Short H-bonds Research Articles

4-Methoxypicolinic Acid N-Oxide: One of the Shortest Hydrogen Bonds Known Characterized by Neutron Diffraction, Inelastic Neutron Scattering, Infrared Spectroscopy, and Periodic DFT Calculations.

The present work focuses on the case of an extremely short intramolecular O-H···O hydrogen bond (H-bond) found in 4-methoxypicolinic acid N-oxide (MPANO). The donor···acceptor separation of 2.403 Å makes the H-bond in MPANO one of the shortest H-bonds known. We elucidated the structure and dynamics of the H-bond by two neutron-based techniques, namely, single-crystal diffraction and inelastic scattering (INS) vibrational spectroscopy. We also utilized conventional infrared (IR) spectroscopy as well as quantum chemical computations on isolated and periodic models. Both the protiated and deuterated variants of MPANO were investigated by INS and IR. All the methods used unequivocally confirm the existence of an extremely short, asymmetric H-bond, with the proton located near yet off the midpoint. The main relevant feature of the IR spectrum is an extremely broad, complex, and red-shifted OH (OD) stretching band spanning between 1800 and 500 cm-1 and centered at about 1360 cm-1, which indicates the presence of extensive anharmonicity and coupling with other H-bond modes. Of the modes characteristic of H-bond dynamics, only the out-of-plane OH (OD) bending can clearly be detected in the INS spectra; it has a relatively high frequency indicative of the strength of the H-bond. The computed structure is in excellent agreement with the diffraction measurement when periodicity is taken into account. The calculated harmonic frequencies show a reasonable match with the observed spectral features, whereby the assignment of the IR and INS spectra is facilitated. The hydrogen stretching frequency, however, appears to be significantly overestimated, on account of the limitations of the harmonic approximation and the complex nature of the short H-bond.

Effects of alkyl chain length and halide anion on hydrogen bonding, electrochemical transport properties and double layer capacitance in eutectic solvents

Eutectic solvents (ESs) have been widely studied due to their tunable solvation and physical properties. The properties of eutectic solvents that are formed from mixtures of hydrogen bond acceptors (HBAs) and hydrogen bond donors (HBDs) are governed by a complex hydrogen (H) bonded network among their constituents. However, an understanding of the key factors effecting the H-bonding structure and dynamics, and the resulting physical and electrochemical properties at interfaces is still in its infancy. In this study, the influence of the cation’s alkyl chain length and the anion chemical identity on the characteristics of the H-bonding network in eutectic solvents were examined. In particular, ESs formed from mixtures of tetraalkylammonium salts as HBAs and ethylene glycol as the HBD were studied. Complementary spectroscopic studies revealed the changes in the solvation environment as a function of the ammonium chain length (C1, C2, C3, and C4) and the anion type (Cl−, Br−, I−). With an increase in the alkyl chain length or a decrease in the electronegativity of halide anions (X−), it was found that the O–H bond length shortens and the H···X− bond elongates in the H-bond (O–H···X−) structure, thus resulting in an increase of the solvated ion size and a decrease in the bulk ionic conductivity. Further, double layer capacitance measurements showed that increased capacitance was correlated with shorter H-bonds in anion solvates.

Syntheses, Structures, and Corrosion Inhibition of Various Alkali Metal Carboxylate Complexes.

Complexes of the alkali metals Li-Cs with 3-thiophenecarboxylate (3tpc), 2-methyl-3-furoate (2m3fur), 3-furoate (3fur), 4-hydroxycinnamate (4hocin), and 4-hydroxybenzoate (4hob) ions were prepared via neutralisation reactions, and the structures of [Li2(3tpc)2]n (1Li); [K2(3tpc)2]n (1K); [Rb(3tpc)(H2O)]n (1Rb); [Cs{H(3tpc)2}]n (1Cs); [Li2(2m3fur)2(H2O)3] (2Li); [K2(2m3fur)2(H2O)]n (2K); [Li(3fur)]n(3Li); [K(4hocin](H2O)3]n (4K); [Rb{H(4hocin)2}]n.nH2O (4Rb); [Cs(4hocin)(H2O)]n (4Cs); [Li(4hob)]n (5Li); [K(4hob)(H2O)3]n (5K); [Rb(4hob)(H2O)]n (5Rb); and [Cs(4hob)(H2O)]n (5Cs) were determined via X-ray crystallography. Bulk products were also characterised via XPD, IR, and TGA measurements. No sodium derivatives could be obtained as crystallographically suitable single crystals. All were obtained as coordination polymers with a wide variety of carboxylate-binding modes, except for dinuclear 2Li. Under conditions that normally gave coordinated carboxylate ions, the ligation of hydrogen dicarboxylate ions was observed in 1Cs and 4Rb, with short H-bonds and short O…O distances associated with the acidic hydrogen. The alkali-metal carboxylates showed corrosion inhibitor properties inferior to those of the corresponding rare-earth carboxylates.

