Molecular Modeling - American Chemical Society

Chapter 19

NMR and Molecular Modeling Evidence for Entrapment of Water in a Simple Carbohydrate Complex 1

P. Irwin, Gregory King, Thomas F. Kumosinski, P. Pieffer, J. Klein , and L. Doner Downloaded by MONASH UNIV on October 26, 2012 | http://pubs.acs.org Publication Date: December 14, 1994 | doi: 10.1021/bk-1994-0576.ch019

Eastern Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture, 600 East Mermaid Lane, Philadelphia, PA 19118 During structural investigations of a glucuronic acid derivative dissolved in DMSO we recognized a water activity-dependency between the NOESY cross-peaks of H O and the carbohydrate's hydroxyl protons. The -OHHO first order exchange rate constant increased from 0.32 to 11.14 s as the molar ratio of H O:sugar increased from only ca. 4 to 5. The latter finding indicated that the -OHHO proton exchange process, which is proportional to the translational diffusion of water, diminished as H O approached the concentration which exists in the crystalline structure and was, presumably, entrapped by our glucuronic acid derivative forming a stable complex. Supporting this, a significant upfield shift in the resonance frequencies of the hydroxyl (-OH Δδ = 86.33 Hz) protons was observed (CH Δδ = 0.25 Hz) when water was removed by reaction with 2,2dimethoxypropane. Molecular dynamics calculations (100 ps) on the energy-minimized carbohydrate-water complex confirm the presence of 2-3 near neighbor H O molecules associated with the polar functional groups. In fact, the computationally-derived weighted average distance of all water molecules adjacent to the -OH groups was found to be inversely proportional to the individual -OH Δδs. 2

2

-1

2

2

2

ave

ave

2

Knowledge about the interactions between carbohydrates and water is of some consequence because important chemical and physical properties are imparted by the way these compounds coexist. DMSO is a good solvent for understanding these interactions because carbohydrates retain much of their H 0-induced confor mation (1) in DMSO and one can specifically observe, assign and study a carbohydrate's hydroxyl exchange with small quantities of H 0 because the -OH resonance frequencies are dissimilar. In this chapter we present spin-lattice relaxation, 2D NMR, chemical shift and molecular dynamics evidence that N-phenyl vV-phenyl-/3-D-glucopyranosylamine) 2

2

1

Current address: Department of Field Crops, Volcani Center, Bet Dagan, Israel 0097-6156/94/0576-0342$08.00/0 © 1994 American Chemical Society In Molecular Modeling; Kumosinski, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

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IRWIN ET AL.

Entrapment of Water in a Simple Carbohydrate Complex

343

uronamide's (W-phenyl uronamide; Figure 1) waters of crystallization are tightly bound to the polar functional groups of the sugar moiety even after extreme dilution in DMSO.

Materials and Methods Sample Preparation. N-phenyl uronamide was synthesized and purified as described previously (2,3). D-glucopyranuronic acid was dissolved in H 0 (1.5 g/25 mL). Aniline (2 mL) was dropped slowly into the stirring rnixture and the pH adjusted to 4.75 on a Radiometer (reference to brand or firm name does not constitute endorsement by the U. S. Department of Agriculture over others of a similar nature not mentioned) pH stat. Approximately 3 g of l-ethyl-3-[3-(dimemylamino)propyl]carbodnmide (EDC) was added to the solution and the pH stat activated causing 0. IN HC1 to be delivered to the reaction rnixture to maintain the pH at ca. 4.75 (4-7). When no more titrant was needed to maintain a constant pH the reaction was complete. At this point an insoluble off-white precipitate had formed and was subsequently washed with H 0 to remove unreacted aniline or EDC. Excess water was removed by washing the precipitate with chilled EtOH. The acid sugar derivative was then dissolved in hot EtOH and 2-4 mm needle-like crystals formed overnight at room temperature. For production of the anhydrous form, the above procedure was repeated except that a small amount of 2,2dimethoxypropane was added to the EtOH to react with unwanted water (e.g., H 0 + 2,2-dimethoxypropane -> 2MeOH + acetone).

