356

Note

CARBOHYDRATE RJSEARCH

Mutarotation of D-glucose and D-mannose in aqueous solution*

D-Glucose (1) and o-marmose (2) exhibit pseudo-first-order kinetics during mutarotation, as monitored by the changes in optical rotation (o.r.) in a suitable solvent. Each of these hexoses is available as the a- and the /l-pyranose in crystalline form. Both anomers of each sugar exhibit the same kinetics of mutarotation; hence, it would be expected that, at equilibrium in water, these two hexoses are present in only two major forms, namely, the a- and the @-pyranoses. Optical rotation studies, however, cannot exc!ude the possible presence of other tautomers (e.g., the a- and /&furanoses) which, due to compensation, might not contribute to the optical rotatory data. Recently, the trimethylsilylation procedure’ for preparing volatile derivatives of sugars has been appliedze3 to the measurement, by g.l.c., of the individual tautomers of D-galactose in solution in pyridine. Bentley and Botlock4 developed a trimethyl- silylation procedure, and prepared pertrimethylsilyl derivatives of sugars without apparent change in the position of the anomeric equilibrium that exists in water.

Identification of the pertrimethylsilyl derivatives of tautomers separated by g.1.c. is a prerequisite to use of the procedure of Bentley and Botlock4 for the study of the tautomerization of sugars. The pertrimethylsilyl derivatives of D-glucopyranose and D-mannopyranose tautomers obtained by g.l.c.3 have now been characterized by elementary analysis, polarimetry, and p.m.r. and i.r. spectrometry. For both of these hexoses during mutarotation and at equilibrium in aqueous solution, the kinetic and thermodynamic data obtained by g.l.c.4 have been compared with those calculated from o.r.** for the same sample, in order to determine the extent to which the trimethylsilylation procedure alters the position of the reaction, either during mutarotation or at equilibrium.

The gas-liquid chromatograms of the pertrimethylsilyl derivatives of mutaro- tated o-glucopyranose and D-mannopyranose showed two peaks (see Fig. 1). For characterization, each compound was isolated by preparative g.1.c. as previously described3. The data used to establish the identity of each peak, in descending order of elution from an (ethylcyano)siliconz column are shown in Table I. The molecular rotations found are consistent with those reported for the peracetylated tautomers of rr-glucopyranose and D-mannopyranose6. The 1CKLMHz p.m.r. spectra of the four

*Presented, in part, at the 155th meeting of the American Chemical Society at San Francisco, California, in April, 1968. **The rate and equilibriumconstants werecalculated according to the procedure described by Isbeli and PigmarS.

Carbohyd. Res., 9 (1969) 3X5-360

NOTE 357

compounds in chloroform-d showed three general types of protons. (a) The first signal of three sets occurred at z 4.5-5.5, and is assigned to the anomeric proton.

c

JO 14 18

TIME {Minufes]

22

2A

L

4.5 7.5 JO.5 73.5

TIME (Minutes)

Fig. 1. Gas-liquid chromatogram of per-U-trimethylsilylated tautomers of o-glucose (1) and o-mannose (2) in aqueous solution at the equilibrium.

TABLE I

DATA CHARACTERIZING TEE TRIMETHYISILYL TETRA-O-(TRIhlETlIYLSILYL)-D-HEXOSLDES STUDIED

PertrimethykiIyf ether of trimeth_vIsiIyl

[alz3 degZes

IM] loo-MHz, p.m.r. data EIementary analysis

Chemical shift Jr,2 coupling of w-1 (Hz) C H Si

signal, t percenta found

a-o-Glucopyranoside +80.5 +43,500 4.6 (doublet) 3.0 46.71 9.61 25.53 P-D-Glucopyranoside + 4.6 + 2,530 5.2 (doublet) 7.0 46.69 9.58 25.29 a-o-Mannopyranoside +40.5 +21,870 5.1 (doublet) 1.2 46.06 9.70 25.08 @-o-Mannopyranoside -20.1 - 10,870 5.3 (singlet) 0.0 46.62 9.59 25.58

Walt. for CsH7Os(SiMea)s; C, 46.66: H, 9.63: Si, 25.92%.

According to Lemieux et al.‘, the anomeric proton of an a-D-aldohexopyranose in the CI (D) conformation resonates at a iower chemical shift than for the #?-D anomer in the same conformation, owing to interaction of this proton of the former with the p-orb&& of the ring-oxygen atom. For the four compounds studied, the signals given by the anomeric proton and their fist-order coupling-constants (Jr,*) are in good agreement with the anomeric configurations assigned. (6) The second set of signals, at r 5.5-7.0, is assigned to the remaining methine and methylene protons. (c) The third set, resonating at about z 10, is assigned to protons of the O-trimethyIsiIy1 groups. The integrated areas of these three sets of proton signals were in the ratios of 1:6:45,

Carbohyd. Res., 9 (1969) 356-360

358 NO’IE

consistent with the view that the pairs of compounds isolated are the pertrimethyl- silyl ethers of the anomers of trimethylsilyl D-glucopyranoside and D-mannopyranos- ide, respectively. Moreover, i.r. analysis failed to reveal any bands attributable to. hydroxyl or carbony absorption. The results of elementary analysis of these four compounds were in agreement with the values calculated for C,H,O,(SiMes)s. The structural and configurational assignments for each compound were verified by trimethylsilylation immediately after dissolution of each pure, crystalline compound (1 and 2) in water; g.1.c. then revealed only one major peak for the compound having the configuration assigned.