Distinction between low-barrier hydrogen bond and ordinary hydrogen bond: a case study of varying nature of charge assisted hydrogen bonds of diglycine perchlorate crystal

Hydrogen bonding is a complex phenomenon that is a resultant of many energy components like the electrostatic, dispersive, covalent, charge cloud overlap repulsion etc, nature of hydrogen bond (H-bond) depends on which of these components play a dominant role. Low barrier hydrogen bond (LBHB) constitutes a special category of hydrogen bonds characterized by near delocalization of proton between donor and acceptor groups of the H- bond unlike an ordinary hydrogen bond (OHB) having proton clearly localized near the donor group. The significance of LBHBs in macromolecular interactions has been highly controversial, despite may attempts the existence and potential importance of protein LBHBs remains debatable. In order to answer questions like whether or not a distinct class of LBHBs exists and if they do exist under what conditions they are formed and how do they behave differently from OHBs, a detailed study of H-bonding in Diglycine Perchlorate (DGPCl) crystal containing five unique hydrogen bonded glycinium-glycine pairs is undertaken. All O-H–O bonds of DGPCl are between the carboxyl (-COOH) and carboxylate (-COO−) groups with slightly different electron distributions resulting in observable variations in the H-bond geometries, this is an indication of varying strength of these short strong H-bonds. It is found that LBHB nature of the five O-H—O bonds between glycinium-glycine pairs (P1-P5) varies as P1 < P4 < P2 < P3 < P5. This study gives an experimental evidence of the existence of LBHBs and demonstrates that the behaviour of LBHBs is very different from that of strong OHBs.

Mechanism of Asparagine-Mediated Proton Transfer in Photosynthetic Reaction Centers.

In photosynthetic reaction centers from purple bacteria (PbRCs), light-induced charge separation leads to the reduction of the terminal electron acceptor quinone, QB. The reduction of QB to QB•- is followed by protonation via Asp-L213 and Ser-L223 in PbRC from Rhodobacter sphaeroides. However, Asp-L213 is replaced with nontitratable Asn-L222 and Asn-L213 in PbRCs from Thermochromatium tepidum and Blastochloris viridis, respectively. Here, we investigated the energetics of proton transfer along the asparagine-involved H-bond network using a quantum mechanical/molecular mechanical approach. The potential energy profile for the H-bond between H3O+ and the carbonyl O site of Asn-L222 shows that the proton is predominantly localized at the Asn-L222 moiety in the T. tepidum PbRC protein environment, easily forming the enol species. The release of the proton from the amide -NH2 site toward Ser-L232 via tautomerization suffers from the energy barrier. Upon reorientation of Asn-L222, the enol -OH site forms a short low-barrier H-bond with Ser-L232, facilitating protonation of QB•- in a Grotthuss-like mechanism. This is a basis of how asparagine or glutamine side chains function as acceptors/donors in proton transfer pathways.

Common Mechanism of Activated Catalysis in P-loop Fold Nucleoside Triphosphatases-United in Diversity.

To clarify the obscure hydrolysis mechanism of ubiquitous P-loop-fold nucleoside triphosphatases (Walker NTPases), we analysed the structures of 3136 catalytic sites with bound Mg-NTP complexes or their analogues. Our results are presented in two articles; here, in the second of them, we elucidated whether the Walker A and Walker B sequence motifs—common to all P-loop NTPases—could be directly involved in catalysis. We found that the hydrogen bonds (H-bonds) between the strictly conserved, Mg-coordinating Ser/Thr of the Walker A motif ([Ser/Thr]WA) and aspartate of the Walker B motif (AspWB) are particularly short (even as short as 2.4 ångströms) in the structures with bound transition state (TS) analogues. Given that a short H-bond implies parity in the pKa values of the H-bond partners, we suggest that, in response to the interactions of a P-loop NTPase with its cognate activating partner, a proton relocates from [Ser/Thr]WA to AspWB. The resulting anionic [Ser/Thr]WA alkoxide withdraws a proton from the catalytic water molecule, and the nascent hydroxyl attacks the gamma phosphate of NTP. When the gamma-phosphate breaks away, the trapped proton at AspWB passes by the Grotthuss relay via [Ser/Thr]WA to beta-phosphate and compensates for its developing negative charge that is thought to be responsible for the activation barrier of hydrolysis.

