Great question! If I would be to GUESS, I would point at deoxyribose and the maximized number of hydrogen bonds capable to establish with the environment in order to stabilize the DNA molecule. Certainly still an open debate though...

The normal B-form right-handed double helix is energetically favored under most conditions because of the chirality of its substituent nucleotides.

Under certain circumstances, however, it is possible to observe DNA having a left-handed double helical structure, called Z-DNA.

http://en.wikipedia.org/wiki/Z-DNA

Dear Adam,

Z-DNA is relatively unstable. I think that the twisting preferences are right handed. My new paper "Theory of Everything" provides an explanation as follows on page 4:

When electron runs into Proton, it applies Z twists to proton; when it comes out of Proton, it applies S twist to Proton.  

There are fifteen electrons for a Phosphorus atom. One of electron is unpaired. The unpaired unstable electron can twist the nucleus when nuclei are moving. The preferred twisting direction is Z twist as collision twist and electron twist (when run into the Proton) are in same Z twist direction.

To understand how an electron interacts with a nucleus, there are two other papers "Unified field theory and topology of atom" and "Cubic Atom and Crystal Structures".

Article Unified field theory and topology of atom

Article Cubic Atom and Crystal Structures

Article Theory of Everything

Hi Zhiliang,

As a biochemist, I have to say that I have trouble connecting your statements above to the structural chemistry of macromolecules, which is determined by the arrangement of the chemical bonds between atoms. In some cases, including DNA, some of these bonds are chiral (for example, 2-deoxyribose is the D isomer), and it is this chirality that determines the handedness of the macromolecule.

People who interest themselves in the origin of life often wonder why deoxyribose is the D-isomer and why amino acids in proteins are the L-isomer (except for glycine, which is not chiral). The other isomers can be made chemically, and bacteria produce some D-amino acids enzymatically by racemizing L-amino acids, so it is not the case that the L-deoxyribose and D-amino acids are impossible. In fact, there is no energetic difference between isomers. Amino acids will spontaneously racemize over long periods of time, which can be used to date ancient biological samples.

Dear Adam,

Oxygen and Carbon are relatively neutral. Right handed torque force applicable mainly on Phosphorous and Nitrogen when there is a movement. The twisting momentum of an unpaired electron (P and N, electron is odd number and electrons are not paired) is right handed when it collides with nucleus, then, it moves out left handed. Without movements, the forces are cancelled out. Once the structure is settled, there is no energy difference between the L and D isomers.

I did not study the issue at deoxyribose and amino acids level. My speculation is that the natural process is 50-50, while right handed Torque forces tip the equation slightly. We do need some models to describe relationship between preference and force. i.e., preferred handedness enjoys higher formation speed in preferred environment or initial states. 

As for left handedness of RNA, it can be explained as folding of single right handed string into left handed string.

 @Adam: I think Zhiliang is asking what he considers a rhetorical question, if you read his question + follow-up sentence.

@Zhiliang: After considering what you write (reproduced below), it appears the concept of chemical bonding is seriously violated.  The phosphorus atom in DNA, which comes as a phosphate group, has no unpaired electron - all of its five valence electrons are tied up in chemical bonding.  Maybe this somehow makes sense in your physics construct, but I'm sure chemists would not accept such a conjecture.

(It was written above: "There are fifteen electrons for a Phosphorus atom. One of electron is unpaired. The unpaired unstable electron can twist the nucleus when nuclei are moving. The preferred twisting direction is Z twist as collision twist and electron twist (when run into the Proton) are in same Z twist direction.")

Professor Kaoru Aou,

The unpaired unstable electron can twist the nucleus when nuclei are moving

In the above statement, "unpaired unstable electron" is an electron of an atom without its "companion". The term "paired" is not chemical bonding pair. It is an odd electron that rotates nucleus of moving (unbonded) atom without chemical bonding.

A chemical bonding can give atom with even electron count an "unpaired" electron. As result, a right handed Torque applies to the moving (free) neutral atom.

Dear Zhiliang Cao,

Does your theory of why nature favors right-handed helices stand in support of any existing theories (/speculations) on the matter? – Such as those mentioned here:

http://www.nature.com/news/force-of-nature-gave-life-its-asymmetry-1.15995

['Force of nature gave life its asymmetry', Nature, Sept. 2014]

Or is it offered up as an alternative to existing theories/speculations?

Please elaborate.

Thanks!

Dear Line Brandt,

Sorry for late response. My theory is an alternative to the existing theories.

Thanks.