Thursday, November 16, 2006

JPCB Update

It's been a while but here are some highlights from the past couple of months. This is only up to issue 43 more to come.

Interesting application ...
Generalized Two-Dimensional Correlation Analysis of NMR and Raman Spectra for Structural Evolution Characterizations of Silk Fibroin
Bing-Wen Hu, Ping Zhou, Isao Noda, and Qing-Xia Ruan
J.Phys.Chem.B (2006)110, 18046
Generalized two-dimensional (2D) correlation spectroscopy was used to characterize the structural evolution of silk fibroin as the pH changed from 6.8 to 4.8, demonstrating that the conformational transitions of silk fibroin are induced step by step as the pH decreases. 2D homo- and hetero-spectral correlation spectroscopy was used to establish the relationship between information extracted from NMR and Raman spectroscopy. This novel method reveals the structural evolution using two probes with different frequency scales (105-9 Hz for nuclear spin motion and 1012-14 Hz for molecular vibration motion), reflecting the different spatial scale sensitivity to the molecular conformational change. The transition order is identified as silk I state (helix dominant) silk I intermediate state silk II intermediate state silk II state (-sheet dominant), as the pH decreases. The results may rationalize the silkworm spinning process, which undergoes the conformational transition steadily from the soluble helix state to the insoluble -sheet state as the pH decreases from the posterior to anterior glands.

Low-Frequency Cooperative Dynamics in L-, D-, and DL-Alanine Crystals: A 13C and 15N Cross-Polarization Magic-Angle-Spinning NMR Study
Sabyasachi Sen, Ping Yu, Subhash H. Risbud, Reay Dick, and David Deamer
J.Phys.Chem.B (2006)110, 18058.
Knowledge of the dynamical changes in molecular configurations in various amino acid structures over a wide range of time scales is important since such changes may influence the structural transformations and the diverse biological functionalities of proteins. Using the temperature dependence of the rotating-frame NMR spin-lattice relaxation times T1 of protons as a probe, we have investigated the low-frequency (~60-100 kHz) dynamics in the crystal structures of L-, D-, and DL- alanine (C12H28O8N4) polymorphs. The proton relaxation times T1 were obtained from 13C 1H and 15N 1H cross-polarization magic-angle-spinning NMR experiments over a temperature range of 192-342 K. The data reveal that the time scales of these low-frequency dynamical processes are distinctly different from the localized, high-frequency rotational motion of methyl and amine groups. The strongly asymmetric T1 versus temperature curves and the subtle dynamical differences between the DL-alanine and the L- and D-enantiomorphs indicate that these low-frequency processes are cooperative in nature and are sensitive to molecular packing.

Direct Solid-State NMR Spectroscopic Evidence for the NH4AlF4 Crystalline Phase Derived from Zeolite HY Dealuminated with Ammonium Hexafluorosilicate
Hsien-Ming Kao and Pai-Ching Chang
J.Phys.Chem.B (2006)110, 19104.
Multinuclear 1H, 19F, and 27Al MAS (magic angle spinning) and corresponding 2D HETCOR (heteronuclear correlation) NMR spectroscopy, in combination with powder XRD measurements, provide the direct evidence for the NH4AlF4 crystalline phase, which was formed from zeolite HY dealuminated with an aqueous (NH4)2SiF6 solution at 80 C. The NH4AlF4 crystalline phase exhibits a characteristic second-order quadrupolar-induced 27Al NMR line shape spreading from 0 to -90 ppm (in a magnetic field of 11.7 T) and two 19F resonances at -151 and -166 ppm in the 19F NMR spectrum. An 27Al quadrupolar coupling constant (CQ) of 9.5 MHz and an asymmetry parameter () of 0.1 were identified, for the first time, for the NH4AlF4 crystalline phase observed. On the basis of the 19F{27Al} TRAPDOR (transfer population in double resonance) NMR results, the 19F resonances at -151 and -166 ppm are therefore assigned to 19F spins associated with the fluorines in the terminal Al-F and the bridging Al-F-Al groups, respectively.