Importance of Nuclear Quantum Effects for Molecular Cocrystals with Short Hydrogen Bonds.

Many efforts have been recently devoted to the design and investigation of multicomponent pharmaceutical solids, such as salts and cocrystals. The experimental distinction between these solid forms is often challenging. Here, we show that the transformation of a salt into a cocrystal with a short hydrogen bond does not occur as a sharp phase transition but rather a smooth shift of the positional probability of the hydrogen atoms. A combination of solid-state NMR spectroscopy, X-ray diffraction, and diffuse reflectance measurements with density functional theory calculations that include nuclear quantum effects (NQEs) provides evidence of temperature-induced hydrogen atom shift in cocrystals with short hydrogen bonds. We demonstrate that for the predictions of the salt/cocrystal solid forms with short H-bonds, the computations have to include NQEs (particularly hydrogen nuclei delocalization) and temperature effects.

Path integral Monte Carlo simulations of the geometrical effects in KDP crystals

Path integral Monte Carlo (PIMC) simulations with very simple models were used in order to unveil the physics behind the isotope effects in H-bonded ferroelectrics. First, we studied geometrical effects in the H-bonds caused by deuteration with a general three-site model based on a back-to-back double Morse potential plus a Morse potential between oxygens, fitted to explain different general features for a wide set of H-bonded compounds. Our model results show the Ubbelohde or geometrical effect (GE), i.e., the expansion of the H-bond with deuteration, in agreement to what is observed in H-bonded ferroelectrics with short H-bonds. Moreover, adjusting the potential parameters to ab initio results, we have developed a 1D model which considers the bilinear proton-proton interaction in mean-field to study nuclear quantum effects that give rise to the GE in KDP crystals. PIMC simulations reveal that protons tunnel more efficiently than deuterons along the 1D chain, giving rise to a strong attraction center that pulls the oxygens together. This mechanism, which is based on the correlation between tunneling and geometrial modifications of the H-bonds, leads to a strong GE in the ordered phase of the chain at low temperature which is in good agreement with the experimental data.

Protonation Equilibrium in the Active Site of the Photoactive Yellow Protein.

The role and existence of low-barrier hydrogen bonds (LBHBs) in enzymatic and protein activity has been largely debated. An interesting case is that of the photoactive yellow protein (PYP). In this protein, two short HBs adjacent to the chromophore, p-coumaric acid (pCA), have been identified by X-ray and neutron diffraction experiments. However, there is a lack of agreement on the chemical nature of these H-bond interactions. Additionally, no consensus has been reached on the presence of LBHBs in the active site of the protein, despite various experimental and theoretical studies having been carried out to investigate this issue. In this work, we perform a computational study that combines classical and density functional theory (DFT)-based quantum mechanical/molecular mechanical (QM/MM) simulations to shed light onto this controversy. Furthermore, we aim to deepen our understanding of the chemical nature and dynamics of the protons involved in the two short hydrogen bonds that, in the dark state of PYP, connect pCA with the two binding pocket residues (E46 and Y42). Our results support the existence of a strong LBHB between pCA and E46, with the H fully delocalized and shared between both the carboxylic oxygen of E46 and the phenolic oxygen of pCA. Additionally, our findings suggest that the pCA interaction with Y42 can be suitably described as a typical short ionic H-bond of moderate strength that is fully localized on the phenolic oxygen of Y42.

Controlling Light-Induced Proton Transfer from the GFP Chromophore.

