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Optical Properties of Sugars. 4. Circular Dichroism of Methyl Aldopyranosides' a Richard G. Nelsonlb and W. Curtis Johnson, Jr.*lC Contribution f r o m the Department of Biochemistry and Biophysics, Oregon State University, Corvallis, Oregon 97331. Received April 11, 1975
Abstract: Circular dichroism spectra in the vacuum ultraviolet to 165 nm are presented for aqueous solutions of 12 methyl aldopyranosides. Difference spectra of homomorphic and epimeric pairs reveal that the circular dichroism of these sugars bears more similarity to the spectra of aldopyranoses than is initially apparent. For a given anomer, changes on going from the pyranose to the corresponding methyl pyranoside are very similar. Also, changes in circular dichroism spectra which occur when a hydroxymethyl group is added to C-5 are similar for corresponding pyranose and methyl pyranosides. These difference spectra verify the use of the pairwise principle and show that the methyl pyranosides investigated have the same conformation as the corresponding pyranoses where comparisons are possible. With these data it is possible to tentatively assign specific chromophores to the first three bands in methyl pyranosides as well as the first band in the pyranoses. Apparently, the first band (-185 nm) in methyl pyranosides is due to the ring oxygen, the second (-175 nm) to the methoxy group, and the third (below 165 nm) at least in part to the methoxy group. The signs of the second and third bands are correlated with configuration about the anomeric carbon. The first band in the pyranoses (-180 nm) is apparently due to the ring oxygen. The ring oxygen transition is red shifted considerably when a hydroxymethyl on C-5 or methoxy group on C-1 shields the chromophoric nonbonding electrons from the hydrogen-bonding solvent.
Methyl aldopyranosides, unlike the aldopyranoses studied in our third paper2 in this series, do not undergo mutarotation upon dissolution. Having a fixed conformation a t C- 1 has the advantage that it simplifies circular dichroism (CD) measurements, but these molecules have a methyl acetal group whose optical properties are expected to be different from those of the hemiacetal found in the aldopyranoses. Here we report the C D spectra of 12 methyl aldohexo- and pentopyranosides in aqueous solution to about 165 nm (Figure 1). W e investigate the effects the 0 - 5 , C-1,O-1 system has on the spectra of the simple monosaccharides and obtain additional experimental data on the C D changes associated with anomerization, epimerization, and the addition of a hydroxymethyl group in homomorphic pairs of sugars. All but two of these pyranosides predominantly adopt the usual C 1 chair conformation according to Reeves' classical study of stereospecific complex formation with cuprammonium reagent.3 Reeves' assignments are corroborated by empirica14s5 and semiempirica16 free-energy calculations as well as H-1, H-2 coupling constants' for the majority of the sugars that we have investigated. Aqueous solutions of the two other pyranosides studied, @-D-ribosideand a-D-lyxoside, are believed to contain substantial amounts of both the C1 and 1C conforme r ~ . ~ . ~ The optical rotatory dispersion (ORD) of anomeric pairs of tetrahydropyranyl ethers, 2-deoxy- and 2,6-dideoxy-~glycopyranosides, have been reported by Klyne et al. to 200 nm.Io They proposed that the plain dispersion curves they observed in the far-uv were due to the acetal chromophore since the sign of the rotation was directly correlated with the anomeric configuration. Listowsky et aI.I1 investigated the O R D of a number of methyl glycosides to 185 nm. They suggested that the optical rotation in this region is predominantly associated with ring oxygen absorption and that the sign and magnitude of the rotation are determined primarily by the stereochemical arrangement of groups about this oxygen atom. Subsequently, Listowsky and EnglardI2 measured the C D spectra of several monosaccharides to 188 nm. Their results showed that the sign of the first C D band did not depend on the configuration about the anomeric carbon. This tended to confirm the idea that the first band was due to the ring oxygen. The data presented here allow us to tentatively assign C D bands in the spectra of the methyl pyranosides to specific chromophores. Finally, these conclusions allow us to assign
Journal of the American Chemical Society / 98.14
/
chromophoric groups to C D transitions for the pyranoses.
Experimental Section Materials. The source and rotatory properties of the 12 methyl aldopyranoside sugars studied are listed in Table I. Three aldopyranosides (methyl a-D-lyxopyranoside,methyl a-D-ribopyranoside,and methyl 2-deoxy-a-D-glucopyranoside)were synthesized in our laboratory from their corresponding aldopyranoses following the general technique of Cadotte et aI.l3 Procedure and Apparatus. These details are the same as in previous work.2 Results C D spectra for the 12 methyl aldopyranosides in aqueous solution are presented in Figures 2-8. The spectrum of methyl a-D-xyloside (Figure 2) shows that at least three transitions contribute to its C D spectrum between 200 and 165 nm. The sugar has a very weak negative band at about 185 nm, a weak positive band at about 174 nm, and a more substantial negative band which peaks below 165 nm. Presumably the C D spectrum of methyl @-D-xylosidealso consists of three C D bands. The long-wavelength band is again negative and is seen as a long, very low intensity tail on the second C D band. The second and third bands are similar in intensity but opposite in sign to those found for methyl a-D-xyloside. The signs of these two short wavelength bands are correlated with the configuration about the anomeric center for all the methyl aldopyranoside spectra. This suggests that the chromophore responsible for these bands must be in a t least approximately enantiomorphic environments in the a and p anomers. The C D spectrum of methyl a-D-glucoside (Figure 3) also contains a t least three C D bands. In contrast to methyl a-Dxyloside, the first band is positive and appears as a low-intensity tail at the long wavelength end of the spectrum. The second and third bands are similar to those of the xyloside but increased in intensity. Methyl @-D-glucosideonly clearly exhibits two bands, a positive low-intensity band at 182 nm and a more intense positive band whose maximum lies below 165 nm. Of course the C D spectrum could well involve three or more component C D bands. Certainly one does not expect the addition of a hydroxylmethyl group at the C-5 carbon of a methyl xyloside to decrease the number of C D bands present. While the C D spectra of the methyl glucosides are predominantly positive, changing the C-4 hydroxyl from equatorial to axial to produce the methyl D-galactosides results in
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OCH3
P-D-xylopyranoside
Q -D-xylopyranoside
+ Hok CHZOH
1
025
I
