2D-IR spectroscopy of the sulfhydryl band of cysteines in the hydrophobic core of proteins.

We investigate the sulfhydryl band of cysteines as a new chromophore for two-dimensional IR (2D-IR) studies of the structure and dynamics of proteins. Cysteines can be put at almost any position in a protein by standard methods of site-directed mutagenesis and, hence, have the potential to be an extremely versatile local probe. Although being a very weak absorber in aqueous environment, the sulfhydryl group gets strongly polarized when situated in an alpha-helix inside the hydrophobic core of a protein because of a strong hydrogen bond to the backbone carbonyl group. The extinction coefficient (epsilon=150 M(-1) cm(-1)) then is sufficiently high to perform detailed 2D-IR studies even at low millimolar concentrations. Using porcine (carbonmonoxy)hemoglobin as an example, which contains two such cysteines in its wild-type form, we demonstrate that spectral diffusion deduced from the 2D-IR line shapes reports on the overall-breathing of the corresponding alpha-helix. The vibrational lifetime of the sulfhydryl group (T1 approximately 6 ps) is considerably longer than that of the much more commonly used amide I mode (approximately 1.0 ps), thereby significantly extending the time window in which spectral diffusion processes can be observed. The experiments are accompanied by molecular dynamics simulations revealing a good overall agreement.

Numerical studies of intermolecular multiple quantum coherences: high-resolution NMR in inhomogeneous fields and contrast enhancement in MRI.

A fast, efficient numerical algorithm is used to study intermolecular zero-quantum coherences (iZQCs) and double-quantum coherences (iDQCs) in two applications where the three-dimensional structure of the magnetization is important: high-resolution NMR in inhomogeneous fields and contrast enhancement in MRI. Simulations with up to 2 million coupled volume elements (256 x 256 x 32) show that iZQCs can significantly narrow linewidths in the indirectly detected dimension of systems with inhomogeneous fields and explore the effects of shape and orientation of the inhomogeneities. In addition, this study shows that MR images from iZQC and iDQC CRAZED pulse sequences contain fundamentally new contrast, and a modified CRAZED pulse sequence (modCRAZED) can isolate the contrast from chemically inequivalent spins.

Electron solvation in two dimensions.

Ultrafast two-photon photoemission has been used to study electron solvation at two-dimensional metal/polar-adsorbate interfaces. The molecular motion that causes the excess electron solvation is manifested as a dynamic shift in the electronic energy. Although the initially excited electron is delocalized in the plane of the interface, interactions with the adsorbate can lead to its localization. A method for determining the spatial extent of the localized electron in the plane of the interface has been developed. This spatial extent was measured to be on the order of a single adsorbate molecule.

Intermolecular zero-quantum coherence imaging of the human brain.

The first intermolecular zero-quantum coherence (iZQC) MR images of the human brain at 4T are presented. To generate iZQC images, a modified echo-planar imaging pulse sequence was used which included an additional 45 degrees RF pulse and a correlation gradient. The observability and nonconventional contrast of human brain iZQC images at 4T is demonstrated. Axial images are presented for various pulse sequence parameters, and a zero-quantum relaxation map is obtained.