Dalton Transactions Blog

Hypochlorous acid (HOCl) is a weak acid, formed from the reaction of chlorine with water. In addition to its use as a reagent in organic chemistry, it has significant biological relevance. HOCl is generated in biological systems in a reaction between chloride ions and hydrogen peroxide, catalysed by the enzyme myeloperoxidase.

This enzyme is secreted by phagocytes (cells which help protect the body by ‘ingesting’ bacteria) when they are activated during an immune response. Hypochlorite (ClO^–), the conjugate base of HOCl, is extremely toxic to bacteria and plays a vital role in assisting the activated phagocytes with killing a wide range of pathogens.¹

Excess production of HOCl in a living system can have a detrimental effect, as HOCl can react with many different biological molecules, including DNA, cholesterol and proteins, leading to changes in their biological properties. An example of this is the reaction of hypochlorous acid with unsaturated bonds in lipids, which produces a species called a chlorohydrin. This disrupts the formation of the essential lipid by-layers which form around cells.

Excess hypochlorous acid has been implicated in conditions such as inflammatory diseases, neurodegeneration and cancers.² In order to fully understand the role of HOCl in these biological processes, accurate detection methods must be developed to monitor the molecule in living cells.

Several ‘HOCl-recognising’ molecules have been found to be effective sensors of hypochlorous acid. When conjugated with a fluorophore, these probes can successfully ‘recognise’ HOCl by reacting with it, however their application in vivo is still limited due to their excitation wavelengths being in the ultraviolet region of light.³ Sensors with adsorption (or emission) in the visible light range are more desirable for clinical diagnostic applications. 

In one recent paper in Dalton Transactions, Yuan and co-workers combine an excellent HOCl-recognising moiety: 4-amino-3-nitrol phenol) and a ruthenium(II)-2,2-bipyridyl complex, which is well known to exhibit visible light adsorption and emission, into one compound to create a luminescent probe for HOCl.

The resulting complex [Ru(bpy)₂(AN-bpy)][PF₆]₂ is very weakly luminescent but, upon reaction with HOCl in aqueous media, converts to [Ru(bpy)₂(HM-bpy)][PF₆]₂, which has a luminescence signal which is 110-fold stronger.

Impressively, the authors show that when HeLa cells are incubated with [Ru(bpy)₂(AN-bpy)][PF₆]₂ for two hours they remain non-luminescent; when the same cells are subsequently treated with HOCl for thirty minutes, a bright red luminescence is observed, clearly demonstrating the potential for using this ruthenium complex as an in vivo, luminescent detector of hypochlorous acid. 

To find out more, read the article using the link below:

Development of a functional ruthenium(II) complex for probing hypochlorous acid in living cells
Dalton Trans. 2014, DOI:10.1039/C4DT00179F

Dr C. Liana Allen is currently a post-doctoral research associate in the group of Professor Scott Miller at Yale University, where she works on controlling the enantio- or regioselectivity of reactions using small peptide catalysts. Liana received her Ph.D. in organic chemistry at Bath University with Professor Jonathan Williams, where she worked on developing novel, efficient syntheses of amide bonds.

References  

 ¹ J. M. Albrich, C. A. McCarthy, J. K. Hurst, Prot. Nat. Acad. Sci., 1981, 78, 210.
²  T. I. Kim, S. Park, Y. Choi, Y. Kim, Chem.-Asian J., 2011, 6, 1358
³ Y. Xiao, R. Zhang, Z. Ye, Z. Dai, H. An, J. Yuan, Anal. Chem., 2012, 84, 10785.

Direct capture of CO₂ from the air presents significant challenges, one of which is the separation of CO₂ from other gases. Currently, a lot of waste carbon dioxide is captured at the emission source – fossil fuel-powered energy plants – by a ‘filter’ that traps the CO₂ as it travels up a chimney. This method can prevent up to 90% of a power plant’s carbon emissions from entering the atmosphere, however the process requires a lot of energy and the captured gas still needs to be transported to a suitable storage area. New technologies for selective capture of carbon dioxide from the air are still in their infancy, but could offer more efficient ways to trap CO₂ anywhere on the planet, not just at the sources of emission. 

Figure 1: Double chain MOF

Metal organic frameworks (MOFs), also known as coordination polymers, are compounds containing metal ions which are coordinated to organic molecules to form extensive two- or three-dimensional structures. The uniform and controllable porosity of these materials has already been exploited as potential devices for gas capture and storage.² In one recent paper in Dalton Transactions, Kim and co-workers synthesise novel MOFs using cobalt and zinc metal ions coordinated to porphyrin-based molecules. The resulting MOFs display an interesting 1D ‘double chain’ arrangement of molecules (Figure 1). These ‘double chains’ pack together tightly, forming hydrogen bonds between the ‘chains’, resulting in a stable solid state structure with defined pores. Gas sorption experiments revealed that these new MOFs both show high uptake of CO₂ gas compared with nitrogen (N₂), hydrogen (H₂) and methane (CH₄), (Figure 2), making these types of materials excellent candidates for selective CO₂ capture from the air. 

