Twisted amino group

Figure 12.6 Geometry-optimized forms of cytosine anion in the keto and enol forms, AMI calculations. Note the twisted amino group and hent N—H bond in the keto anion and the bent O—H bond in the enol form.

The ortho hydrogen atoms surrounding the central carbon atom show considerable steric overlap. Therefore, it can be assumed that the three aryl groups in the dye are not coplanar, but are twisted in such a fashion that the shape of the dye resembles that of a three-bladed propeller (9). Substitution in the para position of the three aryl groups determines the hue of the dye. When only one amino group is present, as in fuchsoriimine hydrochloride [84215-84-9] = 440 nm (2), the shade is a weak orange-yeUow. 

In 1,8-bis(dimethylamino)naphthalene steric and lone pair interactions are minimised in a structure (Einspahr et al., 1973) having the amino groups twisted and displaced above and below the plane of the naphthalene ring, as in [53] in which the almost planar naphthalene ring is shown as a dashed line. In l,8-bis(dimethylamino)-2,7-dimethoxynaphthalene, steric interactions involving the methoxy substituents bring about a further distortion as in [54] (Woolf, 1980). 

Rettig and Gleiter81 have studied the dependence of intramolecular rotation in 4-cyano-IVTV-dialkylanilines 6-12 on the twist angle by fluorescence, UV absorption and PE spectroscopic measurements. The twist angles were determined from the split of the first and the third IP. While in molecules 6, 8 and 9, 11 and 12 the twist of the amino group... 

Fig. 1. Potentials and reaction path for PIPBN. In its formula, a component (amino group twist) of the reaction coordinate q is given by the arrow [3].

Bunker et al. also reported approximately three aminofullerene derivatives (3, 32, 33, Fig. 25) that show intramolecular electron transfer from the amino group to the singlet excited state of the fullerene core [189], Although the intramolecular electron transfer shares some characteristics with the classical twisted intramolecular charge transfer in molecules represented by p-N,N-dimethylaminobenzonitrile the amino-C6o derivatives are in fact better classified as redoxdyads [189],... 

Meyer, W. P. Martin, J. C. Substituent effects of alkoxy and amino groups directly bonded to cationic carbon in the perpendicularly twisted geometry. 2-Oxa- and 2-aza-l-adamantyl tosy-lates./. Am. Chem. Soc. 1976, 98, 1231-1241. 

The polypeptide chains of proteins do not remain in a flat plane. Instead, as a protein is formed, the polypeptide chain starts to twist and curl up. It folds and coils like a rope that can be bundled in many different shapes. This coiling and folding determines the protein s secondary structure. The secondary structure is maintained by chemical bonds between the carboxyl groups and the amino groups in the polypeptide backbone. There are many secondary structure patterns, but the two most common are the a-helix, and the p-sheet. 

These amino acids form hydrophobic (water-repelling) nonpolar regions in proteins. There are three more of this kind with special roles. Phenylalanine and tryptophan have aromatic rings and, though they are still hydrophobic, they can form attractive 7t-stacking interactions with other aromatic molecules. Enzyme-catalysed hydrolysis of proteins often happens next to one of these residues. Proline is very special. It has its amino group inside a ring and has a different shape from all the other amino acids. It appears in proteins where a bend or a twist in the structure is needed. 

A majority of theoretical studies indicates the amino-group twists as the reaction coordinate along which the LE —> ICT transition occurs. For example, a recent... 

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