside.
parametrization originally optimized for saccharide fragrnentsI6 was used unmodified for the present work. We have, however, ineluded a solvent correction term equal to (n2 + 2)/3, where n is the refractive index of water, and a scaling factor (1.69) obtained by fitting the results for six hexaopyranasides to experimental data. The latter may represent contributions from magnetic transition moments omitted in the Kirkwood theory.21 For each hexopyranoside, the calculation was carried out for the three hydroxymethyl group rotational conformers, gg, gt, and tg. The average ORD for the three was evaluated by using the estimated weights proposed by Lemieux and Brewer,’ as follows: [MI = 0.60[M], 0.30[M], + O.1O[MIrg
+
for the glucose and mannose pyranosides and [MI = 0.45[M], 0.45[MIrg + 0.10[MIgg
+
for galactose compounds.
Results Table I displays the calculated and observed2s NaD molar rotations, and Figures 1 and 2 show the calculated ORD compared with the experimental data of Listowsky et al.24 Discussion
The comparison of calculated NaD molar rotations with observed values (Table I) is relatively good for methybgluco- and galactopyranosides and for methyl-@-D-mannopyranoside. The calculated weighted average over hydroxymethyl group conformers was obtained by using Lemieux and Brewer’s proposed statistical weights.’ It would take only slight variations of those weights to bring the calculated and observed values into exact correspondence (Table I), but the approximate nature of the theory ( 2 5 ) Bates, F. J. ‘Polarimetry, Saccharimetry and the Sugars”; Narf. Bur. Sfand. (U.S.) Circ. 1942, No. C440.
J . Phys. Chem. 1989,93, 3436-3439
3436
TABLE 11: Observed Molar Rotation at 300 nm Adjusted for Long-Wavelength CD Contributions; Comparison with Calculated Values (Units of dee cm2 dmol-I)
compound hexopyranosides
[MI%*
methyl-0-D-glucopyranoside methyl-@-D-galactopyranoside methyl-ol-D-mannopyranoside methyl-a-D-glucopyranoside methyl-a-D-galactopyranoside
pentopyranosides methyl-@-D-arabinopyranoside methyl-0-D-xylopyranoside methyl-a-D-xylopyranoside
-310 -30 +760 1490 + 1950
-460 +53 +780 +1530 +2420
-1850 -450 + 1240
-2090 -440 +1310
+
-344 +lo2 +I201 1694 +2220
+
-1469 -287 +I876
least-squares deviation from the observed CD. The KronigKramers transform of the fitted C D was then subtracted from the observed ORD. This procedure gives the background dispersion from higher energy transitions. The results at 300 nm are shown in Table 11. Table I1 indicates that, for the four compounds represented in Figure 1, the long-wavelength CD accounts for the sign of the discrepancy between the calculated and observed ORD. In two
cases the procedure overadjusts and in two cases it underadjusts. For three of the compounds represented in Figure 2, the adjustment (Table 11) is in the correct direction but is much too small to account for the discrepancy. For the fourth compound, methyl-P-D-arabinopyranoside, the adjustment is in the wrong direction. Other factors must be playing a role in these cases (see above). Overall, the agreement between calculated and observed ORD reported here can be taken as support for the theoretical model. The theory, in spite of its necessarily approximate nature, represents a conceptual advance by describing the connection between saccharide molecular structure and optical activity explicitly in terms of the high-energy electronic transitions and vacuum-UV CD features which are intrinsically related.
Acknowledgment. This work was supported by USPHS Grant
GM-24862. Registry No. Methyl-@-D-mannopyranoside,22277-65-2; methyl-@methyl-0-D-galactopyranoside, 1824-94-8; D-glucopyranoside,709-50-2; methyl-a-D-mannopyranoside, 617-04-9;methyl-a-D-glucopyranoside, 97-30-3; methyl-a-D-galactopyranoside, 3396-99-4; methyl-@-D-arabinomethyl-P-o-xylopyranoside, 612-05-5; methyl-apyranoside, 5328-63-2; D-xylopyranoside, 91-09-8.
