Gas-Phase Infrared and NMR Investigation of the Conformers of Diacetone Diperoxide (DADP).

Gas-phase infrared measurements of diacetone diperoxide (DADP) indicate a chair conformation with less than 5% of the predicted twist conformer. Vibrational frequencies are very similar to those previously measured in the solid state. Solution NMR measurements using 2D exchange spectroscopy (EXSY) also set a very low maximum limit on the equilibrium population of the twist conformer, with a room-temperature free-energy difference in excess of 14.5 kJ/mol. These experimental results are in accord with high-level quantum calculations incorporating full thermochemistry and solvation effects, which indicate a free-energy difference in the range of 14.7-17.5 kJ/mol in polar solvents.

Polarization of core orbitals and computation of nuclear quadrupole coupling constants using Gaussian basis sets.

Most standard Gaussian basis sets for first row atoms, even large sets designed to converge on a 'complete basis set' limit, systematically overestimate the electric field gradient at nuclear sites for first row atoms, resulting in errors of up to 15% in the computation of nuclear quadrupole coupling constants. This error results from a failure to include tight d functions, which permit the core 1s orbitals to distort under the influence of the field of the nuclear quadrupole. Augmentation of standard basis sets with a single set of single-exponent d functions, matched to the reciprocal square of the nominal 1s radius, reduces these errors by up to 90%.

13C NMR reveals no evidence of n-π* interactions in proteins.

An n = π* interaction between neighboring carbonyl groups has been postulated to stabilize protein structures. Such an interaction would affect the (13)C chemical shielding of the carbonyl groups, whose paramagnetic component is dominated by n = π* and π = π* excitations. Model compound calculations indicate that both the interaction energetics and the chemical shielding of the carbonyl group are instead dominated by a classical dipole-dipole interaction. A set of high-resolution protein structures with associated carbonyl (13)C chemical shift assignments verifies this correlation and provides no evidence for an inter-carbonyl n = π* interaction.

Thermal averaging of the spin-rotation coupling in small molecules leads to an isotropic NMR shielding.

It has been known for over 70 years that nuclear spins couple to molecular rotation via a Zeeman interaction. This spin-rotation coupling can be observed as a discrete splitting in molecular beam magnetic resonance experiments, but is quenched by molecular collisions at higher pressures. We show that because of differential thermal population of M(J) levels at high magnetic fields, the spin rotation coupling retains a small isotropic component at high field. For all but the smallest molecules at very low temperature, the residual coupling is temperature independent and linear in the magnetic field; it therefore closely mimics the chemical shift. The 'super spin rotation' shift may in the future be a necessary correction to ultra - high precision computations of the NMR chemical shielding of small molecules in gases and liquids.

Solid state 2H NMR analysis of furanose ring dynamics in DNA containing uracil.

DNA damage has been implicated in numerous human diseases, particularly cancer, and the aging process. Single-base lesions, such as uracil, in DNA can be cytotoxic or mutagenic and are recognized by a DNA glycosylase during the process of base excision repair. Increased dynamic properties in lesion-containing DNAs have been suggested to assist recognition and specificity. Deuterium solid-state nuclear magnetic resonance (SSNMR) has been used to directly observe local dynamics of the furanose ring within a uracil:adenine (U:A) base pair and compared to a normal thymine:adenine (T:A) base pair. Quadrupole echo lineshapes, , and relaxation data were collected, and computer modeling was performed. The results indicate that the relaxation times are identical within the experimental error, the solid lineshapes are essentially indistinguishable above the noise level, and our lineshapes are best fit with a model that does not have significant local motions. Therefore, U:A base pair furanose rings appear to have essentially identical dynamic properties as a normal T:A base pair, and the local dynamics of the furanose ring are unlikely to be the sole arbiter for uracil recognition and specificity in U:A base pairs.

Low temperature 65Cu NMR spectroscopy of the Cu+ site in azurin.

