Rotation glycosyl

FIGURE 12.14 Comparison of the deoxy-guanosine conformation in B- and Z-DNA. In B-DNA, the Cl -N-9 glycosyl bond is always in the anti position (lefi). In contrast, in the left-handed Z-DNA structure, this bond rotates (as shown) to adopt the syn conformation. 

Optical Rotations and Melting Points of Glycosyl Di-D-fructose Dianhydrides and Their Per-0-acetyl Derivatives... 

Fischer projection of acyclic form, 56-57 glycosides, 132-135 C-glycosyl compounds, 139-140 N-glycosyl derivatives, 137-139 glycosyl halides, 136-137 glycosyl residues, 125 isotopic substitution and isotopic labelling, 91 me so forms, 59 optical rotation, 59 parent structure choice, 53... 

The physical properties of the synthetic glycosyl derivatives of l-asparagine, L-serine, and L-threonine are reported in Tables I-V. Derivatives characterized otherwise, but without m.p. and optical rotation, have also been included. Whenever more than one reference is given, the physical constants are taken from the references printed in bold letters. The abbreviations used in the m.p. column are as follows foam., foaming dec., decomposing and soft., softening. 

P212121 Z = 4 D, = 1.362 R = 0.162 for 2112 intensities. The structure is very similar to that found308 for air-dried vitamin B12 crystals. Two water molecules move into phosphate oxygen-atom positions when the phosphate in the precursor is removed, and one acetamido group in contact with these water molecules in the vitamin is rotated out of the way in the phosphate. The disposition of the a-D-glycosyl bond between the D-ribosyl group and the 5,6-dimethylbenzimidazole is anti (—45°), and the conformation of the D-ribosyl group is 2T3 (P = 352.1 rm = 47.1). The orientation about the exocyclic, C-4 -C-5 bond is g+ (53°). 

Fluorescence probes possessing the PyU base 46 selectively emit fluorescence only when the complementary base is adenine. In this case, the chromophore of is extruded to the outside of the duplex because of Watson-Crick base pair formation, and exposed to a highly polar aqueous phase. On the contrary, the duplex containing a PyU/N (N = G, C and T) mismatched base pair shows a structure in which the glycosyl bond of uridine is rotated to the syn conformation. In this conformation, the fluorophore is located at a hydrophobic site of the duplex. The control of base-specific fluorescence emission is based on the polarity change in the microenvironment where the fluorophore locates are dependent on the l>yU/A base-pair formation. 

Structural variation in DNA reflects three things the different possible conformations of the deoxyribose, rotation about the contiguous bonds that make up the phosphodeoxyribose backbone (Fig. 8-18a), and free rotation about the C-l -N-glycosyl bond (Fig. 8-18b). Because of steric constraints, purines in purine nucleotides are restricted to two stable conformations with respect to deoxyribose, called syn and anti (Fig. 8-18b). Pyrimidines are generally restricted to the anti conformation because of steric interference between the sugar and the carbonyl oxygen at C-2 of the pyrimidine. 

Minor variations of the backbone and glycosyl rotations from the fixed values used in the sample computations above produce a variety of theoretically acceptable double helices. As evident from the partial list of structures in Table I, these structures include several 10-fold duplexes similar to the B-DNA models from fiber diffraction studies as well as the larger 13-fgld complex. Despite the large fluctuations in h from 1.7 to 4.3 A, the bases associate at standard separation distances (3 ft < < 4 A and 2.8 X < < 3.0 A) and orientations (A < 30° and < 30°) in all cases. In order to avoid severe steric contacts at small values of h, the bases may tilt up to values of n = 45° with respect to the standard orientation (n = 90°) perpendicular to the helix axis. 

Because the glycosyl links and the associated sugar-phosphate backbone are related, in the Watson-Crick duplex, by a pseudodyad perpendicular to the helix axis, the two strands are oriented antiparallel. They are parallel in the Hoogsteen duplex because the glycosyl links are related by rotation about the helix axis. 

Glycoside or glycosyl compound Boronate Melting point (°C) Md (degrees) Rotation solvent References... 

Figure 5. Regression of rotated component 1 of mean aroma attribute scores of glycoside hydrolysates on red free glycosyl-glucose concentration (G-G) of the juices and skin extracts that the glycosides were isolated from. The symbols used are explained in the caption to Figure 2. For sample codes see Table II.

The first aeetylated glycosyl fiuoride derivative was prepared by Brauns in 1923 and, in subsequent papers, he explored the synthesis of a number of poly-O-acetylglycosyl fluorides. In addition, Brauns prepared the other poly-O-acetylglycosyl halides of the same carbohydrates and investigated the proportionality relations which exist between their optical rotations and the diameters of the respective halogen atoms. ... 

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