Helix, alpha , double

N. T., Lhomme J., Helene C. Sequence-specific recognition, photocrosslinking and cleavage of the DNA double helix by an oligo-[alpha]-thymidylate covalently linked to an azidoproflavine derivative. Nucleic Acids Res. 1987 15 7749-7760. 

At the time Pauling was working on the structure of proteins culminating eventually in his discovery of the alpha-helix. Yet this declaration sounds as if he were anticipating the mechanism of DNA replication via the double helix. It came, however, only in 1953, and it was not Pauling, but Watson and Crick, who discovered it. 

There are also structural differences RNA is usually single-stranded (alpha helix) while DNA is usually double-stranded (double helix). (Figure 1.5)... 

The NMR structure of ASYN bound to SDS micelles, commonly used for membrane mimicking, revealed a break in the helix, resulting in two antiparallel alpha-helices [132]. This model was confirmed by distance measurements exploiting SDSL EPR utilizing 13 different ASYN double mutants each containing 2 spin-labeled cysteines (horseshoe model. Fig. 12) [137]. 

Figure A3.2 The structure of an alpha-helix. Oxygen atoms are shown in red and nitrogens in blue. In this spiral arrangement of the polypeptide chain, the C=0 and N—H bonds are approximately parallel to the helix axis and able to form multiple H-bonds, shown here as double dashed lines.

Fig. 1.11 Projection representation of the three-dimensional alpha-helix model. The peptide linkage CN bonds are shown exaggeratedly as double bonds. The hydrogen bonds are depicted by dashed lines (see text)...

Above, we narrated the story of alpha helix to stress the merits of utilizing the theory of resonance in this pivotal discovery. There were other theories developed at the same time. Erich Hiickel s (Fig. 1.12) studies of the double bond and of aromatic... 

Willey et al. (1) have monitored carboxypeptidase digestion of cytochrome f (Cyt f) in intact thylakoid membranes. They have presented a model of the orientation of Cyt f in the thylakoid membrane where residues 251-270 form a transmembrane alpha-helix, residues 271-285 are in the stroma and the remainder of the molecule resides in the intra-thylakoid space. It has been postulated that the solubility of purified Cyt f, from members of the Cruciferae, is due to the loss of the transmembrane alpha-helix during purification. Turnip Cyt f is soluble and, upon electrophoresis in the presence of SDS (SDS-PAGE), is resolved into diffuse double bands with Mp of 28 and 32 Kd. The predicted Mp of sequenced cytochromes f is 31 Kd and a loss of 4 Kd from tne C-terminus would include all of the presumed membrane spanning alpha-helix. The remaining Cyt f would be more negative by 2 units. 

The double helix structure of a DNA has been imaged by AFM. That is shown in Fig. 21.9. A DNA molecule looks like a right-handedly wound ropes. The ropes are the (alpha) helically coiled double strand. These images (Figs. 21.6-21.9) give us... 

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