(65)Cu central-transition NMR spectroscopy of the blue copper protein azurin in the reduced Cu(I) state, conducted at 18.8 T and 10 K, gave a strongly second order quadrupole perturbed spectrum, which yielded a (65)Cu quadrupole coupling constant of +/-71.2 +/- 1 MHz, corresponding to an electric field gradient of +/-1.49 atomic units at the copper site, and an asymmetry parameter of approximately 0.2. Quantum chemical calculations employing second order Møller-Plesset perturbation theory and large basis sets successfully reproduced these experimental results. Sensitivity and relaxation times were quite favorable, suggesting that NMR may be a useful probe of the electronic state of copper sites in proteins.

Estimating protein-ligand binding affinity using high-throughput screening by NMR.

Many of today's drug discovery programs use high-throughput screening methods that rely on quick evaluations of protein activity to rank potential chemical leads. By monitoring biologically relevant protein-ligand interactions, NMR can provide a means to validate these discovery leads and to optimize the drug discovery process. NMR-based screens typically use a change in chemical shift or line width to detect a protein-ligand interaction. However, the relatively low throughput of current NMR screens and their high demand on sample requirements generally makes it impractical to collect complete binding curves to measure the affinity for each compound in a large and diverse chemical library. As a result, NMR ligand screens are typically limited to identifying candidates that bind to a protein and do not give any estimate of the binding affinity. To address this issue, a methodology has been developed to rank binding affinities for ligands based on NMR screens that use 1D (1)H NMR line-broadening experiments. This method was demonstrated by using it to estimate the dissociation equilibrium constants for twelve ligands with the protein human serum albumin (HSA). The results were found to give good agreement with previous affinities that have been reported for these same ligands with HSA.

The (14)N quadrupole coupling in hexamethylene triperoxide diamine (HMTD).

Using high-field NMR, we have determined the magnitude of the nuclear quadrupole interaction in hexamethylene triperoxide diamine (HMTD), the explosive allegedly used in the London bombings of July 2005. The experimental quadrupolar coupling constant, 5.334 MHz, is in good agreement with quantum chemical calculations. The predicted single zero-field transition frequency should lie in a relatively empty part of the (14)N nuclear quadrupole resonance (NQR) spectrum; the spin relaxation rate is reasonably fast.

Slow turning reveals enormous quadrupolar interactions (STREAQI).

We introduce a new solid-state NMR method, which uses very slow sample rotation to visualize NMR spectra whose width exceeds feasible spectrometer bandwidths. It is based on the idea that if we reorient a tensor by a known angle about a known axis, the shifts in the NMR frequencies observed across the spectral width allow us to reconstruct the entire tensor. Called STREAQI (Slow Turning Reveals Enormous Anisotropic Quadrupolar Interactions), this method allows us to probe NMR nuclei that are intractable to current methods. To prove the concept and demonstrate its promise we have implemented the method for several 79Br containing samples with quadrupolar coupling constants in the range of 10-50 MHz.

Deuterium quadrupolar tensors of L-histidine hydrochloride monohydrate-d7.

A deuterium NMR study at 14 T of a single crystal of L-histidine hydrochloride monohydrate has determined the deuteron quadrupole coupling constants CQ, asymmetry parameters eta, and electric field gradient orientation for the imidazolium and primary ammonium groups and for a water of crystallization. The imidazolium deuterons, which have very long relaxation times, have quite different coupling constants, reflecting different hydrogen bonding but nearly identical orientations, with the most distinct principal axis in both cases nearly parallel to the N-D vector. The -ND3 groups undergo 3-fold hops about the C-N bond axis and have typical quadrupole couplings; the D2O undergoes 2-fold hops, leading to a tensor with a large asymmetry parameter. With appropriate corrections for vibrational averaging, density functional cluster calculations give an excellent fit to the imidazolium tensor magnitudes and orientations.

Origins of isotopomeric polymorphism.

