N-Acetyl-L-phenylalanine.
The structure of the title compound, C(11)H(13)NO(3), is characterized by a two-dimensional infinite network of intermolecular N-H.O and O-H.O hydrogen bonds.
Journal Article↗
Biomedical subjects
A Ramamoorthy
Publications and source records attributed to A Ramamoorthy.
Experimental aspects of multidimensional solid-state NMR correlation spectroscopy.
The experimental parameters critical for the implementation of multidimensional solid-state NMR experiments that incorporate heteronuclear spin exchange at the magic angle are discussed. This family of experiments is exemplified by the three-dimensional experiment that correlates the (1)H chemical shift, (1)H-(15)N dipolar coupling, and (15)N chemical shift frequencies. The broadening effects of the homonuclear (1)H-(1)H dipolar couplings are suppressed using flip-flop (phase- and frequency-switched) Lee-Goldburg irradiations in both the (1)H chemical shift and the (1)H-(15)N dipolar coupling dimensions. The experiments are illustrated using the (1)H and (15)N chemical shift and dipolar couplings in a single crystal of (15)N-acetylleucine.
Hydrogen Bonding↗
Analysis of dipolar-coupling-mediated coherence transfer in a homonuclear two spin-12 solid-state system
Homonuclear dipolar-mediated coherence transfer (DCT), a through-space transfer of magnetization between like spins, can yield otherwise difficult-to-obtain structural information for macromolecules by measuring the internuclear distances between two sites of interest. The behavior of a spin-12 system under DCT is analyzed in detail by computing the time development of the density matrix using the product operator formalism. The effect of coherence transfer (CT) via the homonuclear isotropic scalar coupling on DCT is examined. Analytical and computational results that yield useful information on the frequencies, first-maxima, and first-zero of CT for a uniaxially oriented or a single-crystal solid-state system are presented. The results predict that the evolution of the spin angular momentum operators under the homonuclear dipolar coupling Hamiltonian leads to "cylindrical mixing" unlike "isotropic mixing" due to the strong scalar coupling Hamiltonian. These results will find relevance in both the design of RF pulse sequences for the structural studies of uniaxially oriented biological solids and the interpretation of solution NMR results from proteins embedded in partially oriented bicelles. Copyright 1999 Academic Press.
Journal Article↗
Conformational studies of human islet amyloid peptide using molecular dynamics and simulated annealing methods.
Molecular dynamics simulations and simulated annealing in vacuum, model aqueous solution, and simulated membrane were used to analyze the conformational preferences of a segment spanning 20-29 residues of human islet amyloid polypeptide, [referred to as IAPPH (20-29)]. Molecular dynamics simulations were conducted at 300 K on IAPPH (20-29). The minimum energy conformers obtained in model aqueous solution and vacuum exhibited similar structures. Even in the absence of any constraints on peptide bonds, trans conformation was preferred consistently by all the peptide bonds. Analysis of the minimum energy conformers indicated that IAPPH (20-29) showed a strong preference for turn structures in all the environments. These turn structures were stabilized by the formation of hydrogen bonds between the backbone amide and carbonyl groups. A good agreement was found between the results obtained from the molecular dynamics simulation and solid-state nmr experimental studies.
Amino Acid Sequence↗
A simple one-dimensional solid-state NMR method to characterize the nuclear spin interaction tensors associated with the peptide bond.
We propose a simple one-dimensional RF pulse sequence for the study of chemical shift and heteronuclear dipolar coupling tensors of oriented as well as unoriented solids. An off-resonance RF decoupling of protons during the signal acquisition of less sensitive nuclei is used to suppress homonuclear 1H-1H dipolar interactions. This method is experimentally demonstrated on peptide samples selectively labeled with 15N isotope.
Algorithms↗
Complete resolution of the solid-state NMR spectrum of a uniformly 15N-labeled membrane protein in phospholipid bilayers.
Complete resolution of the amide resonances in a three-dimensional solid-state NMR correlation spectrum of a uniformly 15N-labeled membrane protein in oriented phospholipid bilayers is demonstrated. The three orientationally dependent frequencies, 1H chemical shift, 1H-15N dipolar coupling, and 15N chemical shift, associated with each amide resonance are responsible for resolution among resonances and provide sufficient angular restrictions for protein structure determination. Because the protein is completely immobilized by the phospholipids on the relevant NMR time scales (10 kHz), the linewidths will not degrade in the spectra of larger proteins. Therefore, these results demonstrate that solid-state NMR experiments can overcome the correlation time problem and extend the range of proteins that can have their structures determined by NMR spectroscopy to include uniformly 15N-labeled membrane proteins in phospholipid bilayers.
