Conformation of the ascorbic acids in aqueous solution

Conformation of the ascorbic acids in aqueous solution

Journal of Molecular Structure, 142 (1986) Elsevier Science Publishers B.V., Amsterdam 387-390 - Printed CONFORMATION IN AQUEOUS J.GUILLEME, Facul...

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Journal of Molecular Structure, 142 (1986) Elsevier Science Publishers B.V., Amsterdam

387-390 - Printed

CONFORMATION

IN AQUEOUS

J.GUILLEME, Facultad

OF THE ASCORBIC

ACIDS

E. DIEZ*, M.M.GOMEZ,

de Ciencias,

M.SECUNDINO

Universidad

C-II,

387 in The Netherlands

SOLUTION

and

L.GARRIGOS

AutBnoma

de Madrid,

28049_Madrid(Spain)

ABSTRACT The complete analysis of the H-l and C-13 NMR spectra of L-ascorbic acid 1 and D-araboascorbic acid 2 has been carried out for acidic aqueous solutions. Conformational equilibrium of the side chain of 1 and 2 was determined from the vicinal H-H and C-H couplings. The obtained results are compared with previous models and with those provided by molecular mechanics calculations. Assuming a constant J(C,H)/J(H,H) ratio of 0.58 between equivalent vicinal C-H and H-H couplings the two methylene protons were assigned and the generalized Karplus equation was applied to the C-H couplings.

INTRODUCTION The ascorbic C-5 giving

acid molecule

contains

rise to two pairs of enantiomers.

bit acid 2 have the same configuration C-5. They are diasteomers by several

authors

of L-ascorbic

(ref.l-6)

the conformational

(ref.6).

conformational clarification

because

L-ascorbic

of the biological models

for the C-H coupling

and chemical

(ref.l,4)

importance for

and for the H-H

determination

of the

for the side chain of 1 and 2 in order to get some

Hi31

0022-2860/86/$03.50

at

acid 1 in water

about this question.

l.R=H(S),R?=O(S)

C-4 and

have been studied

have been proposed

The aim of this paper is the reliable

equilibrium

in configuration which

of the side chain of L-ascorbic

values

at carbons

acid 1 and D-araboascor-

NMR properties

C). Three different

behaviour

centres

at C-4 but differ

with different

acid (Vitamin

based in the empirical couplings

two asymmetric

H(5’);

2.R=0i5)H15’1,Ri=H

0 1986 Elsevier Science Publishers

B.V.

(5)

388 TABLE

1

H-H and C-H coupling

constants

(J, Hz) for compounds

J(Hn,Hm)a Comp.

J(Cn,Hm)

1

2

Comp.

1 C3

H4,H5

1.9

:':

H4

H5,H6 H5,H6' H6,H6'

-5 8

:*;

H5 H6 H6'

6'7 -1118

-11:4

aJ(H4,H6)

1 and 2.

= J(H4,H6')

2 C3

1 C4

2 C4

-5.7

153.1

154.1

1'5 0'0 0:O

-2.0 ;:!

1 C5

2 C5

1 C6

2 C6

-0.1

-3.1 24 3:2

145.0 -3.4 -1.6

145.7 -3.2

143.1

144.5

= 0.0 Hz

NMR RESULTS Proton

and carbon spectra

100 spectrometer Table

of compounds

and analysed

1 and 2 were recorded with a Varian

by using an iterative

fitting

computer

1 shows the final values for the H-H and C-H coupling

mental methods The effect

and spectral

analysis

on the proton spectra

is small due to the strong coupling determined

with a precision

will be described of the coupling between

of specie (ref.6).

