J. Am. Chem. Soc., 130 (3), 945 -954, 2008.
Quantifying Weak Hydrogen Bonding in Uracil and 4-Cyano-4'-ethynylbiphenyl: A Combined Computational and Experimental Investigation of NMR Chemical Shifts in the Solid State
Anne-Christine Uldry, John M. Griffin, Jonathan R. Yates, Marta Pérez-Torralba, M. Dolores Santa María, Amy L. Webber, Maximus L. L. Beaumont, Ago Samoson, Rosa María Claramunt, Chris J. Pickard, and Steven P. Brown*
Weak hydrogen bonding in uracil and 4-cyano-4'-ethynylbiphenyl, for which single-crystal diffraction structures reveal close CH···O=C and CCH···NC distances, is investigated in a study that combines the experimental determination of 1H, 13C, and 15N chemical shifts by magic-angle spinning (MAS) solid-state NMR with first-principles calculations using plane-wave basis sets. An optimized synthetic route, including the isolation and characterization of intermediates, to 4-cyano-4'-ethynylbiphenyl at natural abundance and with 13C13CH and 15NC labeling is described. The difference in chemical shifts calculated, on the one hand, for the full crystal structure and, on the other hand, for an isolated molecule depends on both intermolecular hydrogen bonding interactions and aromatic ring current effects. In this study, the two effects are separated computationally by, first, determining the difference in chemical shift between that calculated for a plane (uracil) or an isolated chain (4-cyano-4'-ethynylbiphenyl) and that calculated for an isolated molecule and by, second, calculating intraplane or intrachain nucleus-independent chemical shifts that quantify the ring current effects caused by neighboring molecules. For uracil, isolated molecule to plane changes in the 1H chemical shift of 2.0 and 2.2 ppm are determined for the CH protons involved in CH···O weak hydrogen bonding; this compares to changes of 5.1 and 5.4 ppm for the NH protons involved in conventional NH···O hydrogen bonding. A comparison of CH bond lengths for geometrically relaxed uracil molecules in the crystal structure and for geometrically relaxed isolated molecules reveals differences of no more than 0.002 Å, which corresponds to changes in the calculated 1H chemical shifts of at most 0.1 ppm. For the CCH···NC weak hydrogen bonds in 4-cyano-4'-ethynylbiphenyl, the calculated molecule to chain changes are of similar magnitude but opposite sign for the donor 13C and acceptor 15N nuclei. In uracil and 4-cyano-4'-ethynylbiphenyl, the CH hydrogen-bonding donors are sp2 and sp hybridized, respectively; a comparison of the calculated changes in 1H chemical shift with those for the sp3 hybridized CH donors in maltose (Yates et al. J. Am. Chem. Soc. 2005, 127, 10216) reveals no marked dependence on hybridization for weak hydrogen-bonding strength.
J. Am. Chem. Soc., 130 (3), 918 -924, 2008.
Solid-State 19F NMR Spectroscopy Reveals That Trp41 Participates in the Gating Mechanism of the M2 Proton Channel of Influenza A Virus
Raiker Witter, Farhod Nozirov, Ulrich Sternberg, Timothy A. Cross, Anne S. Ulrich,* and Riqiang Fu*
The integral membrane protein M2 of influenza A virus assembles as a tetrameric bundle to form a proton-conducting channel that is activated by low pH. The side chain of His37 in the transmembrane -helix is known to play an important role in the pH activation of the proton channel. It has also been suggested that Trp41, which is located in an adjacent turn of the helix, forms part of the gating mechanism. Here, a synthetic 25-residue peptide containing the M2 transmembrane domain was labeled with 6F-Trp41 and studied in lipid membranes by solid-state 19F NMR. We monitored the pH-dependent differences in the 19F dipolar couplings and motionally narrowed chemical shift anisotropies of this 6F-Trp41 residue, and we discuss the pH activation mechanism of the H+ channel. At pH 8.0, the structural parameters implicate an inactivated state, while at pH 5.3 the tryptophan conformation represents the activated state. With the aid of COSMOS force field simulations, we have obtained new side-chain torsion angles for Trp41 in the inactivated state (1 = -100 ± 10, 2 = +110 ± 10), and we predict a most probable activated state with 1 = -50 ± 10 and 2 = +115 ± 10. We have also validated the torsion angles of His37 in the inactivated state as 1 = -175 ± 10 and 2 = -170 ± 10.