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Does it mean clockwise and anticlockwise?

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The signs refer to the direction of rotation. Anticlockwise (right handed helix) is positive whereas clockwise (left handed helix) is negative. The direction of rotation is observed from the side where the helix is ascending towards you (perpendicular to the ascent of helix).

In other words, curl your right hand and extend the thumb out. When viewed from above (thumb points towards you), the fingers curl anticlockwise. This is a right handed (RH) helix. Conversely, you can use your left hand to understand left handed (LH) helices such as that of Z-DNA.


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Obtained from http://biomedapps.curtin.edu.au/biochem/tutorials/prottute/helixfigures.htm. Which, in-turn is modified from Lehninger et al (1993) Principles of Biochemistry 2nd Edition Worth Publishers


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This figure depicts a RH helix. Taken from Wikipedia.


These definitions can be defined in a converse sense too and you would find many web sources that do like that. For example, if you look from below your hand such that the thumb points away from you, then the fingers curl clockwise. However, in physics and mathematics, positive angles are conventionally measured anti-clockwise. So my definition here would be suitable with respect to your question.

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