Ola Persson

Ola undervisar på Biologiprogrammet, Gastronomiprogrammet och lärarutbildningens inriktning och specialisering i kemi. Ola undervisar framför allt i organisk kemi, biokemi, miljökemi och livsmedelskemi.

Forskning

1. Studies of electron transfer processes radical cations and free radicals.

Introduction

Radical cations and free radicals are generally very reactive and thus difficult to investigate. We have previously found a method that can be used to generate and stabilize radical cations. This means that we have a tool that can be used to explore the chemistry of radical cations, for instance their reactions with nucleophiles and also with neutral radicals.

Free radicals have become a concept soon known to every man, not least due to the idea that free radicals are involved in the process of aging. The most used method to study free radicals is spin trapping. We have previously in a number of papers shown that results from spin trapping experiments should be interpreted with care. Spin adducts may be formed via non-conventional mechanisms that do not involve any originally trapped radical. The most serious property of the compounds used as spin traps is their reactivity towards nucleophiles. Such reactions will produce intermediates that are easily oxidized, yielding spin adducts.

I will now give a more detailed summary of our most significant results concerning radical cations and spin trapping, and in the future plans presented below some of the projects planned will be outlined.

Summary of results

I. Formation and studies of radical cations.

No universal method for generating radical cations exists, explaining the search for new combinations of oxidants and solvents. We have found a method that hitherto is the best for generating radical cations. The system consists of various thallium(III) oxidants and 1,1,1,3,3,3-hexafluoropropan-2-ol (HFP) as the solvent. The generated radical cations show a remarkable persistency with half-lives generally increased with a factor 10-100 compared to the previously known best methods. Also, the experiments do not require low temperature but can conveniently be performed at room temperature. The quality of the EPR spectra is normally excellent.

We have shown that the main explanation to the superiority of our system is the ability of HFP to suppress nucleophilic activity. However, in many cases also thallium(III) is crucial since this oxidant will slowly oxidize the substrate and thereby provide a steady-state concentration of such radical cations which otherwise would be too reactive to study even in HFP. The mechanism of the electron transfer between thallium(III) and the organic compound is still not fully understood and will be the subject of further studies.

Using our method we have for instance been able to characterize the radical cations of methylsubstituted dibenzothiophenes and dibenzofurans. These studies also provided results that could be compared with theoretical calculations of spin densities. The correlation was found to be excellent. HFP was also used as the solvent when producing radical cations with tetranitromethane as the oxidant. This reaction leads to the formation of a triad consisting of the radical cation, the nucleophile trinitromethanide ion and nitrogen dioxide. Since HFP strongly attenuates nucleophilic reactivity, the reaction between the radical cation and the neutral radical nitrogen dioxide could conveniently be studied.

II. Non-conventional mechanisms for the generation of spin adducts.

The trapping of radicals by certain nitrones or nitroso compounds (spin traps) is often used to demonstrate the intervention of radicals in organic and biochemical reactions. However, the interpretation of the results should be made with care.

Under strongly oxidizing conditions the radical cation of the spin trap may form and its subsequent reaction with a nucleophile will yield a spin adduct. This mechanism is called inverted spin trapping and should be kept in mind, especially when spin trapping experiments are made under the influence of light since excited states may then form and act as strong electron acceptors.

Another, and more serious, pathway which will generate spin adducts is the direct addition of a nucleophile to the spin trap. An intermediate hydroxylamine is then formed, and this class of compounds is generally easily oxidized. Even rather mild oxidants will then lead to the formation of spin adducts. The mechanism is called the Forrester-Hepburn mechanism and is the most serious one considering proper interpretation of spin adduct formation. This is especially true in biochemical-biological systems where a variety of nucleophiles and oxidants generally are present.

In order to prevent the non-conventional spin trapping mechanisms, we have shown that HFP might be used as the solvent when performing the spin trapping studies since nucleophilic activity then is strongly curtailed. Thus, both inverted spin trapping and the Forrester-Hepburn mechanism are suppressed. Finally it should be mentioned that we have found HFP also to be very useful for electrochemical studies.

