Adding water to sugar: A spectroscopic and computational study of α- and β-phenylxyloside in the gas phase

Abstract

The gas phase structures of phenyl alpha- and beta-d-xylopyranoside (alpha- and beta-pXyl) and their mono-hydrates have been investigated using a combination of resonant two-photon ionization (R2PI), ultra-violet hole-burning and resonant infrared ion dip spectroscopy, coupled with density functional theory (DFT) and ab initio computation. The hole-burning experiments indicate the population of a single conformer only, in each of the two anomers. Their experimental and calculated infrared spectra are both consistent with a conformational assignment corresponding to the computed global minimum configuration. All three OH groups are oriented towards the oxygen atom (O1) on the anomeric carbon atom to form an all trans(ttt) counter-clockwise chain of hydrogen bonds. The mono-hydrates, alpha- and beta-pXyl(H(2)O) each populate two distinct structures in the molecular beam environment, with the water molecule inserted between OH4 and OH3 or between OH3 and OH2 in alpha-pXyl(H2O), and between OH2 and O1 in either of two alternative orientations, in beta-pXyl(H2O). In all of the mono-hydrated xyloside complexes, the water molecule inserts into the weakest link of the sugar molecules' hydrogen-bonded chain of hydroxy groups, creating a single extended chain, strengthened by co-operativity. The all-trans configuration of the xylose moiety is retained and the mono-hydrate structures correspond to those calculated to lie at the lowest relative energies.

Ribose Found in the Gas Phase

“…Because of the geometry constraints of the tetrahydropyran chair the stronger hydrogen-bond contacts are between hydroxy groups in positions Ax-Ax (1.9-2.1 ), and decrease for Ax-Eq or Eq-Ax (2.2-2.4 ) and Eq-Eq (2.4-2.5 ) orientations, as shown in Figure 4 and Supporting Information Figure S1. Similar hydrogen-bond patterns were observed in the hexoses and pentoses [12] (although in these previous cases all O3-H groups were equatorial). While 4 C 1 chairs have most of the hydroxy groups equatorial, the stability of 1 C 4 chairs with all hydroxy groups axial raises additional questions on the magnitude of plausible stereoelectronic contributions, such as the anomeric effect, [31,13] which would modulate conformational populations favoring b forms in 1 C 4 and a forms in 4 C 1 .…”

“…global minimum, cc-b-pyr 1 C 4 , and three other structures exhibit a 1 C 4 -chair conformation, contrasting with the most stable 4 C 1 -chairs observed in the a-/b-aldo-hexoses and its deoxy derivatives detected with electronic spectroscopy: phenylglucose, phenylgalactose, phenylmannose, phenylfucose, and aldopentose phenylxylose. [12] However the stabilization of the 1 C 4 -chair is not large, since the second 4 C 1 ribose conformation is practically isoenergetic with the global minimum (DG = 0.0(6) kJ mol À1 ; MP2: DG = 0.2 kJ mol À1 ). The two chair conformations are associated with different (Ax/Eq) hydroxy orientations, which in turn, favor different intramolecular hydrogen bonds between vicinal or across-ring hydroxy groups.…”

“…To answer these questions we conducted a rotational study of ribose using microwave spectroscopy. A large volume of structural information on chromophore-tagged sugars has been collected by the Simons group [11][12][13] using UV-UV and IR-UV laser spectroscopy techniques and theoretical calculations. [14] However, laser spectroscopy provides only vibrational resolution and band assignment is not always unambiguous.…”

Microwave Spectroscopy of Biomolecular Building Blocks

“…Gas phase structures of phenyl α-and β-D-xylopyranoside have been investigated by UV-UV and IR-UV laser spectroscopic techniques coupled with theoretical calculations [270]. The authors hypothesized that the substitution of the anomeric OH group for a phenoxy group compatible with the UV excitation scheme has little effect in the conformational behavior.…”

Monosaccharide-Water Complexes: Vibrational Spectroscopy and Anharmonic Potentials

“…Xylose·D 2 O. The earlier experimental investigations of α-PhXyl·D 2 O 8 (and H 2 O) 28 identified two distinct structures, with very similar IRID spectra, one of which, associated with the more strongly populated structure, is shown in Figure 4. It was tentatively assigned to one or other of the two lowest energy structures, cc.ins(3,2) I or cc.ins(4,3) II, 8 calculated (at the MP2/6-311++G(d,p) level) to be separated by only 0.4 kJ mol −1 .…”