Solvent Effects on the Soret Absorption Band of Nickel Protoporphyrin I X Dimethyl Ester Containing the Four-Coordinate Metal Orland W. Kolling Chemistry Department, Southwestern College, Winfield, Kansas 67156 (Received: June 24, 1988;
In Final Form: November 10, 1988)
Comparisons were made between observed solvent effects upon the Soret peak for two cases of four-coordinatemetalloporphyrins, nickel protoporphyrin IX dimethyl ester (NiPPDME) and zinc tetraphenylporphyrin (ZnTPP). Solvents included in this study were members of the aromatic polar and apolar, aprotic dipolar and highly dipolar, and hydrogen-bonding classes in the Chastrette-Purcell generalized classification system. With reference to hydrocarbon environments the Soret peak of ZnTPP exhibits a systematic red shift with increasing dipolarity and Lewis basicity of the solvent while that for NiPPDME is blue-shifted. A more exact analysis of the empirical blue shift for the Soret band of MPPDME indicates that the composite solvent effect arises from the variable mixing of so1ute:solvent dipole-dipole orientational and hydrogen-bondinginfluences. Simple distortional contributions predicted by reaction field models for the solvent-induced spectral shift are not detected for the Soret band of NiPPDME.
The synthetic metalloporphyrins containing various transition-metal atoms have been extensively investigated because of their somewhat unique redox behaviors, structural characteristics, and electronic spectra, as well as their mimicry of the biological functions of the very important iron porphyrins. The pronounced Soret absorption peak for the metalloporphyrins is influenced significantly by axial ligation and the solvent environment, just as are the redox potentials for metalloporphyrins measured with the DME in nonaqueous For the latter, there appear to be some qualitiative trends in the shifts in half-wave potentials for a given redox couple which parallel the dielectric constants and/or Gutmann donor numbers (DN) for the Although the limited systematic studies of axial ligand and solvent effects upon the Soret band have not yielded an interpretive consensus, it is clear that those metalloporphyrins which contain a restricted 4-fold coordination of the central metal atom provide ( 1 ) Bottomley, L.; Kadish, K. Inorg. Chem. 1981, 20, 1348. (2) Kadish, K.;Shine, L. Inorg. Chem. 1982, 21, 3623. (3)Kelly, S.;Kadish, K. Inorg. Chem. 1982, 21, 3631. (4) Rillema, D.et al. J . Am. Chem. SOC.1982, 104, 1276. (5)Antipas, A.; Gouterman, M. J . Am. Chem. SOC.1983, 105, 4896. (6)Lexa, D.et al. J . Am. Chem. SOC.1984, 106, 6321. (7)Lexa, D.;Momenteau, M.; Saveant, J.; Xu, F. J . Am. Chem. SOC. 1986, 108, 6937.
0022-3654II 8 9 i12093-3436$01 S O ,I O -
the simplest and most suitable initial models for the analysis of variables to be attributed to the solvent environment. In a major work on the (tetraphenylporphinato)zinc(II) (ZnTPP) case Nappa and Valentine8 concluded that the red shift of the Soret band is due to ligation of a fifth position on Zn(I1) rather than solvation even though the size of the red shift is not generally related to -AH of complex formation as a measure of the strength of that Zn-ligand bond. On the other hand, Vogel and Stahlbush9 reported a semiquantitative correspondence between the Soret red shift and the enthalpy of adduct formation for ZnTPP:donor pairs in cyclohexane. However, such enthalpy comparisons are useful only for those series in which the AS values for adduct formation remain small and relatively constant.I0 Linear free energy relationships between the stability constants of ZnTPP:donor complexes and the pK, of the ligand have been observed by Kadish et al." in dichloromethane as the solvent. Because in some instances the Soret band shifts are electronic responses to ligation effects on the free energy change for the ground state to excited state transition, the variables associated (8) Nappa, M.; Valentine, J. J . Am. Chem. SOC.1978, 100, 5075. (9)Vogel, G.;Stahlbush, J. Inorg. Chem. 1977, 16, 950. (IO)Jones, R.;Staley, R. J . Am. Chem. SOC.1982, 104, 2296. ( 1 1) Kadish, K. et al. Inorg. Chem. 1981, 20, 1274.
0 1989 American Chemical Society