III. Spin trapping of radicals from the reactions between donor and acceptor olefins.

The spontaneous free radical copolymerization between styrenes and electrondeficient olefins were studied in order to elucidate the origin of the initiating radicals. Several suggestions are found in the literature, including electron transfer from the donor to the acceptor olefin or alternatively formation of a tetramethylene diradical. When studying the reactions between various styrenes and acceptor olefins all our results were consistent with an initial formation of a diradical.

Future plans

I. Formation and studies of radical cations.

We have shown that the combination of thallium(III) oxidants and HFP provides an excellent method for the generation of radical cations. Future plans in this project include the generation and characterization of new radical cations, especially of the non-alternant type. This will make possible the comparison of experimentally determined spin densities and calculated ones using high level ab initio theory.

We know that the stability of the radical cations can be explained by the strongly attenuated reactivity of nucleophiles in HFP. What we do not know for certain, is how the thallium(III) oxidants bring about the electron transfer. The method works for aromatic compounds both with and without nuclear positions. The reaction presumably occurs via thalliation followed by homolytic cleavage of the organo-thallium compound. This will then give a strongly oxidizing thallium(II) intermediate. More work needs to be done in this project to further strengthen the proposed electron transfer mechanism.

A similar project deals with the oxidizing properties of trifluroacetic acid (TFA), another commonly used solvent for the generation of radical cations. It has been suggested that TFA by itself can act as an oxidant forming trifluoroacetaldehyde. However, this reaction is not very likely considering the electrochemical properties of carboxylic acids. A more probable explanation is that dissolved dioxygen is the true oxidant. We plan to further investigate the oxidations claimed to be caused by TFA, using 19F-NMR spectroscopy and an oxygen-free atmosphere.

It will also be of great interest to investigate the reactions of radical cations with nucleophiles. These studies will provide experimental results that can be used to test the results from theoretical calculations of charge and spin densities of the radical cations. We also aim to study the reactions between radical cations and neutral radicals such as TEMPO and nitrogenoxide. This is an area hitherto almost unexplored except for the reaction between radical cations and nitrogen dioxide. Perhaps these studies also will reveal reactions of synthetic value due to different regiochemistry in the products compared to products formed in polar reactions.

II. Non-conventional mechanisms for the generation of spin adducts.

We have previously shown that the compounds used as spin traps are reactive towards nucleophiles and may give spin adducts via non-conventional mechanisms. These studies will be continued with emphasis on the mechanism of formation of boron-, phosphorous- and nitrogen-centered spin adducts. The results will be of particular interest since biochemical studies have shown that easily identified N-centered spin adducts form during the metabolism of certain drugs (for instance 5,5-diphenyl-hydantoin). We plan to elucidate whether these studies are representatives of proper spin trapping or whether a non-conventional mechanism for generating spin adducts may explain the results.

2. Sockerarter i tardigrader

Tardigrader (björndjur) har en fantastisk förmåga att överleva vid ogynnsamma betingelser såsom torka och kyla. Detta tros vara möjligt genom att uppbyggnaden av djurens cellmembran förändras genom att vattenmolekyler ersätts med olika sockermolekyler, bl.a. trehalos. På så sätt förstörs inte cellmembranen och därmed kan djuren under lång tid befinna sig i ett vilande tillstånd tills livsbetingelserna förbättras. Fenomenet tros kunna vara av stort intresse för att ge exempelvis förbättrade möjligheter vid organtransplantation.
I samband med ett examensarbete har vi utarbetat en ny och enklare metod för extraktion av sockerarterna hos tardigrader. Vi har hittills kunnat visa att trehalos inte är någon universell skyddsmolekyl som används av alla tardigrader. I den pågående studien jämför vi kolhydrat-innehållet i arter från olika klasser av tardigrader. Projektet utförs i samarbete med docent Ingemar Jönsson.

Publikationer