Rotations of amino acids

N-NMR relaxation 49 protein mobility and 46 -49 rotation of amino acid side chains and 47, 48... 

The y-dispersion is solely due to water and its relaxational behavior near about 20 GHz. A minor additional relaxation (6) between 3 and y-dispersion is caused in part by rotation of amino acids, partial rotation of charged side groups of proteins, and the relaxation of protein bound water which occurs somewhere between 300 and 2000 MHz. 

The specific rotation of amino acids in aqueous solution is strongly influenced by pH. It passes through a minimum in the neutral pH range and rises after addition of acids or bases (Table 1.3). There are various possible methods of separating the racemates which generally occur in amino acid synthesis (cf. 1.2.5). Selective crystallization of an over-saturated solution of racemate after seeding with an enantiomer is used, as is the fractioned crystallization of diastereomeric salts or other derivatives,... 

Peptides and proteins are composed of amino acids polymerized together through the formation of peptide (amide) bonds. The peptide bonded polymer that forms the backbone of polypeptide structure is called the a-chain. The peptide bonds of the a-chain are rigid planar units formed by the reaction of the oc-amino group of one amino acid with the a-carboxyl group of another (Figure 1.1). The peptide bond possesses no rotational freedom due to the partial double bond character of the carbonyl-amino amide bond. The bonds around the oc-carbon atom, however, are true single bonds with considerable freedom of movement. 

The unit cell considered here is a primitive (P) unit cell that is, each unit cell has one lattice point. Nonprimitive cells contain two or more lattice points per unit cell. If the unit cell is centered in the (010) planes, this cell becomes a B unit cell for the (100) planes, an A cell for the (001) planes a C cell. Body-centered unit cells are designated I, and face-centered cells are called F. Regular packing of molecules into a crystal lattice often leads to symmetry relationships between the molecules. Common symmetry operations are two- or three-fold screw (rotation) axes, mirror planes, inversion centers (centers of symmetry), and rotation followed by inversion. There are 230 different ways to combine allowed symmetry operations in a crystal leading to 230 space groups.12 Not all of these are allowed for protein crystals because of amino acid asymmetry (only L-amino acids are found in proteins). Only those space groups without symmetry (triclinic) or with rotation or screw axes are allowed. However, mirror lines and inversion centers may occur in protein structures along an axis. 

The dynamic behavior of the model intermediate rhodium-phosphine 99, for the asymmetric hydrogenation of dimethyl itaconate by cationic rhodium complexes, has been studied by variable temperature NMR LSA [167]. The line shape analysis provides rates of exchange and activation parameters in favor of an intermo-lecular process, in agreement with the mechanism already described for bis(pho-sphinite) chelates by Brown and coworkers [168], These authors describe a dynamic behavior where two diastereoisomeric enamide complexes exchange via olefin dissociation, subsequent rotation about the N-C(olefinic) bond and recoordination. These studies provide insight into the electronic and steric factors that affect the activity and stereoselectivity for the asymmetric hydrogenation of amino acid precursors. 

The use of square plates allows two-dimensional chromatography to be performed using two different mobile phases. The second migration is performed after rotating the plate a quarter turn (Fig. 5.3). A typical application of this approach, although seldom used for quantitative analysis, is the separation of amino acids. 

A protein is a linear sequence of amino acids linked together by peptide bonds. The peptide bond is a covalent bond between the oi-amino group of one amino acid and the a-carboxyl group of another. The peptide bond has partial double bond character and is nearly always in the trans configuration. The backbone conformation of a polypeptide is specified by the rotation angles about the Ca-N bond phi, (j>) and Ca-C bond psi, amino acid residues. The sterically allowed values of 0 and yr are visualized in a Ramachandran plot. When two amino acids are joined by a peptide bond they form a dipeptide. Addition of further amino acids results in long chains called oligopeptides and polypeptides. 

Solid-state NMR does not have the size limit because it does not rely on the fast rotational tumbling of molecules. However, line widths are larger such that overlap becomes an issue limiting the number of amino acids that could be assigned to presently around 100. Still, partial information could be extracted from much larger systems approaching several tens of kD. Solid-state NMR relies on detection of heteronuclei, so that labeling of the protein is mandatory. 

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