Green Fluorescent Protein (GFP) is known to undergo excited-state proton transfer (ESPT). Formation of a short H-bond favors ultrafast ESPT in GFP-like proteins, such as the GFP S65T/H148D mutant, but the detailed mechanism and its quantum nature remain to be resolved. Here we study in vacuo, light-induced proton transfer from the GFP chromophore in hydrogen-bonded complexes with two anionic proton acceptors, I- and deprotonated trichloroacetic acid (TCA- ). We address the role of the strong H-bond and the quantum mechanical proton-density distribution in the excited state, which determines the proton-transfer probability. Our study shows that chemical modifications to the molecular network drastically change the proton-transfer probability and it can become strongly wavelength dependent. The proton-transfer branching ratio is found to be 60 % for the TCA complex and 10 % for the iodide complex, being highly dependent on the photon energy in the latter case. Using high-level ab initio calculations, we show that light-induced proton transfer takes place in S1 , revealing intrinsic photoacid properties of the isolated GFP chromophore in strongly bound H-bonded complexes. ESPT is found to be very sensitive to the topography of the highly anharmonic potential in S1 , depending on the quantum-density distribution upon vibrational excitation. We also show that the S1 potential-energy surface, and hence excited-state proton transfer, can be controlled by altering the chromophore microenvironment.

Crossover from hydrogen to chemical bonding.

Hydrogen bonds (H-bonds) can be interpreted as a classical electrostatic interaction or as a covalent chemical bond if the interaction is strong enough. As a result, short strong H-bonds exist at an intersection between qualitatively different bonding descriptions, with few experimental methods to understand this dichotomy. The [F-H-F]- ion represents a bare short H-bond, whose distinctive vibrational potential in water is revealed with femtosecond two-dimensional infrared spectroscopy. It shows the superharmonic behavior of the proton motion, which is strongly coupled to the donor-acceptor stretching and disappears on H-bond bending. In combination with high-level quantum-chemical calculations, we demonstrate a distinct crossover in spectroscopic properties from conventional to short strong H-bonds, which identify where hydrogen bonding ends and chemical bonding begins.

The nature of short hydrogen bonds

Chemical Physics Hydrogen bonding (H-bonding) unquestionably plays an important role in chemical and biological systems and is responsible for some of their unusual properties. Strong, short H-bonds constitute a separate class that, owing to their elusive characterization, has remained a point of contention over the past several decades. Using femtosecond two-dimensional infrared spectroscopy in conjunction with quantum chemical calculations, Dereka et al. demonstrate a powerful way to investigate the nature of short H-bonding (see the Perspective by Bonn and Hunger). Their quantitative characterization of multiple coupled motions in the model system of bifluoride anion [F-H-F]− in aqueous solution reveals several distinctive features of a crossover from conventional to short, strong H-bonding. Science , this issue p. [160][1] see also p. [123][2] [1]: /lookup/doi/10.1126/science.abe1951 [2]: /lookup/doi/10.1126/science.abf3543

Protein secondary structure motifs: A kinematic construction.

The kinematic geometry of protein backbone structures, constrained by either single or multiple hydrogen bonds (H-bonds), possibly in a periodic array, is discussed. These structures include regular secondary structure elements α-helices and β-sheets but also include other short H-bond stabilized irregular structural elements like β-turns. The work here shows that the variations observed in such structures have simple geometrical correlations consistent with constrained motion kinematics. A new classification of the ideal helices is given, in terms of the parameter α, the angle at a Cα atom to its two neighboring Cα 's along the helix, and shown how it can be generalized to include nonideal helices. Specifically, we derive an analytical expression of the backbone dihedrals, (ϕ, ψ), in terms of the parameter α subject to the constraint that the peptide planes are parallel to the helical axis. Helices constructed in this way exhibit near-vertical alignment of the C = O and N - H units and are the canonical objects of this study. These expressions are easily modifiable to include perturbations of parameters relevant to nonplanar peptide units and noncanonical angles. The addition of a second parameter, ε0 , inclination of successive peptide planes along a helix with respect to the helical axis leads to a generalization of the previous expression and provides an efficient parametrization of such structures in terms of coordinates consistent with H-bond parameters. An analogs parametrization of β-turns, using inverse kinematic methods, is also given. Besides offering a unifying viewpoint, our results may find useful applications to protein and peptide design.

