Category Archives: DNA

Stability of Unnatural DNA – @PCCP #CompChem

As is the case of proteins, the functioning of DNA is highly dependent on its 3D structure and not just only on its sequence but the difference is that protein tertiary structure has an enormous variety whereas DNA is (almost) always a double helix with little variations. The canonical base pairs AT, CG stabilize the famous double helix but the same cannot be guaranteed when non-canonical -unnatural- base pairs (UBPs) are introduced.


Figure 1

When I first took a look at Romesberg’s UBPS, d5SICS and dNaM (throughout the study referred to as X and Y see Fig.1) it was evident that they could not form hydrogen bonds, in the end they’re substituted naphtalenes with no discernible ways of creating a synton like their natural counterparts. That’s when I called Dr. Rodrigo Galindo at Utah University who is one of the developers of the AMBER code and who is very knowledgeable on matters of DNA structure and dynamics; he immediately got on board and soon enough we were launching molecular dynamics simulations and quantum mechanical calculations. That was more than two years ago.

Our latest paper in Phys.Chem.Chem.Phys. deals with the dynamical and structural stability of a DNA strand in which Romesberg’s UBPs are introduced sequentially one pair at a time into Dickerson’s dodecamer (a palindromic sequence) from the Protein Data Bank. Therein d5SICS-dNaM pair were inserted right in the middle forming a trisdecamer; as expected, +10 microseconds molecular dynamics simulations exhibited the same stability as the control dodecamer (Fig.2 left). We didn’t need to go far enough into the substitutions to get the double helix to go awry within a couple of microseconds: Three non-consecutive inclusions of UBPs were enough to get a less regular structure (Fig. 2 right); with five, a globular structure was obtained for which is not possible to get a proper average of the most populated structures.

X and Y don’t form hydrogen bonds so the pairing is pretty much forced by the scaffold of the rest of the DNA’s double helix. There are some controversies as to how X and Y fit together, whether they overlap or just wedge between each other and according to our results, the pairing suggests that a C1-C1′ distance of 11 Å is most stable consistent with the wedging conformation. Still much work is needed to understand the pairing between X and Y and even more so to get a pair of useful UBPs. More papers on this topic in the near future.

Unnatural DNA and Synthetic Biology

Ever since I read the highly praised article by Floyd Romesberg in Nature back in 2013 I got really interested in synthetic biology. In said article, an unnatural base pair (UBP) was not only inserted into a DNA double strand in vivo  but the organism was even able to reproduce the UBPs present in subsequent generations.


Romesberg’s Nucleosides. No Hydrogen bonding is formed between them!

Inserting new unnatural base pairs in DNA works a lot like editing a computer’s code. Inserting a couple UBPs in vitro is like inserting a comment; it wont make a difference but its still there. If the DNA sequence containing the UBPs can be amplified by molecular biology techniques such as PCR it means that a polymerase enzyme is able to recognize it and place it in site, this is equivalent to inserting a ‘hello world’ section into a working code; it will compile but it’s pretty much useless. Inserting these UBPs in vivo means that the organism is able to thrive despite the large deformation in a short section of its genetic code, but having it replicated by the chemical machinery of the nucleus is an amazing feat that only a few molecules could allow.

The ultimate goal of synthetic biology would be to find a UBP which codes effectively and purposefully during translation of DNA.This last feat would be equivalent to inserting a working subroutine in a program with a specific purpose. But not only could the use of UBPs serve for the purposes of expanding the genetic code from a quaternary (base four) to a senary (base six) system: the field of DNA origami could also benefit from having an expansion in the chemical and structural possibilities of the famous double helix; marking and editing a sequence would also become easier by having distinctive sections with nucleotides other than A, T, C and G.

It is precisely in the concept of double helix that our research takes place since the available biochemical machinery for translation and replication can only work on a double helix, else, the repair mechanisms get activated or the DNA will just stop serving its purpose (i.e. the code wont compile).

My good friend, Dr. Rodrigo Galindo and I have worked on the simulation of Romesberg’s UBPs in order to understand the underlying structural, dynamical and electronic causes that made them so successful and to possibly design more efficient UBPs based on a set of general principles. A first paper has been accepted for publication in Phys.Chem.Chem.Phys. and we’re very excited for it; more on that in a future post.

#RealTimeChem – Happy birthday DNA!

What a happy coincidence -if indeed it was- that #RealTimeChem week happened to coincide with the sixtieth anniversary of the three seminal papers published in Nature on this day back in 1953, one of which was co-authored by J. Watson and F. Crick; of course I mean the publication for the first time of the structure of deoxyribose nucleic acid, or DNA, as we now call it.

You can get the original Nature papers from 1953 here at: (costs may apply)

Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid 737


Molecular Structure of Nucleic Acids: Molecular Structure of Deoxypentose Nucleic Acids 738

Molecular Configuration in Sodium Thymonucleate 740

Nature’s podcast released two episodes (called ‘pastcast’) to celebrate DNA’s structure’s birthday, one of them is an interview with Dr. Raymond Gosling who in 1953 worked under Dr. Rosalind Franklin at King’s College London in diffractometry of biological molecules. If you haven’t listened to them you can get them here at Of course, the history around the discovery of DNA’s structure is not without controversy and it has been long argued that the work of Franklin and Gossling didn’t get all deserved credit from Watson and Crick. In their paper W&C acknowledge the contribution of the general nature of DNA from the unpublished results by Franklin’s laboratory but that is as far as they went, they didn’t even mention photo 51 which Crick saw at Wilkins laboratory, who in turn got it from Gossling at Franklin’s suggestion. Still, no one can deny that the helical structure with which we are now familiar is their work, and more importantly the discovery of the specific pairing, which according to Gossling was a stroke of genious that probably couldn’t have happened in his own group, but without Franklin’s diffraction and Gossling’s crystallization  there was little they could do. Details about the process used to crystallize DNA can be heard in the aforementioned podcast, along with an inspiring tale of hard work by Dr. Gossling. Go now and listen to it, its truly inspiring.

For me it was not the story of a helix, that I was familiar with; it was the story of the specific pairing of two hélices
– Dr. Raymond Gosling

Famous Photo 51 by Dr. Rosalind Franklin and Raymond Gosling (Source: Wikipedia)

The iconic Photo 51 by Dr. Rosalind Franklin and Raymond Gosling (Source: Wikipedia)

Above, the iconic Photo 51 taken by Franklin and Gossling (have you ever noticed how most scientists refer to Franklin just as Rosalind but no one refers to Watson as James? Gender bias has a role in this tale too) To a trained crystallographer, the helical symmetry is evident from the diffraction pattern but going from Photo 51 to the representation below was the subject of hard work too.

Modern DNA representation (Source: Wikipedia)

Modern DNA representation (Source: Wikipedia)

There are million of pages written during the last 60 years about DNA’s structure and its role in the chemistry of life; the nature of the pairing and the selectivity of base pairs through hydrogen bond interactions, an interaction found ubiquitously in nature; water itself is a liquid due to the intermolecular hydrogen-bonds, which reminds us about the delicate balance of forces in biochemistry making life a delicate matter. But I digress. Millions of pages have been written and I’m no position of adding a meaningful sentence to them; however, it is a fascinating tale that has shaped the course of mankind, just think of the Human Genome Project and all the possibilities both positive and negative! DNA and its discovery tale will continue to amaze us and inspire us, just like in 2011 it inspired the Genetech company to set a Guiness World Record with the largest human DNA helix.

Genetech SF, Cal. USA (Source

Genetech SF, Cal. USA (Source

Happy birthday, DNA!

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