KOTES
April, 1963 Acknowledgment.--The authors wish to thank the donors of the Petroleum Research Fund, administered by the American Chemical Society, for their support of the research which led to this paper.
ON THE CONFORMATIOK OF THE D-GLUCOPYRASOSE RISG IN MALTOSE ASD IN HIGHElt POLWNERS OF D - G L U C ~ S E BY V. S. R. RAOAND JOSEPH F. FOSTER Departmbnt of Chemzstry, Purdue Unzversaty, LaJauette, Indzana Reeeaved October 18, 1968
I n considerations of the configuration in solution of amylose and other polysaccharides the question of ring conformation of the monomer units is of utmost importance.' Thus 130110 and Szejdli' have pointed out that if the D-glucose residues in amylose exist in the C1 conformation the chain should be relatively rigid and possess an essentially helical configuration. Of the two possible chair forms and innumerable boat forms it has been suggested from stability considerations that either the C1, B1, or 3B conformations are most probable.2 While it seems to have been generally accepted that D-glucose (in common with most monosaccharides), simple D-glucosides, and the glucose units of cellobiose and cellulose exist exclusively in the C1 conformation, there have been several suggestions that the same is not true for maltose and higher polysaccharides of the amylose series. Reeves3 called attention to the difficulty of forming a-1,4-glucosidic bonds between two glucose residues both of which are in the C1 conformation. He further suggested, on the basis of the incomplete reaction of amylose with cupraminonium reagent and as an explanation for the decrease in optical rotation in alkaline solution, that approximately half of the glucose residues of amylose exist in a boat conformation. This conclusion has been questioned by Greenwood and RossottL2 who favored the C1 conformation. Since methyl p-maltoside also shows a decrease in rotation of alkali Reeves further suggested that the non-reducing residue of maltose exists in a boat conformation.3 B e n t l e ~ ,on ~ the basis of comparative studies of the rates of oxidative bromination and hydrolysis, also concluded that the non-reducing unit of maltose possesses a boat conformation, while the reducing unit of maltose and both units of cellobiose have the usual C1 conformation. It would appear that clear answers to such questions should be attainable through nuclear magnetic resonance (n.m.r.) spectroscopy. It has been shown that in sugars5and in acetylated sugars6the signal due to the anomeric proton appears a t lower field than that of any of the other casbon-bonded hydrogen atoms. Furthermore, it seems clear from those same studies that the signal for an equatorial anomeric proton (HI,) occurs a t a somewhat lower field by about 0.5-0.7 p.p.m. than for an axial ansmeric proton (Hla). I n addition, the dihedral angle between the anomeric proton and (1) J. Hollo and J. Szejdli, D i e Starke, 13, 222 (1961). (2) C. T. Greenwood and 13. Rossotti, J Pclynaer SCL.27, 481 (1958) (3) R. E. Reeves, J . A m . Chem. Sue., 7 6 , 4595 (1954); A n n . Rev. Bzochena., 27, 15 (1958). (4) R. Bentley, J A m . Chem. Soc., 81, 1962 (1969). ( 5 ) R . W. Lenz and J. P. Heeschen, J . Polymer. Scz , 61,247 (1961). (6) R. U. Lemieux, R. K. Kullnig, H. J. Bernstein. and W.G . Schneider, J . A m . Chem. Sue., 79, I005 (1957); 80, 6098 (1958).
95 1
the hydrogen on the adjacent carbon atom may be obtained from the magnitude of the splitting of the corresponding absorption peak through application of the Karplus eq~ation.~.' The n.m.r. spectra of maltose and a number of related compounds have been determined a t 60 Me./ sec. with a Varian A-60 n.m.r. spectrometer employing 10-20ojo by weight solutions of the carbohydrates in D20. The assignment of r values was made by taking the water peak (5.2 r ) as an internal standard. Results are shown in Table I. I n the case of cellobiose two peaks have been observed in the lower field region a t 4.70 and 5.38 r . From intensity considerations it seems clear that the peak a t 5.38 is due to the anomeric proton of the non-reducing unit plus the anomeric proton a t the reducing end when it is in the axial position @-anomer). The peak at 1.70 is due to the anomeric proton at the reducing end when it is in the equatorial position (a-anomer). These two peak positions approximately coincide with those of the P- and a-anomers, respectively, of D-glucose and the methyl D-glucosides. The dihedral angles deduced using the modified Karplus equation, namely, 5-1 and 160' for the peaks a t 4.70 and 5.38 T , respectively, while not in agreement with the expected angles for the C1 conformation (60 and 180') do agree well with the corresponding values deduced in the same way for a- and P-glucose and the methylglucosides. Hence these results are in agreement with Bentley's conclusion and with the recent demonstration by Jacobson, Wunderlich and Lipscornb8 that both glucose units in cellobiose exist in the C1 conformation. I n the case of maltose three peaks are seen in the lower field range a t 4.58, 4.74, and 5.30. From the intensity of these peaks it can be concluded that the peak a t 4.58 is due to the anomeric proton (HI) of the ncmreducing unit. Because of mutarotation the signal due to the protons Hl of the reducing glucose units appears a t two places, 4.74 and 5.30. The dihedral angles obtained from the splitting of these peaks are 54 and 154O, respectively. From the dihedral angles and the peak locations, it may be concluded that the reducing glucose unit in maltose is in the C1 conformation. The dihedral angle obtained from the splitting of the peak at 4.58 is about 55", in accord with either the C1 or B1 but not the 3B conformation. The T value of this peak is slightly lower than that of a-glucose (4.78). If this unit were in the B1 conformation this proton should be in an essentially axial position and it might be expected that its signal would appear at a much higher T value (above 5.0). However, this cannot be stated with certainty in the absence of suitable test compounds known to exist in a boat conformation. It is, nevertheless, very unlikely that the glucose ring would occur in a perfect B1 conformation due to repulsion between the protons a t C1 and C4, which would be very close in this conformation. Any twisting to relieve this repulsion would lead to a considerable change in dihedral angle. Bentley4 has suggested that the conformaiion is between that of the idealized B1 and 3B, which seems very unlikely from the observed dihedral splitting. Furthermore, due to the ready interconvertability of the boat forms it would be (7) M. Karplus. J . Chem. P h y s . , S O , 11 (1959). (8) R. A. Jacobson, J. A. Wunderhch, and W. N. Lipsoomb, Acta Cryst., 14, 598 (1961).
Vol. 67
NOTES
TABLE I: N.M.R.RESULTS
--
SCJMMdRY OF
Dihedral Coupling, const. J B ~ H P , angle (degrees) cyoles/sec.
2.4 T .d
P-&thyl-o-glu eoside
7.7
58 151
Chemical shifts
c
protonsa-
-Anomerio
His
Hie
4 78
6 ;',I:
152
5.54
Cellobiose
values)
-(T
Other protons----
6.25 f i , 25
6,41
fi,2:
