Dequan Xiao

A donor-nanotube paradigm for nonlinear optical materials.

Studies of the nonlinear electronic response of donor/acceptor substituted nanotubes suggest a behavior that is both surprising and qualitatively distinct from that in conventional conjugated organic species. We find that the carbon nanotubes serve as both electronic bridges and acceptors, leading to a donor-nanotube paradigm for the effective design of large first hyperpolarizabilities. We also find that tuning the donor orientation, relative to the nanotube, can significantly enhance the first hyperpolarizability.

Modulating Unimolecular Charge Transfer by Exciting Bridge Vibrations

Ultrafast UV-vibrational spectroscopy was used to investigate how vibrational excitation of the bridge changes photoinduced electron transfer between donor (dimethylaniline) and acceptor (anthracene) moieties bridged by a guanosine-cytidine base pair (GC). The charge-separated (CS) state yield is found to be lowered by high-frequency bridge mode excitation. The effect is linked to a dynamic modulation of the donor-acceptor coupling interaction by weakening of H-bonding and/or by disruption of the bridging base-pair planarity.

Amphiphilic Adsorption of Human Islet Amyloid Polypeptide Aggregates to Lipid/Aqueous Interfaces

Many amyloid proteins misfold into β-sheet aggregates upon interacting with biomembranes at the onset of diseases, such as Parkinsons disease and type II diabetes. The molecular mechanisms triggering aggregation depend on the orientation of β-sheets at the cell membranes. However, understanding how β-sheets adsorb onto lipid/aqueous interfaces is challenging. Here, we combine chiral sum frequency generation (SFG) spectroscopy and ab initio quantum chemistry calculations based on a divide-and-conquer strategy to characterize the orientation of human islet amyloid polypeptides (hIAPPs) at lipid/aqueous interfaces. We show that the aggregates bind with β-strands oriented at 48° relative to the interface. This orientation reflects the amphiphilic properties of hIAPP β-sheet aggregates and suggests the potential disruptive effect on membrane integrity.

Chiral sum frequency generation for in situ probing proton exchange in antiparallel β-sheets at interfaces.

Studying hydrogen/deuterium (H/D) exchange in proteins can provide valuable insight on protein structure and dynamics. Several techniques are available for probing H/D exchange in the bulk solution, including NMR, mass spectroscopy, and Fourier transform infrared spectroscopy. However, probing H/D exchange at interfaces is challenging because it requires surface-selective methods. Here, we introduce the combination of in situ chiral sum frequency generation (cSFG) spectroscopy and ab initio simulations of cSFG spectra as a powerful methodology to probe the dynamics of H/D exchange at interfaces. This method is applied to characterize H/D exchange in the antiparallel β-sheet peptide LK7β. We report here for the first time that the rate of D-to-H exchange is about 1 order of magnitude faster than H-to-D exchange in the antiparallel structure at the air/water interface, which is consistent with the existing knowledge that O-H/D dissociation in water is the rate-limiting step, and breaking the O-D bond is slower than breaking the O-H bond. The reported analysis also provides fundamental understanding of several vibrational modes and their couplings in peptide backbones that have been difficult to characterize by conventional methods, including Fermi resonances of various combinations of peptide vibrational modes such as amide I and amide II, C-N stretch, and N-H/N-D bending. These results demonstrate cSFG as a sensitive technique for probing the kinetics of H/D exchange in proteins at interfaces, with high signal-to-noise N-H/N-D stretch bands that are free of background from the water O-H/O-D stretch.

Excited-state intramolecular hydrogen transfer (ESIHT) of 1,8-dihydroxy-9,10-anthraquinone (DHAQ) characterized by ultrafast electronic and vibrational spectroscopy and computational modeling.

We combine ultrafast electronic and vibrational spectroscopy and computational modeling to investigate the photoinduced excited-state intramolecular hydrogen-transfer dynamics in 1,8-dihydroxy-9,10-anthraquinone (DHAQ) in tetrachloroethene, acetonitrile, dimethyl sulfoxide, and methanol. We analyze the electronic excited states of DHAQ with various possible hydrogen-bonding schemes and provide a general description of the electronic excited-state dynamics based on a systematic analysis of femtosecond UV/vis and UV/IR pump-probe spectroscopic data. Upon photoabsorption at 400 nm, the S2 electronic excited state is initially populated, followed by a rapid equilibration within 150 fs through population transfer to the S1 state where DHAQ exhibits ESIHT dynamics. In this equilibration process, the excited-state population is distributed between the 9,10-quinone (S2) and 1,10-quinone (S1) states while undergoing vibrational energy redistribution, vibrational cooling, and solvation dynamics on the 0.1-50 ps time scale. Transient UV/vis pump-probe data in methanol also suggest additional relaxation dynamics on the subnanosecond time scale, which we tentatively ascribe to hydrogen bond dynamics of DHAQ with the protic solvent, affecting the equilibrium population dynamics within the S2 and S1 electronic excited states. Ultimately, the two excited singlet states decay with a solvent-dependent time constant ranging from 139 to 210 ps. The concomitant electronic ground-state recovery is, however, only partial because a large fraction of the population relaxes to the first triplet state. From the similarity of the time scales involved, we conjecture that the solvent plays a crucial role in breaking the intramolecular hydrogen bond of DHAQ during the S2/S1 relaxation to either the ground or triplet state.

