Chemical environment stereochemistry

To solve the structure of proteins by using NMR, a series of nuclear magnetic resonant peak positions of proteins in solutions is measured. From the peak shifts due to chemical environments, the stereochemical configurations of the atoms which can be attributed to the peaks can be calculated, such as the bond lengths, bond angles, and dihedral angles. According to the stereochemistry information obtained, the structures of the macromolecules can be constructed. However, due to the limitations in the resolutions of NMR... 

NMR spectra are constituted by a series of resonance signals, each of them originated by the magnetically equivalent nuclei that undergo to the NMR experiment (i.e. hydrogen nuclei for H NMR spectroscopy). Their position in the spectrum (chemical shift) is peculiar of the chemical group to which the nucleus is bound and of its chemical environment (chemical neighbourhood— both directly linked and spatially displaced, stereochemistry), their intensities... 

Racemisation is a chemical reaction, and its rate is different for each type of amino acid. An important fact is that this process is affected by many factors that influence the rate of change of the amino acids stereochemistry [106]. The main parameters affecting the racemisation process include the amino acid structure, the sequence of amino acids in peptides, the bound state versus the free state of the amino acids, the pH in the environment, the concentration of buffer compounds, the contact of the sample with clay surfaces... 

The exact local magnetic field acting on a given nucleus is dependent on its electronic environment, i.e., the kind and number of the surrounding atoms determine its chemical shift (<5), and stereochemistry plays an important role22. <5 Values in this section are referenced to commonly agreed standards tetramethylsilane for 2H and 13C, ammonia for 15N, water for l70, and 85% aqueous phosphoric acid for 31P. 

The compound 35 possesses exo- and air-stereochemistry of the methyl and the benzoate groups based on the nuclear Overhauser effect (NOE) data. Based on the C-N distance (r), the Woodward parameter (h) and the sum at the N-atom (SN) value from X-ray data, N-l is in a pyramidal environment. N-3 is on maximum resonance with the Jt-framework of the adjacent C=0 group. This is reflected in a shorter C(2)-N(3) bond (1.375(2) A, compared to the C(2)-N(l) bond distance [1.438(2)A]. JZN values are as follows N-3 = 359.99°, and ]N-1 =311.74° and, = 0,601 A. The X-ray crystallographic data support some of the conclusions derived from AMI calculations, viz. N-l and N-3 atoms are in chemically distinct environments and form C-N bonds of different strengths. This would have implications on the... 

Significant characteristics of the porphyrin iron monoxide are seen in the chemical reactivity. Naphthalene is converted initially to the corresponding arene oxide on treatment with P 450 (19), consistent with a molecular mechanism of oxygen transfer from an iron monoxide to the aromatic nucleus. Retention of stereochemistry in the P-450 catalyzed hydroxylation of d ethylbenzene also supports the molecular mechanism. The unusually large kinetic isotope effect observed for the P-450 oxidation of dideutero 1,3-diphenylpropane, kJkD = 11, demonstrates that C—H cleavage is involved in the rate determining step (20), probably in a very unusual environment, not incompatible with a molecular mechanism. 

Considerable progress has been made in the development of theories that can predict the complete ROA spectrum, provided that a good normal coordinate analysis is available, and this leads us to the hope that it might be possible eventually to deduce the total stereochemistry (absolute configuration, conformation, bond lengths and angles) of a chiral molecule in a chemically relevant environment from the measured ROA spectrum (or indeed from the infrared CD spectrum). 

The prediction of chemical shifts in H-NMR spectroscopy is usually more problematic than in C-NMR. Experimental conditions can have an influence on the chemical shifts in H-NMR spectroscopy and structural effects are difficult to estimate. In particular, stereochemistry and 3D effects have been addressed in the context of empirical H-NMR chemical shift prediction only in a few specific situations [81,82]. Most of the available databases lack stereochemical labeling, assignments for diastereo-topic protons, and suitable representations for the 3D environment of hydrogen nuclei [83]. This is the point where local RDF descriptors seemed to be a promising tool. 

---