The complex formed between 4-methylpyridine and pentachlorophenol (4MPPCP) crystallises in a triclinic space group. If the same complex is synthesized from deuterated pentachlorophenol, it crystallizes in an entirely different monoclinic polymorph. Using solid-state NMR of samples synthesized with a full range of deuteration levels, crystallized from solution or the melt, and in the presence or absence of seeds, we have confirmed that the isotopomers indeed have different thermodynamically stable crystal structures. The roots of this phenomenon of isotopomeric polymorphism apparently lie in the differences in hydrogen bonding between the polymorphs. The triclinic form has a relatively short hydrogen bond. High-field solid-state NMR shows both the 1H chemical shift and the 2H electric quadrupole coupling of the hydrogen involved in the bond to be strongly temperature-dependent, indicating a low-lying excited state of the hydrogen bond longitudinal vibration. Inelastic neutron scattering of isotopomers of 4MPPCP has allowed us to identify the three orthogonal vibrational modes of the hydrogen in the hydrogen bond, at 29.7, 145, and 205 meV (240, 1168, and 1651 cm(-1)). The longitudinal mode is the lowest in energy, and it indicates a slightly asymmetric low-barrier double-well potential. Intrinsic to such potentials is a very small difference in zero-point energies (ZPEs) between the protonated and deuterated forms. As a contrast, the monoclinic form has a comparatively normal hydrogen bond, in which the proton and deuteron ZPEs should be different by approximately 500 cm(-1). A scenario can be envisaged where the triclinic protonated form is lower in energy than the monoclinic protonated form, but the triclinic deuterated form is higher in energy than the monoclinic deuterated form. This evidently accounts for the difference in relative stabilities of the two forms upon isotope substitution.

Helical chirality in hexamethylene triperoxide diamine.

The primary explosive hexamethylenetriperoxide diamine has previously been found to exist in the solid state as a racemic mixture of helically chiral, threefold symmetric enantiomers; another enantiomeric pair of low-energy conformers has been predicted, but has never been observed. We show by solution 2D NMR at 14 T, in achiral solution and by addition of chiral shift reagents, that all four optically isomeric conformers coexist at slow equilibrium on the NMR timescale at room temperature, and can be observed. Calculations of the 1H and 13C NMR chemical shifts using gauge-including atomic orbital methods are in excellent agreement with experiment; thermochemical calculation of the free energies in solution are in somewhat worse agreement, but correctly predict the relative stability of the conformers. Analysis of the effects of chiral shift reagents on the NMR spectra suggests that discrimination between chiral isomers is primarily around the molecular equator, around which the enantiomeric gauche O--O linkages are arrayed.

Effects of Zero-Point and Thermal Vibrational Averaging on Computed NMR Properties of a Model Compound for Purine Nucleosides.

A method for the incorporation of thermal averaging into the calculation of nuclear magnetic resonance properties is given. These properties are computed using density functional theory, anharmonic first-order perturbation corrections to a normal-mode analysis, and standard statistical mechanical averaging. The method is applied to the calculation of chemical shieldings and spin-spin coupling constants (J couplings) of 1'-imidazolyl-2'-deoxy-ß-ribofuranose (IDR), a model compound for purine nucleosides, at the B3LYP/6-311++G(2d,p) level of theory. Thermal averaging causes substantial changes in the values of computed parameters. The calculated harmonic normal modes of IDR are also investigated; we find reasonable agreement with published results from vibrational spectroscopy on DNA fragments. Finally, the calculated magnetic and structural data regarding the reported hydrogen bond between H8 on the imidazole ring and O5' on the sugar ring are investigated; we find that such data do not strongly support the formation of a hydrogen bond between these two atoms.

Orientation of single crystals using linear approximations to NMR transits.

We have derived analytical expressions for determining the orientation of high-symmetry single crystals from line-crossings in a single rotation plot. We demonstrate the utility of the method using the strontium-87 resonance in strontium nitrate. Employing our new method, which we call orientation of single crystals using linear approximations to NMR transits (OSCULANT), in combination with fourth-order perturbation theory, we obtain a highly accurate value for the quadrupole coupling constant, and an estimate for the chemical shielding anisotropy.

Isotopomeric polymorphism.

The complex formed between 4-methylpyridine and pentachlorophenol (4MPPCP) crystallizes in a triclinic space group. If the same complex is synthesized from deuterated pentachlorophenol (4MPPCP-d1), it crystallizes in an entirely different monoclinic polymorph. Solid-state NMR of samples synthesized with a full range of deuteration levels, crystallized from solution or the melt, and in the presence or absence of seeds confirms that the isotopomers indeed have different thermodynamically stable crystal structures. The phenomenon is apparently due to very different hydrogen bond strengths between the two polymorphs.