1 differ

difference

of 0.5 Hz from the frequencies

and J(C4,H6’)

J(C4,H6) contrary,

with a precision

can be interchanged

the combinations

J(C4,H6)+J(C4,H6')

MOLECULAR

MECHANICS

The molecules

from the 600 MHz spectrum

Exploration

acids 1 and 2 consist of a roughly planar

of the conformational

have been performed

which

geometries

~(2)=r(C4-C5-C6-06),

in staggered

to 69 isomers

positions.

for molecule

ed changing

surface

isomers

having all the torsion

constant

and ~(4)=

final energy minima 2. There are 13

differ less than 2 Kcal/mol

lowest ones. For these isomers, energy minimizations

the dielectric

in

extensive

~(3)=~(H5'-05-C5-C6),

The different

which

requires

using the MM2 force field (ref.7).

1 and 64 for molecule

for 1 and 22 for 2 with energies

lac-

rotational

space for these molecules

energy

were all the 81 rotational

angles T(~)=T(C~-C~-C~-C~),

the respective

On the

and have been

side chain having four single bonds providing

energy minimizations

correspond

the spectra.

are well determined

on the potential

=T(H~'-06-C6-C5)

affecting

below.

order to locate the minima

The starting

J(C4,H6)-

CALCULATIONS

of the ascorbic

tone ring and an acyclic states.

of

of only 2.0 Hz and the values of

without

used to assign H6 and H6' as it is discussed

isomers

it could be

and intensities

for the vicinal couplings

less than 0.15 Hz from those derived

could be determined

Experi-

J(H5,H6)-J(H5,H6')

In the case of the carbon spectra the coupling differences

J(C4,H6’),

isomeric

constants.

in detail elsewhere.

H6 and H6'. However,

four pairs of small outer lines. In fact, our values

XL-

program.

E from the default

from

were repeat-

value of 1.5 to 3.0 and

TABLF 2 Populations ( % ) for rotarcers about the C4-C5 and C5-C6 bonds of molecules and 2 determined by NMR and calculated by the MM2 method (see text).

I

Rotamers

for the C4-C5 bond

Rotamers

for the C5-C6 bond

Compound

1

Compound

1

Compound

2

Compound

2

T

G-

G+

53 12

39 32 20 16 13

49 15 73 73 72

Method T

;Fj;b”

G-

G+

T

G-

G+

II!,' 89-O 54

T

G-

G+

0-z:

56 35

26 51

18 14

MM2(~=1.5)

5

86

9

21

44

35

9

77

14

MM~(E=~) MM2(~=25)

5 7

86 85

9 8

23 29

::

4391

:63

::

12 12

87 36-O0 64-87

aPopulations

0-i:

for the C4-C5 rotamers

obtained

from J(H4,H5),

bAllowed population ranges for the C4-C5 rotamers Populations for the C5-C6 rotamers corresponding H6 and H6'.

to 25. The populations are given

calculated

for rotamers

I: 15

J(C6,H4)

and J(C3,H5)

calculated from J(H4 H5) to reversed assignation of

about the C4-C5 and C5-C6 bonds

in Table 2.

DISCUSSION The vicinal rotation

coupling

around

vicinal

rotamers

Ji

vicinal

values

lized Karplus

the three rotamers

the reversed couplings

can be calculated

equation

assignation,

corrected

combination

around

with the empirical 7.5 Hz calculated

Ji are calculated

of the

isomeric

by applying

using eqn.(l)

from two observed

state

of the indi-

can be calculated

corresponding

vicinal

couplings. populations

the values

The resulting

to compounds

Such are

for the vicinal Karplus

values for the

1 and 2 are 5.6 and

1, in reasonable

values of 5.3 and 5.6 Hz. The respective

of

1, (a), and for

using the generalized

of H6 and H6' given in Table

the assignation

the genera-

the mol fractions

of H6 and H6' shown in Table

by a factor of 0.58 (ref.9).

reversing

about

= I

(b). From these populations

J(C4,H6)+J(C4,H6’)

rotamers

orientation

Xi by

the C5-C6 bond. The resulting

and J(C4,H6')

4.9 Hz for the assignment

the relative

populations

XT+XG_+XG+

2 for the assignment

J(C4,H6)

The three possible

Under a three rotational

(ref.8). Therefore,

is the case for rotation

on the internal

J is related to the couplings

for the H-H couplings

equation

given in Table

coupling

and their relative

J = XTJT + XG-JG_ + XG+JG+ ; Reliable

information

T, G-, and Gt, delineating

oxigen substituents.

model each measured vidual

J give direct

the C4-C5 and C5-C6 bonds.

each bond are denoted respective

constants

agreement

values of 7.3 and

of H6 and H6' are far larger than

390 the empirical

ones.