Dynamics of Benzene Guest Inside a Self-Assembled Cylindrical Capsule: A Combined Solid-State 2H NMR and Molecular Dynamics Simulation Study
Alexandra R. Albunia, Carmine Gaeta, Placido Neri, Alfonso Grassi, and Giuseppe Milano
J.Phys.Chem.B (2006)110, 19207.
The reorientational dynamics of benzene-d6 molecules hosted into the cavity of a cavitand-based, self-assembled capsule was investigated by Molecular Dynamics (MD) simulations and temperature-dependent solid-state 2H NMR spectroscopy. MD simulations were preliminarily performed to assess the motional models of the guest molecules inside the capsules. An in-plane fast reorientation of the benzene guest around the C6 symmetry axis (B1 motion), characterized by correlation times of the order of picoseconds, was predicted with an activation barrier (~8 kJ/mol) very similar to that found for neat benzene in the liquid state. An out-of-plane reorientation corresponding to a nutation of the C6 symmetry axis in a cone angle of 39 (B2 motion, 373 K) with an activation barrier (~39 kJ/mol) definitely larger than that of liquid benzene was also anticipated. In the temperature range 293-373 K correlation times of the order of a nanosecond have been calculated and a transition from fast to slow regime in the 2H NMR scale has been predicted between 293 and 173 K. 2H NMR spectroscopic analysis, carried out in the temperature range 173-373 K on the solid capsules containing the perdeuterated guest (two benzene molecules/capsule), confirmed the occurrence of the B1 and B2 motions found in slow exchange in the 2H NMR time scale. Line shape simulation of the 2H NMR spectral lines permitted defining a cone angle value of 39 at 373 K and 35 at 173 K for the nutation axis. The T1 values measured for the 2H nuclei of the encapsulated aromatic guest gave correlation times and energetic barrier for the in-plane motion B1 in fine agreement with theoretical calculation. The experimental correlation time for B2 as well as the corresponding energetic barrier are in the same range found for B1. A molecular mechanism for the encapsulated guest accounting for the B1 and B2 motions was also provided.

15N MAS NMR Studies of Cph1 Phytochrome: Chromophore Dynamics and Intramolecular Signal Transduction
Thierry Rohmer, Holger Strauss, Jon Hughes, Huub de Groot, Wolfgang Gärtner, Peter Schmieder, and Jörg Matysik
J.Phys.Chem.B (2006)110, 20580.
Solid-state nuclear magnetic resonance (NMR) is applied for the first time to the photoreceptor phytochrome. The two stable states, Pr and Pfr, of the 59-kDa N-terminal module of the cyanobacterial phytochrome Cph1 from Synechocystis sp. PCC 6803 containing a uniformly 15N-labeled phycocyanobilin cofactor are explored by 15N cross-polarization (CP) magic-angle spinning (MAS) NMR. As recently shown by 15N solution-state NMR using chemical shifts [Strauss, H. M.; Hughes, J.; Schmieder, P. Biochemistry 2005, 44, 8244], all four nitrogens are protonated in both states. CP/MAS NMR provides two additional independent lines of evidence for the protonation of the nitrogens. Apparent loss of mobility during photoactivation, indicated by the decrease of line width, demonstrates strong tension of the entire chromophore in the Pfr state, which is in clear contrast to a more relaxed Pr state. The outer rings (A and D) of the chromophore are significantly affected by the phototransformation, as indicated by both change of chemical shift and line width. On the other hand, on the inner rings (B and C) only minor changes of chemical shifts are detected, providing evidence for a conserved environment during phototransformation. In a mechanical model, the phototransformation is understood in terms of rotations between the A-B and C-D methine bridges, allowing for intramolecular signal transduction to the protein surface by a unit composed of the central rings B and C and its tightly linked protein surroundings during the highly energetic Pfr state.

First-Principles Calculations within Periodic Boundary Conditions of the NMR Shielding Tensor for a Transition Metal Nucleus in a Solid State System: The Example of 51V in AlVO4
L. Truflandier, M. Paris, C. Payen, and F. Boucher
J.Phys.Chem.B (2006)110, 21403.
We present the first density functional theory based calculations of NMR shielding parameters for a transition metal nucleus using periodic boundary conditions. These calculations employ the gauge-including projected augmented-wave pseudopotential approach. The quality of this method is discussed by comparing experimental and calculated chemical shift tensor eigenvalues for the quadrupolar 51V nucleus in the diamagnetic solid-state compound AlVO4. Furthermore, the combination of shielding tensor with fast and accurate projector augmented-wave electric field gradient tensor calculations allows us to determine the relative orientation of these two tensors.

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