Article pubs.acs.org/JPCA
Monosaccharide-Water Complexes: Vibrational Spectroscopy and Anharmonic Potentials Lin Jin,† John P. Simons,‡ and R. Benny Gerber*,†,§ †
Department of Chemistry, University of California, Irvine, California 92697-2025, United States Department of Chemistry, Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QZ, U.K. § Institute of Chemistry and The Fritz Haber Research Center, The Hebrew University, Jerusalem 91904, Israel ‡
S Supporting Information *
ABSTRACT: Ab initio vibrational self-consistent field (VSCF) calculations are used to predict the vibrational spectra of an extended series of monosaccharide·D2O complexes, including glucose, galactose, mannose, xylose, and fucose in their α and β anomeric forms, and compared with recently published experimental data for their (phenyl-tagged) complexes. Anharmonic VSCF-PT2 frequencies are calculated directly, using ab initio hybrid HF/MP2 potentials, to assess their accuracy in reproducing the vibrational anharmonicities and provide a more rigorous basis for vibrational and structural assignments. The average discrepancies between the calculated and experimental frequencies are ∼1.0−1.5%, and the firstprinciples spectroscopic calculations, free of any empirical scaling, yield results of high accuracy. They encourage confidence in their future application to the assignment of other carbohydrate systems, both free and complexed, and an improved understanding of their intra- and intermolecular carbohydrate interactions.
1. INTRODUCTION Identifying carbohydrate structures in differing environments presents complex experimental and computational challenges, not least because of their flexibility and in aqueous environments, the universality of hydrogen-bonded interactions. Although intramolecular hydrogen bonding can be disrupted,1 their preferred conformations may also be influenced by explicit hydration2,3 (or dehydration)which can be revealed by probing their structures spectroscopically. The interrogation of free and hydrated carbohydrates under molecular beam conditions, using double resonance infrared ion dip (IRID) spectroscopy4 generates their mass and conformer selected OH vibrational spectra. Subsequent comparisons with the predictions of force field, density functional theory (DFT), or ab initio electronic structure calculations, then allows their structural assignment. Infrared spectroscopy probes the “vibrational signatures” and computational chemistry translates these into the molecular and conformational structuresprovided the calculations are sufficiently accurate! Applying these techniques, the influence of hydration on the conformational structures of a steadily increasing series of monosaccharides2,3,5−9 and small polysaccharides,2,3,10−12 has been explored, using empirically scaled harmonic vibrational frequency calculations to provide the assignments: these are computationally feasible even for relatively large polysaccharides. The accuracy and reliability of scaling factors are open to doubt, however, particularly when there is strong hydrogen bonding (a common occurrence in carbohydrates and their hydrated complexes). In this situation the potentials may be © 2012 American Chemical Society
strongly anharmonic, allowing coupling between different normal modes.13 The magnitudes of empirical scaling factors, which are used to bring the predicted harmonic frequencies into better coincidence with the observed frequencies, do reflect in part, the consequences of anharmonicity in the potential energy surface, but they have no theoretical basis. Strong hydrogen bonding can also lead to vibrational band broadening and further technical problems can be created by the overlap of multiple OH vibrational bands with increasing hydration number and polysaccharide size. Each of these issues is addressed in the present work, which presents a systematic analysis of the recently published8,9 IR spectra of a series of monosaccharide·D2O complexes isolated in the gas phase, using the vibrational self-consistent field (VSCF) theory approach.14−17 The substitution of D2O by H2O allows the OH vibrational bands associated the carbohydrate, to be separated from the hydrogen bonded OD bands associated with the bound water molecule. The application of the VSCF method allows the anharmonic effects to be computed directly from the potential energy surface, rather than through empirical “adjustments” used to bring the harmonic spectra into better accord with experiment. Special Issue: Jörn Manz Festschrift Received: March 30, 2012 Revised: May 3, 2012 Published: May 4, 2012 11088
dx.doi.org/10.1021/jp303080k | J. Phys. Chem. A 2012, 116, 11088−11094
The Journal of Physical Chemistry A
Article
accuracy in reproducing the vibrational anharmonicities and to provide a more rigorous basis for vibrational and structural assignments.
Several variants of VSCF theory have now been developed15−18 and successfully applied to analyze the vibrational spectra of biological molecules isolated in the gas phase but relatively few have been applied to carbohydrates. The earliest by Gregurick et al.18 in 1999, calculated the VSCF spectrum of glucose associated with a molecular mechanics (Amber) force field, and identified substantial anharmonic effects. More recently, Brauer and co-workers, using potentials based on ab initio calculations, reported new results for glucose and also sucrose,19 while Jin et al. using the same methods, found good agreement between the predicted and observed OH and OD vibrational spectra of the hydrated monosaccharide, O-phenyl β 20 D-xylopyranoside·D2O (β-PhGlc·D2O). This success has encouraged the present, sterner and more systematic test of ab initio VSCF calculations in predicting the vibrational spectra (and structures) of an extended series of monosaccharide·D2O complexes, including those of D-glucose (Glc), D-galactose (Gal), D-mannose (Man), D-xylose (Xyl) and L-fucose (Fuc) in their α and β anomeric forms, and gauging their accuracy against recent experimental data8,9 for their (phenyl-tagged) complexes, see Scheme 1. Aharmonic VSCF-PT2 frequencies
2. METHODS 2.1. Calculation of Harmonic Frequencies and Energies. All the initial structures were taken from the earlier, experimental report.8,9 The harmonic frequencies of the monosaccharide·D2O complexes were calculated at the MP2/ DZP and HF/DZV level (or for α-Gal·D2O, the HF/6-31G(d) level) using the GAMESS program package.21,22 These were subsequently used to provide improved HF anharmonic potentials (discussed in section 2.3). For α-Xyl·D2O and α-Fuc·D2O, MP2/ 6-311++G(d,p) was applied to get more accurate energies. 2.2. VSCF Algorithms. Since the VSCF algorithms have been extensively described elsewhere17,23,24 only a brief overview is presented here. Anharmonic vibrational frequencies were calculated using the VSCF method, included in the GAMESS program package.21,22 At the lowest level, the standard VSCF approximation assumes separable normal coordinates but although it provides a significant improvement over the harmonic approximation, better accuracy requires the incorporation of effects beyond vibrational separability, and the correlation effects between the normal modes are accommodated through the application of second-order perturbation theory.15−17 The resulting approximation, generally known as correlation-corrected VSCF(CC-VSCF), or in the present case, VSCF with perturbation corrections (VSCF-PT2), provides better accuracy and can also be used for moderately large systems.15−17 Several improvements providing shorter computation times, have also shown encouraging results.25−27 The calculation of multidimensional integrals in VSCF, the main computational difficulty in the VSCF algorithm, is costly and to reduce the computational effort, the only contributions to the potential of the system, V(Q1,...QN), that are considered are those from single normal modes, Vdiag j (Qj) and interacting pairs, Vijcoup(Qi,Qj); contributions from the higher order interactions are neglected. The potential thus can be written as
Scheme 1. Schematic Representation of Monosaccharide Structures, R = H, Ph
N
V (Q 1 , ..., Q N ) =
N
N
∑ V jdiag(Q j) + ∑ ∑ Vijcoup(Q i , Q j) j=1
i
i