Deuterium NMR and ab initio studies of strongly hydrogen-bonded molecules.

Two newly synthesized alkylated cyanomalonaldehyde derivatives possess very short intramolecular hydrogen bonds, which are studied by deuterium NMR. Both the dimethyl and di-tert-butyl derivatives have small deuterium quadrupole coupling constants and large asymmetry parameters. In addition, the di-tert-butyl compound, which has one of the shortest O D...O hydrogen bonds yet measured, has a quadrupole coupling which anomalously increases with temperature. The significance of these phenomena is explored using a theoretical model which employs vibrationally averaged ab initio electric field gradient tensors calculated with large basis sets and electron correlation via the multiconfigurational self-consistent field method.

The electric dipole polarity of the ground and low-lying metastable excited states of NF.

NF (nitrogen monofluoride, fluoroimidogen) is isoelectronic with O2, and, like O2, it has a triplet configuration in the ground state, with two low-lying metastable singlet excited states. The dipole moment of the a 1Delta excited state was measured in 1973 to be 0.37 +/- 0.06 D; at the time its polarity was assumed to be normal (i.e., with the negative charge on the fluorine). However, high-level electronic structure calculations, which reproduce with high accuracy the known spectroscopic constants of the ground and excited states of NF, predict a dipole moment of -0.388 D for a 1Delta NF, indicating that, despite the electronegativities, this molecule carries a positive charge on fluorine. The other singlet state is predicted to have an even larger negative dipole moment; the ground-state triplet should have a very small positive moment. Singlet NF resembles in this respect CO and BF, from the N2 isoelectronic series, both of which also have negative dipole moments.

Single-Crystal, Solid-State, and Solution (113)Cd and (77)Se NMR and X-ray Single-Crystal Study of a [Cd(SeR)(2)(N-donor)(2)] Complex.

[Cd(Se-2,4,6-i-Pr(3)-C(6)H(2))(2)(bpy)] (1) has been characterized by X-ray crystallography and studied by solution and solid-state (113)Cd and (77)Se NMR to serve as an analog for biologically occurring [M(S-Cys)(2)(His)(2)] centers. The unit cell parameters for 1 are as follows: a = 21.99(2) Å, b = 21.43(4) Å, c = 16.72(3) Å, V = 7881.3(4) Å(3), Z = 8, orthorhombic space group, Pccn. Two chemically inequivalent cadmiums and seleniums are found per unit cell. The principal values of the cadmium chemical shift tensors are (delta(11) = 752 ppm, delta(22) = 570 ppm, delta(33) = 93 ppm) and (delta(11)= 733 ppm, delta(22) = 547 ppm, delta(33) = 100 ppm) with respect to 0.1 M aq. Cd(ClO(4))(2). Those for selenium are (delta(11) = -950 ppm, delta(22) = -1040 ppm, delta(33) = -1599 ppm) and (delta(11) = -933 ppm, delta(22) = -1069 ppm, delta(33) = -1586 ppm) with respect to solid (NH(4))(2)SeO(4). The orientation of the cadmium chemical shift tensor is similar to that of the previously published sulfur analog.

207Pb NMR of Solid Solutions of Divalent Metal Nitrates.

207Pb NMR spectra of mixed crystals of anhydrous lead and strontium nitrate show up to 13 lines, arising from lead ions with 0-12 Pb(2+) nearest-neighbor cations replaced by Sr(2+). The shifted lines are further split by the multiplicity of inequivalent nearest-neighbor replacements. The average shift per Sr(2+) ion is 21.8 ppm. Similar but somewhat smaller effects are seen in [Pb,Ba](NO(3))(2) mixed crystals. The intensities of the resonances can be fit to Monte Carlo models of Pb(2+) and Sr(2+) ion distributions and are consistent with like nearest-neighbor pairs being 120 J/mol lower in energy than unlike pairs.