Inverse molecular design in a tight-binding framework

The number of chemical species of modest molecular weight that can be accessed with known synthetic methods is astronomical. An open challenge is to explore this space in a manner that will enable the discovery of molecular species and materials with optimized properties. Recently, an inverse molecular design strategy, the linear combination of atomic potentials (LCAP) approach [J. Am. Chem. Soc. 128, 3228 (2006)] was developed to optimize electronic polarizabilities and first hyperpolarizabilities. Here, using a simple tight-binding (TB) approach, we show that continuous optimization can be carried out on the LCAP surface successfully to explore vast chemical libraries of 10(2) to 10(16) extended aromatic compounds. We show that the TB-LCAP optimization is not only effective in locating globally optimal structures based on their electronic polarizabilities and first hyperpolarizabilities, but also is straightforwardly extended to optimize transition dipole moments and HOMO-LUMO energy gaps. This approach finds optimal structures among 10(4) candidates with about 40 individual molecular property calculations. As such, for structurally similar molecular candidates, the TB-LCAP approach may provide an effective means to identify structures with optimal properties.

Low‐Temperature Transformation of Methane to Methanol on Pd1O4 Single Sites Anchored on the Internal Surface of Microporous Silicate

Direct conversion of methane to chemical feedstocks such as methanol under mild conditions is a challenging but ideal solution for utilization of methane. Pd1 O4 single-sites anchored on the internal surface of micropores of a microporous silicate exhibit high selectivity and activity in transforming CH4 to CH3 OH at 50-95 °C in aqueous phase through partial oxidation of CH4 with H2 O2 . The selectivity for methanol production remains at 86.4 %, while the activity for methanol production at 95 °C is about 2.78 molecules per Pd1 O4 site per second when 2.0 wt % CuO is used as a co-catalyst with the Pd1 O4 @ZSM-5. Thermodynamic calculations suggest that the reaction toward methanol production is highly favorable compared to formation of a byproduct, methyl peroxide.

Membrane Permeation Induced by Aggregates of Human Islet Amyloid Polypeptides

Several neurodegenerative diseases such as Alzheimers and Parkinsons diseases as well as nonneuropathic diseases such as type II diabetes and atrial amyloidosis are associated with aggregation of amyloid polypeptides into fibrillar structures, or plaques. In this study, we use molecular dynamics simulations to test the stability and orientation of membrane-embedded aggregates of the human islet amyloid polypeptide (hIAPP) implicated in type II diabetes. We find that in both monolayers and bilayers of dipalmitoylphosphatidylglycerol (DPPG) hIAPP trimers and tetramers remain inside the membranes and preserve their β-sheet secondary structure. Lipid bilayer-inserted hIAPP trimers and tetramers orient inside DPPG at 60° relative to the membrane/water interface and lead to water permeation and Na(+) intrusion, consistent with ion-toxicity in islet β-cells. In particular, hIAPP trimers form a water-filled β-sandwich that induce water permeability comparable with channel-forming proteins, such as aquaporins and gramicidin-A. The predicted disruptive orientation is consistent with the amphiphilic properties of the hIAPP aggregates and could be probed by chiral sum frequency generation (SFG) spectroscopy, as predicted by the simulated SFG spectra.

Highly selective hydrogenation and hydrogenolysis using a copper-doped porous metal oxide catalyst

A copper-doped porous metal oxide catalyst in combination with hydrogen shows selective and quantitative hydrogenolysis of benzyl ketones and aldehydes, and hydrogenation of alkenes. The approach provides an alternative to noble-metal catalysed reductions and stoichiometric Wolff-Kishner and Clemmensen methods.

Ultrafast vibrational frequency shifts induced by electronic excitations: naphthols in low dielectric media.

We study the solvent-induced frequency shifts of the OH-stretching mode of 1-naphthol and 2-naphthol in nonpolar/weakly polar solvents, subject to electronic excitation, with ultrafast UV/mid-infrared pump-probe spectroscopy and theoretical modeling based on Pullins perturbative treatment of vibrational solvatochromic effects. The model is parametrized at the density functional theory (DFT) level, including the B3LYP/TZVP and TD-B3LYP/TZVP descriptions, for the naphthol chromophores in the S(0)- and (1)L(b)-states and accounts for both the static and the optical dielectric response of the solvent on time scales comparable to that of the OH-stretching vibrational motions. The favorable comparison between experimental and theoretical values of the solvent-induced vibrational frequency shifts suggests that the ultrafast dielectric response of the solvent contributes predominantly to the solvatochromic shifts in solvents of moderate polarity where specific solute-solvent interactions are absent.