Torsional twist

The rotational relaxation of DNA from 1 to 150 ns is due mainly to Brownian torsional (twisting) deformations of the elastic filament. Partial relaxation of the FPA on a 30-ns time scale was observed and qualitatively attributed to torsional deformations already in 1970.(15) However, our quantitative understanding of DNA motions in the 0- to 150-ns time range has come from more accurate time-resolved measurements of the FPA in conjunction with new theory and has developed entirely since 1979. In that year, the first theoretical treatments of FPA relaxation by spontaneous torsional deformations appeared. 16 171 and the first commercial synch-pump dye laser systems were delivered. Experimental confirmation of the predicted FPA decay function and determination of the torsional rigidity of DNA were first reported in 1980.(18) Other labs 19 21" subsequently reported similar results, although their anisotropy formulas were not entirely correct, and they did not so rigorously test the predicted decay function or attempt to fit likely alternatives. The development of new instrumentation, new data analysis techniques, and new theory and their application to different DNAs in various circumstances have continued to advance this field up to the present time. 

Fig. 1. Schematic layout of the operating unit of the Weissen-berg rheogoniometer 1 - the liquid studied 2 — rotating cone of the operating unit 3 — round plate ( plane ) 4 — torsion beam 5 — bar which displaces upon torsion twisting 6 — induction translation transducer 7 — opening camera with a thermostated furnace...

Ylanne, J., Scheffzek, K., Young, P., and Saraste, M. (2001a). Crystal structure of the alpha-actinin rod reveals an extensive torsional twist. Structure 9, 597-604. 

Significant inter-ring bond torsional (twisting) coupling to the ground state could occur, resulting in a large radiationless rate constant for Ti —> So... 

Very recently, Stahl et al. reported the first synthesis of a 7-membered NHC ligand [98]. Despite substantial effort, the isolation of the free carbene 21 was not successful. However, palladium complexes of 21 could be formed and structurally characterized. Ligand 21 is C2 symmetric as a result of a torsional twist which is thought to attenuate the antiaromatic character of the 87r-electron carbene heterocycle [101,102]. It will be interesting to see, if the synthesis of conformationally stable analogues and their application in asymmetric catalysis will be feasible. 

The torsions (twisting movements, often involving the neck) can be worsened by activity such as attempts to write or walk. Sometimes they can be relieved by particular movements such as touching the chin to relieve torticollis or touching the brow to relieve blepharospasm. 

Fig. 1-3. Six ideal conformations of dppf arising from Cp. .. Fe. .. Cp torsional twist.

Fig. 1-8. Effects of torsional twist of dppf on metal-metal interaction.

Fig. 1-10. Skeletal freedom of dppf in its bridging state (1) Cp...Fe...Cp torsional twist (2) Cp ring tilting (3) displacement of phosphorus atom from coplanarity with Cp ring (4) rotation of C(Cp)-P bond (5) skewing of ferrocenyl backbone with respect to the M-M bonding axis.

Fig. 1-17. (a) Torsional twist in a twist-boat conformation and (b) bridge-reversal in a half-chair conformation in a metal dppf chelate (adapted from [22]). 

The first dibenzo[<7,/][l,3]azepine-based carbene ligands for transition metals were described, together with their dinaphtho analogues. The heterocyclic framework involved is of interest because the embedded torsional twist results in chiral, C2-symmetric structures <05JOM6143>. 

Figure 4.6 Molecular mechanics components bond stretch (top left), bond angle bend (top right), torsional twist (bottom left), non-bonded interaction (bottom right)...

FIGURE 4.3 Definition of the chain angles 8 (bond angle ring-amine N-ring) and 0 (ring torsional twist). 

Metrics were derived as follows. Macrocyclic centers (Ct) were calculated as the centers of the four nitrogen planes (4-N plane) for each macrocycle. Interplanar angles were measured as the angle between the 4-N least-squares planes. Plane separations were measured as the perpendicular distance from one macrocycle s 4-N least-square plane to the center of the other macrocycle mean-plane separations (MPS) were the average of the two plane separations. Torsional twists were measured as the angle between the two meso carbon to spacer bonds. Uie a-b and c-d distances were obtained according to the convention shown in Fig. 7. Data is reproduced from Ref. (86). 

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