Proton transfer pathway from the oxygen-evolving complex in photosystem II substantiated by extensive mutagenesis

We report a structure-based biological approach to identify the proton-transfer pathway in photosystem II. First, molecular dynamics (MD) simulations were conducted to analyze the H-bond network that may serve as a Grotthuss-like proton conduit. MD simulations show that D1-Asp61, the H-bond acceptor of H2O at the Mn4CaO5 cluster (W1), forms an H-bond via one water molecule with D1-Glu65 but not with D2-Glu312. Then, D1-Asp61, D1-Glu65, D2-Glu312, and the adjacent residues, D1-Arg334, D2-Glu302, and D2-Glu323, were thoroughly mutated to the other 19 residues, i.e., 114 Chlamydomonas chloroplast mutant cells were generated. Mutation of D1-Asp61 was most crucial. Only the D61E and D61C cells grew photoautotrophically and exhibit O2-evolving activity. Mutations of D2-Glu312 were less crucial to photosynthetic growth than mutations of D1-Glu65. Quantum mechanical/molecular mechanical calculations indicated that in the PSII crystal structure, the proton is predominantly localized at D1-Glu65 along the H-bond with D2-Glu312, i.e., pKa(D1-Glu65) > pKa(D2-Glu312). The potential-energy profile shows that the release of the proton from D1-Glu65 leads to the formation of the two short H-bonds between D1-Asp61 and D1-Glu65, which facilitates downhill proton transfer along the Grotthuss-like proton conduit in the S2 to S3 transition. It seems possible that D1-Glu65 is involved in the dominant pathway that proceeds from W1 via D1-Asp61 toward the thylakoid lumen, whereas D2-Glu312 and D1-Arg334 may be involved in alternative pathways in some mutants.

Heat capacity effects associated with urea and tetramethylurea hydration: insight from computer simulation

Molecular dynamic simulation of the hydrophilic urea and hydrophobic tetramethylurea aqueous solutions using the random network model of liquid water reveals that apolar Me groups induce an increase in the population of water molecules with linear and shorter H-bonds in their first hydration shell, whereas the carbonyl oxygen atom causes an opposite effect with elevation in the population of high angle/distance water molecules pairs. This behavior of water is the major reason for opposite changes in the heat capacity of hydration for apolar and polar species.

Energetics of Ionized Water Molecules in the H-Bond Network near the Ca2+ and Cl- Binding Sites in Photosystem II.

In photosystem II, water oxidation occurs at the oxygen-evolving complex (OEC). The presence of a hydronium ion (H3O+) was proposed at the Cl- binding site and Ca2+-depleted OEC. Using a quantum mechanical/molecular mechanical approach, we report the stability of H3O+ in the PSII protein environment. Neither release of the proton from ligand water molecule W2 at the OEC nor formation of H3O+ at Cl- is energetically favorable. In contrast, H3O+ can exist at the Ca2+-depleted OEC. Even when H3O+ exists in Ca2+-depleted PSII, the H-bond network of the redox-active tyrosine (TyrZ) remains unaltered, retaining the unusually short low-barrier H-bond with D1-His190, and the redox potential of TyrZ, Em(TyrZ), remains unaltered. These observations explain why the oxidation of the Ca2+-depleted Mn4O5 cluster by TyrZ (i.e., the S2 to S3 transition) is not inhibited at low pH. It seems likely that Ca2+ plays a role in not only (i) maintaining the H-bond network and facilitating TyrZ oxidation [tuning Em(TyrZ)] but also (ii) providing the valence of +2, decreasing the pKa of the ligand molecule (W1), and facilitating the release of the proton from W1 in the S2 to S3 transition together with Cl-.

The nano-structural inhomogeneity of dynamic hydrogen bond network of TIP4P/2005 water

A method for studying the time dependence of the short-range molecular order of water has been proposed. In the present study, water is considered as a dynamic network between molecules at distances not exceeding 3.2 Å. The instantaneous configurations obtained with the molecular dynamics method have been sequentially analyzed. The mutual orientation of each molecule with its neighboring molecules has been studied and the interaction energy of each pair of neighbor molecules has been calculated. The majority of mutual orientation angles between molecules lie in the interval [0°; 20°]. More than 85% of the molecular pairs in each instantaneous configuration form H-bonds and the H-bond network includes all water molecules in the temperature range 233–293 K. The number of H-bonds fluctuates near the mean value and increases with decreasing temperature, and the energy of the vast majority of such bonds is much higher than the thermal energy. The interaction energy of 80% of the H-bonding molecular pairs lies in the interval [−7; −4] kcal/mol. The interaction energy of pairs that do not satisfy the H-bond angle criterion lies in the interval [−5; 4] kcal/mol; the number of such bonds does not exceed 15% and decreases with decreasing temperature. For the first time it has been found that in each instantaneous configuration the H-bond network contains built-in nanometric structural heterogeneities formed by shorter H-bonds. The fraction of molecules involved in the structural heterogeneities increases from 40% to 60% with a temperature decrease from 293 K to 233 K. Each heterogeneity has a finite lifetime and changeable structure, but they are constantly present during the entire simulation time.