Populations separately

for the three rotamers

about the C4-C5 bond cannot be calculated

from only the alone available

their values can be bounded.

However,

the individual

culated when the two pseudo H-H couplings H4) and J(C3,H5)

couplings,

H-H vicinal

coupling

J(H4,H5)

mol fractions

although

can be cal-

, obtained dividing by 0.58 the J(C6,

are also taken in consideration.

values are given in Table Z,(a), together with the allowed

The resulting ranges calculated

from J(H4,H5),(b). Comparison MM2 method

of data in Table 2 shows that the populations

are in reasonable

for rotamers

According

rotamer

for L-ascorbic

acid 1 are G-(4-5)

for Ogawa et al. from the vicinal

given in Table and G-(5-6). and geminal

show that there is also a substantial rotamersaround

not provide

that J(C4,H6)

precise

Comparing individual favoured.

rotamers,

Paukstelis

concluded

is G-(5-6)

2 the most favoured

This conclusion C-H couplings

populated

rotamers

was first deduced

(ref.1).

T(5-6).

Our results

However,

as Spoormaker

the

and Bye

is well determined.

with the expected

that rotamer T(5-6)

of conformational

though the population

on

as it was already

their combination

but after H6 and H6' were assigned we arrived rotamer

for 1

of the

for 2).

values for these couplings,

vicinal H-H couplings

Under the assumption

(G'(5-6)

= 2.4 Hz. The spectrum at 25.2 MHz does

et al. (ref.6), though

the measured

solution

, and also Ogawa, based their conclusions

= J(C4,H6’)

individual

noted by Paukstelis

about the C5-C6

at the expense

amount of rotamer

the C5-C6 bond are not equally

(ref. 4). These authors

the assumption

in aqueous

by the MM2 method

by the

by NMR, specially

For rotamers

(T(5-6) for 1 and G-(5-6)

to the NMR populations

calculated

those determined

of the one most favoured

for 2) is overestimated

second populated

suggested

with

about the C4-C5 bond and for ~=1.5.

bond the population and G+(5-6)

agreement

equilibrium

values for the is the one most

two solutions

to the conclusion

of T(5-6)

exist

that the main

is also important.

REFERENCES T. S. R. T. C.

Ogawa, J. Uzawa and M. Matsui, Carbohydr. Res., 59(1977)C32-C35. Berger, Tetrahedron, 33(1977)1587-1589. Matusch, Z. Naturforsch. 32b(1977)562-568. Spoormaker and M.J.A. de Bie, Reel. Trav. Chim. Pays-Bas, 98(1979)59-64. Acerete, L. GarrigBs, J. Guilleme, E. Diez and A. Aldaz, Electrochim. Acta

26(1981)1041-1045. J.V. Paukstelis,

D.D. Mueller, P.A. Seib and D.W. Lillard,jr., in P.A. Seib and B.M. Tolbert (Eds.), Ascorbic Acid: Chemistry, Metabolism and Uses, American Chemical Society, Washington, 1982, pp. 125-151. N.L. Allinger, J. Am. Chem. Sot. 99(1977)8127-8134. C.A.G. Haasnoot, F.A.A.M. de Leeuw and C. Altona, Tetrahedron, 36(1980) 2783-2792. T. Spoormaker and M.J.A. de Bie, Reel. Trav. Chim. Pays-Bas., 99(1980) 154-160.