Respiratory pigments are metalloproteins that transport O2, the best known being the bright red/crimson coloured hemoglobin in human blood. The colour derives from Fe2+ at the core of a tetraporphyrin ring. But less well known is blue blood, and here the colour derives from an oxyhemocyanin unit based on Cu1+ (the de-oxy form is colourless) rather than iron. See below for the carapace of a red rock crab.
Archive for the ‘crystal_structure_mining’ Category
Blue blood.
Monday, August 7th, 2023Tunable aromaticity? An unrecognized new aromatic molecule?
Sunday, May 21st, 2023Some time ago in 2010, I showed a chemical problem I used to set during university entrance interviews. It was all about pattern recognition and how one can develop a hypothesis based on this. In that instance, it involved recognising that a cyclic molecule which appeared to have the cyclohexatriene benzene-aromatic pattern 1 was in fact a trimer of carbon dioxide. Perhaps small amounts of this aromatic molecule exist in solutions of fizzy drinks? Analysing these patterns occupied about 10-20 minutes of an interview, and although you might think I was posing a difficult challenge, many students successfully rose to it! Now I revisit, but with a slightly better reality check on a related molecule 2 (cyanuric acid).
Geometries of proton transfers: modelled using total energy or free energy?
Monday, April 18th, 2022Proton transfers are amongst the most common of all chemical reactions. They are often thought of as “trivial” and even may not feature in many mechanistic schemes, other than perhaps the notation “PT”. The types with the lowest energy barriers for transfer often involve heteroatoms such as N and O, and the conventional transition state might be supposed to be when the proton is located at about the half way distance between the two heteroatoms. This should be the energy high point between the two positions for the proton. But what if a crystal structure is determined with the proton in exactly this position? Well, the first hypothesis is that using X-rays as the diffracting radiation is unreliable, because protons scatter x-rays very poorly. Then a more arduous neutron diffraction study is sometimes undertaken, which is generally assumed to be more reliable in determining the position of the proton. Just such a study was undertaken for the structure shown below (RAKQOJ)[cite]10/c3zxh2[/cite], dataDOI: 10.5517/cc57db3 for the 80K determination. 
The substituents had been selected to try to maximise the symmetry of the O…H…N motif via pKa tuning (for another tuning attempt, see this blog). The more general landscape this molecule fits into[cite]10.1039/C1RA00219H[/cite] is shown below:
Protein-Biotin complexes. Crystal structure mining.
Sunday, December 12th, 2021In the previous post, I showed some of the diverse “non-classical”interactions between Biotin and a protein where it binds very strongly. Here I take a look at two of these interactions to discover how common they are in small molecule structures.
First came Molnupiravir – now there is Paxlovid as a SARS-CoV-2 protease inhibitor. An NCI analysis of the ligand.
Saturday, November 13th, 2021Earlier this year, Molnupiravir hit the headlines as a promising antiviral drug. This is now followed by Paxlovid, which is the first small molecule to be aimed by design at the SAR-CoV-2 protein and which is reported as reducing greatly the risk of hospitalization or death when given within three days of symptoms appearing in high risk patients.
More examples of crystal structures containing embedded linear chains of iodines.
Sunday, October 17th, 2021The previous post described the fascinating 170-year history of a crystalline compound known as Herapathite and its connection to the mechanism of the Finkelstein reaction via the complex of Na+I2– (or Na22+I42-). Both compounds exhibit (approximately) linear chains of iodine atoms in their crystal structures, a connection which was discovered serendipitously. Here I pursue a rather more systematic way of tracking down similar compounds.
Herapathite: an example of (double?) serendipity.
Thursday, October 14th, 2021On October 13, 2021, the historical group of the Royal Society of Chemistry organised a symposium celebrating ~150 years of the history of (molecular) chirality. We met for the first time in person for more than 18 months and were treated to a splendid and diverse program about the subject. The first speaker was Professor John Steeds from Bristol, talking about the early history of light and the discovery of its polarisation. When a slide was shown about herapathite[cite]10.1126/science.1173605[/cite] my “antennae” started vibrating. This is a crystalline substance made by combining elemental iodine with quinine in acidic conditions and was first discovered by William Herapath as long ago as 1852[cite]10.1080/14786445208646983[/cite] in unusual circumstances. Now to the serendipity!
More record breakers for the anomeric effect involving C-N bonds.
Saturday, September 4th, 2021An earlier post investigated large anomeric effects involving two oxygen atoms attached to a common carbon atom.
Sterically stabilized cyclopropenylidenes. An example of Octopus publishing?
Sunday, August 15th, 2021Whilst I was discussing the future of scientific publication in the last post, a debate was happening behind the scenes regarding the small molecule cyclopropenylidene. This is the smallest known molecule displaying π-aromaticity, but its high reactivity means that it is unlikely to be isolated in the condensed phase. A question in the discussion asked if substituting it with a large sterically hindering group such as R=Et3C might help prevent its dimerisation and hence allow for isolation of the monomer so that its properties can be studied.
Another very large anomeric effect – with a twist.
Thursday, July 22nd, 2021In the earlier post on the topic of anomeric effects, I identified a number of outliers associated with large differences in the lengths of two carbon-oxygen bonds sharing a common carbon atom.