H/D Isotope Effects on 1H-NMR Chemical Shifts in Cyclic Heterodimers and Heterotrimers of Phosphinic and Phosphoric Acids.

Hydrogen-bonded heterocomplexes formed by POOH-containing acids (diphenylphosphoric 1, dimethylphosphoric 2, diphenylphosphinic 3, and dimethylphosphinic 4) are studied by the low-temperature (100 K) 1H-NMR and 31P-NMR using liquefied gases CDF3/CDF2Cl as a solvent. Formation of cyclic dimers and cyclic trimers consisting of molecules of two different acids is confirmed by the analysis of vicinal H/D isotope effects (changes in the bridging proton chemical shift, δH, after the deuteration of a neighboring H-bond). Acids 1 and 4 (or 1 and 3) form heterotrimers with very strong (short) H-bonds (δH ca. 17 ppm). While in the case of all heterotrimers the H-bonds are cyclically arranged head-to-tail, ···O=P–O–H···O=P–O–H···, and thus their cooperative coupling is expected, the signs of vicinal H/D isotope effects indicate an effective anticooperativity, presumably due to steric factors: when one of the H-bonds is elongated upon deuteration, the structure of the heterotrimer adjusts by shortening the neighboring hydrogen bonds. We also demonstrate the formation of cyclic tetramers: in the case of acids 1 and 4 the structure has alternating molecules of 1 and 4 in the cycle, while in case of acids 1 and 3 the cycle has two molecules of 1 followed by two molecules of 3.

1,2H hyperfine spectroscopy and DFT modeling unveil the demethylmenasemiquinone binding mode to E. coli nitrate reductase A (NarGHI)

The quinol oxidation site QD in E. coli respiratory nitrate reductase A (EcNarGHI) reacts with the three isoprenoid quinones naturally synthesized by the bacterium, i.e. ubiquinones (UQ), menaquinones (MK) and demethylmenaquinones (DMK). The binding mode of the demethylmenasemiquinone (DMSK) intermediate to the EcNarGHI QD quinol oxidation site is analyzed in detail using 1,2H hyperfine (hf) spectroscopy in combination with H2O/D2O exchange experiments and DFT modeling, and compared to the menasemiquinone one bound to the QD site (MSKD) previously studied by us. DMSKD and MSKD are shown to bind in a similar and strongly asymmetric manner through a short (~1.7 Å) H-bond. The origin of the specific hf pattern resolved on the DMSKD field-swept EPR spectrum is unambiguously ascribed to slightly inequivalent contributions from two β-methylene protons of the isoprenoid side chain. DFT calculations show that their large isotropic hf coupling constants (Aiso ~12 and 15 MHz) are consistent with both (i) a specific highly asymmetric binding mode of DMSKD and (ii) a near in-plane orientation of its isoprenyl chain at Cβ relative to the aromatic ring, which differs by ~90° to that predicted for free or NarGHI-bound MSK. Our results provide new insights into how the conformation and the redox properties of different natural quinones are selectively fine-tuned by the protein environment at a single Q site. Such a fine-tuning most likely contributes to render NarGHI as an efficient and flexible respiratory enzyme to be used upon rapid variations of the Q-pool content.

Revealing Single-Bond Anomeric Selectivity in Carbohydrate-Protein Interactions.

The noncovalent binding of proteins to glycans is amazingly selective to the isoforms of carbohydrates, including α/β anomers that coexist in solution. We isolate in the gas phase and study at the atomic level the simplest model system: noncovalent complexes of monosaccharide α/β-GalNAc and protonated aromatic molecule tyramine. IR/UV cold ion spectroscopy and quantum chemistry calculations jointly solve the structures of the two complexes. Although the onsets of the measured UV absorptions of the complexes differ significantly, the networks of H bonds in both complexes appear identical and do not include the anomeric hydroxyl. The detailed analysis reveals that, through inductive polarization, the α- to β-reorientation of this group nevertheless reduces the length of one remote short intermolecular H-bond by 0.03 Å. Although small, this change substantially strengthens the bond, thus contributing to the anomeric selectivity of the binding.