TABLE II

kx RATE CONSTANTS= OF Q Z+ p TRANSFORhUiTION DETERMINED BY G-L-C. AND 0-R.

ka

sugar Rate g.1.c. constant (x IO-)

DGlucopyranose k$- 0.63 ka”-5” 0.39 k$- 1.69 k+=’ 1.02

D-Mannopyranose kl15” 0.48 ka=‘” 1.04 k125” 1.31 k&=’ 2.78

nCaIculated as log10 and expressed in min-1.

ax-

(x 10-9

0.70 0.41 1.69 1.03 0.44 1.02 1.25 2.84

The information obtained from these data indicates that 1A and 1B (see Fig. 1) are trimethylsilyl tetra-O-(trimethylsilyl)-c and -j?-D-glucopyranoside, respectively, and 2A and 2B are trimethylsilyl tetra- C%(trimethylsilyl)-c+ and -j?-D-mannopyrano- side, respectively.

The rate constants of the cz 2 /3 tr an sf ormation, determined by both g.1.c. 2

and or. during mutarotation and at equilibrium, are shown in Table II. The thermo- dynamic properties of the a $ fl t ransformation, calculated from the data given, are summarized in Tables III and IV. The value of dH obtained for D-glucopyranose is in agreement with that calculated from the optical rotatory data of Isbell and P&man* (- 59 cal/mol); however, these values are about 20% of those measured by calori- metryp*lo. The reason for this discrepancy is not yet known. The thermodynamic data of activation, E, and dSf for D-glucopyranose are 5 to 10% lower than those reported by S&mid and Bauer” and Los and Simpson I2 . The value of d Gt for D-glucopyranose is about 10% lower than those reported l1 It is interesting that, from the thermo- . dynamic data, the observed entropy of activation for both of these hexoses is about 20-30 times the entropy dxerence between a- and /?-D-hexopyranoses. Therefore, we suspect that the structure of the transition state is significantly different from that of either of the two pyranose anomers.

Carbohyd. Res., 9 (1969) 356-360

NOTE 359

TABLE III

THERhfODYNAhUCS OF EQUILIBRIUM FOR o( + B TRANSFORhIATION"

D-GIucopyra12ose~ D-MannopyranoseC

g-I-c. 0-r. g.2.c. 0-r.

AG -311 -313 +440 +480 AH -50 -50 +254 +313 AS +0.87 +0.84 -0.64 TO.57

aExpressed in cal. mol-l, except for entropy values, which are given in cal. mol-l .deg-I. bCalculated for 30”. %alculated for 20”.

TABLE IV

kr mmhfo~twmrcs ~FA~TIVATI~N FOR asp TRANSFOR~~ATION~

kr

Sugar g.I.c. 0.r.

kl kz kl ka

AGS +22.11 +22.78 f22.08 +22.78 D-Glucopyranoseb En f18.01 +17.57 f16.09 +16.81

Ass -13.5 - 17.1 -19.8 - 19.6

AGS +21.89 f21.44 +21.92 +21.43 D-Mannopyranosec Ea +17.04 +16.85 +17.75 +17.43

ASt - 16.49 - 15.66 - 14.22 - 13.66

%xpressed in kcal. mol-r, except for entropic values, which are in cal. mol-r.deg-1. bCaIculated at 30’. CCalcuIated at 20”.

The kinetic and thermodynamic data obtained from g.1.c. are in excellent agreement (r = 0.99) with those calculated from optical rotation results. The results of the present study increase the potential usefulness of this g.1.c. technique as a method in the study of tautomerization of sugars, especially for those sugars that exhibit complex mutarotation.

ACKNOWLEDGMENTS

We thank R. M. Butts for the g.1.c. separations, and Got-ton Wood for the p.m.r. determinations. This study was supported, in part, by a grant from the National Institutes of Health.

New York State Agricultural Experiment Station, Cornell University, Geneva, New York 14456 (U. S. A.)

c. Y. LEE T. E. ACREE

R. S. SIIALLENEERGER

Carbohyd. Res., 9 (1969) 356-360

360 NOTE

REFERENCES

1 C. C. SWEELEY, R. BENTLEY, M. M!AKITA, AND W. W. WELLS, J. Amer. Chem. SOL, 85 (1963) 2479. 2 R. S. SIWLENBERGER AND T- E. ACREE, Carbohyd. Res., 1 (1966) 495. 3 T. E. ACREE, R. S. SHALLENBERGER, AND L. R. MATTICR, Corbohyd. Res., 6 (1968) 492. 4 R. BJXTLEY AND N. BOTLOCK, Anal. Biochem., 20 (1967) 312. 5 H. S. kBELL AND W. W. PIGhfAN, J. Res. Nat. Bur. Stand., 18 (1937) 141. 6 F. J. BATES et al., Polarimetry, Saccharinretry and the Sugars, Nat. BIU. Stand. Circ. C440,1942,

pp. 729 and 746. 7 R. U. L~EUX, R. K. KULLNIG, H. J. BERNSTEIN, AND W. G. SCHNEIDER, J. Amer. Chem. Sot.,

80 (1958) 6098. _ 8 H. S. ISBELL AND W. W. PIGMAN, Ref. 6, p. 449. 9 J. M. STURTEVANT, J- Phys. Chem., 45 (1941) 127.

IO M. A. KABAYAMA, D. PAX-I-ER~ON, AND L. PICHE, Can. J. Chem., 36 (1958) 557. I1 H. SCHMID AND G. BAUER, Monatsh., 97 (1966) 866. 12 J. M. Los AND L. B. S~PSON, Rec. Trav. Chim., 76 (1957) 267.

(Received September 7th. 1968)

Corbohyd. Res., 9 (1969) 35fG360