Figure 2: Gas uptake by double-chain MOF

Find out more and download the article now:
CO₂ selective 1D double chain dipyridyl-porphyrin based porous coordination polymers 
Dalton Trans. 2014, DOI:10.1039/C3DT53287A 

References 

¹ IPCC Special Report, ‘Carbon Dioxide Capture and Storage’, IPCC Working Group III, 2005. 
² S. L. James, Chem. Soc. Rev., 2003, 32, 276.

Dr C. Liana Allen is currently a post-doctoral research associate in the group of Professor Scott Miller at Yale University, where she works on controlling the enantio- or regioselectivity of reactions using small peptide catalysts. Liana received her Ph.D. in organic chemistry at Bath University with Professor Jonathan Williams, where she worked on developing novel, efficient syntheses of amide bonds.

How do you combine rare-earth metals, extremely specific energy transfers, and luminescent properties to investigate changes in enzymes? New methods often arise from unique confluences of existing knowledge. In their recent paper, the Natrajan group from the University of Manchester exploit known properties of easily-obtained chemical products to present a clever new biosensory technique .

The unique aspect is the use of upconverting phosphors (UCPs) in combination with enzymes. UCPs are luminescent particles, often based on rare-earth metals, which can be excited by multiple photons absorbed in the near-infra-red region (750-1400 nm wavelengths). Post excitation, they emit a photon of light in the higher-energy visible spectrum, thus the energetic process is known as up-conversion. While enzymes have high specificities and sensitivities to substrates, UCP’s have the advantage of excitation in the near-infra-red region without autofluorescence. In combination, enzymes and UCPs provide several direct advantages over simple biosensory fluorescence measurements.

In the current paper, NaYF4:Yb:Tm was the UCP used to probe the redox properties of the enzyme pentaerythritol tetranitrate reductase (PETNR) and Forster Resonance Energy Transfer (FRET), involving energy transfer between two chromophores, was used to excite the UCP. In this case, transfer of energy from the absorbance band of the flavin mononucleotide core of the PETNR enzyme and the emission band of the UCP, which are very close in wavelength, allow FRET to occur. Since a second emission band in the near-IR region originates from this UCP, this was normalized so that the other band, varying with the enzyme concentration, could be measured against it. When the PETNR underwent a two-electron reduction, it negated its ability to undergo FRET, resulting in the loss of the emission band at 460 nm, rendering the solution colourless. The researchers demonstrated that this new technique can be used with either the full PETNR enzyme or the mononucleotide flavin core alone, indicating that this can be applied to a wider range of systems.

Find out more and download the article now:
Ratiometric detection of enzyme turnover and flavin reduction using rare-earth upconverting phosphors
Dalton Trans., 2014, DOI: 10.1039/C4DT00356J

Ian Mallov is currently a Ph.D. student in Professor Doug Stephan’s group at the University of Toronto. His research is focused on synthesizing new Lewis-acidic compounds active in Frustrated Lewis Pair chemistry. He grew up in Truro, Nova Scotia and graduated from Dalhousie University and the University of Ottawa, and worked in chemical analysis in industry for three years before returning to grad school.

The 2013 Dalton Transactions Lecture awardees delivered their presentations at UC Berkeley last month. Each awardee is provided with an honorarium and a commemorative plaque. 

	Professor Trevor Hayton (UCSB) gave the annual Dalton Transactions Lecture, which is awarded to an exceptional young inorganic chemist in the Americas each year. Previous recipients are: 2012   Teri Odom (Northwestern University) 2011    Daniel Gamelin (U Washington) 2010    Paul Chirik (Princeton University) 2009    Francois Gabbai (Texas A & M University) 2008    Dan Mindiola (Indiana University) 2007    Geoff Coates (Cornell University) 2006    John Hartwig (University of Illinois at Urbana-Champaign) 2005    Kit Cummins (MIT) Professor Hayton has rapidly established himself as a leader in synthetic inorganic chemistry, focusing on actinides and bioinorganic systems. His lecture focused on the synthesis and reactivity of actinide complexes with chalcogenide ligands.  Professor Hayton received his B.Sc. in Chemistry from the University of British Columbia, whereupon he began his Ph.D. research, also at UBC, under the direction of Peter Legzdins. After graduating in 2003, he began a postdoctoral fellowship at Los Alamos National Laboratory before joining the faculty at University of California, Santa Barbara in 2003.
	The inaugural Dalton Transactions Distinguished Lecture was given on February 7 by Professor Phil Power of UC Davis. Professor Power is a world-renowned expert in main group chemistry. His Dalton Transactions Lecture focused on the preparation and structure of low-coordinate main group compounds and their reactivity towards small molecules such as dihydrogen and ethene.  Professor Power received his bachelor’s degree in chemistry from the University of Dublin, Ireland, and his doctorate from the University of Sussex; the latter under the supervision of Mike Lappert. He carried out postdoctoral research at Stanford University before joining the faculty at UC Davis in 1980. He was award the Royal Society of Chemistry Mond Medal in 2005 and elected Fellow of the Royal Society in the same year.

 Congratulations to Professors Hayton and Power for their awards!