Amino Acid Sequence↗
Resolved two-dimensional anisotropic-chemical-shift/heteronuclear dipolar coupling powder-pattern spectra by three-dimensional solid-state NMR spectroscopy.
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Alanine↗
Three-dimensional solid-state NMR correlation experiment with 1H homonuclear spin exchange.
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Hydrogen↗
Three-dimensional solid-state NMR experiment that correlates the chemical shift and dipolar coupling frequencies of two heteronuclei.
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Leucine↗
Four-dimensional solid-state NMR experiment that correlates the chemical-shift and dipolar-coupling frequencies of two heteronuclei with the exchange of dilute-spin magnetization.
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Dipeptides↗
Three-dimensional solid-state NMR spectroscopy of a peptide oriented in membrane bilayers.
A three-dimensional 1H chemical shift/1H-15N dipolar coupling/15N chemical shift correlation spectrum was obtained on a sample of specifically 15N-labeled magainin peptides oriented in lipid bilayers between glass plates in a flat-coil probe. The spectrum showed complete resolution of the resonances from two labeled amide sites in all three dimensions. The three orientationally dependent frequencies associated with each resonance enabled the orientation of the peptide planes to be determined relative to the direction of the applied magnetic field. These results demonstrate the feasibility of multiple-pulse spectroscopy in a flat-coil probe, the ability to measure three spectral parameters from each site in a single experiment, and the potential for resolving among many labeled sites in oriented membrane proteins.
Amides↗
Two-dimensional chemical shift/heteronuclear dipolar coupling spectra obtained with polarization inversion spin exchange at the magic angle and magic-angle sample spinning (PISEMAMAS).
High-resolution two-dimensional 15N chemical shift/1H-15N dipolar coupling polarization inversion spin exchange at the magic angle (PISEMA) spectra of a polycrystalline sample of 15N-acetylvaline were obtained with and without magic-angle sample spinning. These spectra demonstrate the advantages of the PISEMA experiment over conventional approaches to separated local-field spectroscopy, especially the high resolution in the dipolar dimension where the spinning sidebands have uniformly narrow linewidths.
Crystallization↗
Determination of the conformation and stability of simple homopolypeptides using solid-state NMR.
15N CPMAS, 13C CPMAS and 1H CRAMPS spectra of several polypeptide samples were compared to determine the useful features of each technique. 13C CPMAS is the most well-established technique and is useful for quick determination of secondary structure. The 15N nucleus is more sensitive to exact hydrogen-bonding parameters, which complicates interpretation of the spectra. However, it is better for resolving end effects and structural types in short oligomers. 1H CRAMPS spectra are similar to 13C CPMAS in the information obtained, but the resolution is not as good. Using 13C CPMAS, the conformation of polyglycine was investigated in detail. Precipitation from solvents such as DCA or TFA resulted in the rippled beta-sheet structure (PG I), while 3(1)-helix (PG II) was formed by precipitation from aqueous solutions of LiBr. Grinding the sample resulted in an increase in the amount of PG I, indicating that this form is more stable in the solid state. These results agree with previous work on poly(L-alanine) showing that the beta-sheet form is more stable in the solid state. Homopolypeptides with larger side chains did not change conformation upon grinding due to the greater difficulty in disrupting van der Waals interactions and inertia of the large side chains.
Carbon Isotopes↗
A 2D MAS solid-state NMR method to recover the amplified heteronuclear dipolar and chemical shift anisotropic interactions.
A two-dimensional solid-state NMR method for the measurement of chemical shift anisotropy tensors of X nuclei (15N or 13C) from multiple sites of a polypeptide powder sample is presented. This method employs rotor-synchronized pi pulses to amplify the magnitude of the inhomogeneous X-CSA and 1H-X dipolar coupling interactions. A combination of on-resonance and magic angle rf irradiation of protons is used to vary the ratio of the magnitudes of the 1H-X dipolar and X-CSA interactions which are recovered under MAS, in addition to suppressing the 1H-1H dipolar interactions. The increased number of spinning sidebands in the recovered anisotropic interactions is useful to determine the CSA tensors accurately. The performance of this method is examined for powder samples of N-acetyl-(15)N-L-valine (NAV), N-acetyl-15N-L-valyl-15N-L-leucine (NAVL), and alpha-13C-L-leucine. The sources of experimental errors in the measurement of CSA tensors and the application of the pulse sequences under high-field fast MAS operations are discussed.
Carbon Isotopes↗