Parallel and antiparallel

Figure A3.5.2. The Ar photofragment energy spectmm for the dissociation of fiions at 752.5 mn. The upper scale gives the kinetic energy release in the centre-of-mass reference frame, both parallel and antiparallel to the ion beam velocity vector in the laboratory.

Cellulose III. Cellulose III results from treatment of cellulose with Hquid ammonia (ammonia mercerization) or amines. Cellulose III can be made from either Cellulose I or II. When treated with water. Cellulose III can revert to its parent stmcture. Some cellulose III preparations are much more stable than other preparations. The intensities on diffraction patterns from Cellulose III differ slightly depending on whether the Cellulose III was made from Cellulose I or II, and thus these allomorphs are called IIIj or IHjj- Workers studying III concluded, based partiy on the results of I and II, that the packings of IIIj and IIIjj are parallel and antiparallel, respectively (67). IIIjj also is thought to have hydrogen bonds between the corner and center chains. 

Two antiparallel helices, related by space group symmetry, are packed in an orthorhombic unit cell (Fig. 39b). There is substantial interdigitation between the helices so that side chains and main chains are linked by hydrogen bonds, such as 0-4E-0-4D (2,73 A) and 0-4D-0-3F (2.84 A) involving parallel and antiparallel strands, respectively. Plausible sites for sodium ions are near the... 

Warner, F.D. Mitchell, D.R. (1981). Polarity of dynein-microtubule interactions in vitro Cross-bridging between parallel and antiparallel microtubules. J. Cell Biol. 89, 35-44. 

Fig. 15 Possibilities for photochemically induced reactions, using a carbonyl compound as example. U and IT = electron spins parallel and antiparallel respectively.

Fig. 2.27 The two types of extended /1-peptide strands with conformation requirements around the C(a)-C(/1) bonds. (A) Parallel and antiparallel polar sheets with antiperiplanar conformations around the C(a)-C fl) bond are promoted by unlike-fi -ami-no acids with alkyl side-chains. Antiperiplanar side-chains at C(a) and C(/3) occupy positions approximately perpendicular to the amide planes. (B) Extended strands formed by alternating +)-sc and (-)-sc conformations...

Figure 2.6 The 3-sheet. (a) Two segments of (3-strands (antiparallel) forming a (3-sheet via hydrogen bonding. The (3-strand is drawn schematically as a thick arrow. By convention the arrowhead points in the direction of the polypeptide s C terminus, (b) Schematic illustration of a two-strand (3-sheet in parallel and antiparallel modes...

In a basic pulsed NMR experiment (for I = 1/2), when a sample is placed in the applied magnetic field (B0), the nuclear spins distribute themselves between parallel and antiparallel positions, according to Boltzmann distribution [Eq. (11)] (Figure 21 A). The number of spins in the parallel position is slightly greater than that in the antiparallel position. At equilibrium, the spins are processing randomly (i.e., lack phase coherence). The populations... 

Fig. 1. Cross-/] structure of amyloid fibrils. (A) Cartoon representation of a cross-/] X-ray diffraction pattern. The defining features are a meridional reflection at 4.7 A and an equatorial reflection on the order of 10 A. The 4.7-A reflection is generally much brighter and sharper than the reflection at 10 A. (B) The cross-/] core structure of amyloid fibrils. Parallel /(-sheets are depicted, but the structure could equivalendy be composed of antiparallel /(-sheets or a mix of parallel and antiparallel. The 4.7-A spacing of /(-strands within each /(-sheet is parallel to the long fibril axis. The depicted 10-A sheet-to-sheet spacing actually ranges from about 5 to 14 A (Fandrich and Dobson, 2002), depending on the size and packing of amino acid side chains. Amyloid fibrils have diameters on the order of 100 A.

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