Downloaded by MONASH UNIV on October 26, 2012 | http://pubs.acs.org Publication Date: December 14, 1994 | doi: 10.1021/bk-1994-0576.ch019

2

2

2

NMR Spectroscopy. Samples for NMR were prepared in a dry box. N-phenyl uronamide crystals were dissolved in OMSO-d ( > 99.5 atom % H) which had been stored several days with molecular sieve pellets under dry N (the DMSO contained, except when specified, ca. 30 mM H 0 even in the presence of "dry" molecular sieves). Several D M S O - ^ washed molecular sieves were kept in the 5 mm NMR tubes to maintain the sample in a relatively dry state; the NMR tubes were closed and wrapped with parafilm or sealed under vacuum to assist in the exclusion of extraneous H 0 vapor. The samples were stored at 3°C and underwent no obvious degradative process, such as pyranose ring opening and associated Amadori rearrangement (2) or loss of die C amine functionality (Figure 2). Evidence for water activity dependent hydrolysis (2) is provided in Figure 3. Using reverse phase HPLC, one can see that increasing the water concentration from ca. 0 to 50% (v/v) in MeOH increases the first order rate constant by a factor of about 9. It is noteworthy that there was a significant degree of hydrolysis (t - 83 rnin) in absolute MeOH. For these kinetic experiments 3 mg of N-phenyl uronamide were dissolved in 10 mL of either 50% MeOH:H 0 or abs. MeOH and maintained at 40°C. At various times 100 pL of each solution was injected into an HP 1090 HPLC system equipped with a supelco LC-18 reverse phase (15 cm; 5 μτη particle size) column; 50 % MeOH was used as the mobile phase (0.2 mL min" ). The various peaks were checked against 2

6

2

2

2

1

1/2

2

1

In Molecular Modeling; Kumosinski, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

MOLECULAR MODELING

344


para"

Figure 1. labels.

1

Structure and conformation Af-phenyl uronamide H position

CONH—φ

partially insol

Amadori Rearrangement Product(s) Figure 2. Reaction scheme proposed for the formation of N-phenyl uronamide and its Amadori rearrangement product(s). Reproduced with permission from Ref. 2. Copyright 1990, Journal of Carbohydrate Chemistry.



19. IRWIN ET AL.


345

Normalized Concentration

0 H

1

1

0

5

10

1

1

1

1

15

20

25

30

time I min.

Figure 3. Change in the relative concentration of N-phenyl uronamide over time in 100% MeOH (open squares) and 50% MeOH:H 0 (closed squares) at 40°C. 2


MOLECULAR MODELING

346

standards of aniline and Λ^-phenyl-D-glucopyranuronarnide (e.g., Af-phenyl uronamide without the Cj arnine functional group). Before NMR experiments, the 90° pulse was determined for each condition, such as variable temperature or concentration, utilizing standard methods (8). All NOESY spectra were collected on a JEOL GX-400 NMR spectrometer system operated at ca. 400 MHz (9.40 T) using 5 mm probes (J). Computer line broadening was selected to be approximately equal to the digital resolution. These experiments were acquired using a matrix of 128 χ 1024 (tj χ t ), 256 χ 2048 after zero-filling, complex data points which represented a spectral width of 953.1 Hz for either dimension. For each tj spectrum collected, 16 transients were acquired. A sine-bell apodization function was used to process these data. All quantitative 2D Overhauser enhancement matrices were processed without symmetrization. All ROESY (2) data were collected using a JEOL GSX-400 NMR spectrometer with a proton full-power 90° pulse of 10.5 μβ. Acquisition data sets consisted of 2048 complex points for t and 64 acquisitions for each \ data set. A spin-lock field of 3 kHz, 1 kHz off-resonance from the average chemical shifts of the residual H 0 protons and the -OHs, was used for mixing times (r ) of 0.075, 0.2, 0.4 and 0.6 s. The data sets were zero-filled to 4096 t points and 2048 for ί . A phase-shifted sine-bell algorithm was used as the window function. All the -OH resonances ( H ) were integrated and fitted to an exponential function (equation 1)


2

2

x

2

m

2

Ί

2a

4a

(1)

I = Urn I

(2)

0

using a modified Gauss-Newton procedure developed in this laboratory by Dr. William Damert. Proton Tj inversion recovery experiments were performed on JEOL NMR spectrometers operated at either 400 or 270 MHz (9.40 or 6.34 T). Each X value was signal averaged for 64 acquisitions with 16 dummy scans. T experiments (9-11) were performed identically to the above except that the H 0 resonance was irradiated 721.67 Hz upfield from the C - O H (H ) resonance. All peak intensity data were fit to an exponential function (equation 3) utilizing the aforementioned curve-fitting procedure. l s a t

2

4

1

-2e

4a

(3)

Γι

The T -associated pseudo first-order rate constant (# ) calculation was lsat

sat


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IRWIN ET AL.


347

accomplished as shown in equation 4 ΓII*

(4)

x

sat

+

l

\sat

l

\sat

0

where I / I is the ratio of hydroxyl resonance integrals with irradiation on the H 0 resonance and 721.67 Hz downfield, respectively. T is the normal Tj measurement but with spin saturation of H 0 . The correlation time (r ) for the TV-phenyl uronamide Ή 0 complex and individual resonance Tjs (T ) were estimated using equation 5 2

l s a t

2

c

2


H

4r,

JL=_3_ 4

2π

~ 7

6

'

Molecular Modeling - American Chemical Society

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