Indirect detection of selenium-77, tin-119, and tellurium-125 by nuclear magnetic resonance spectroscopy

Abstract

Nuclear magnetic resonance experiments were performed on a series of selenium model compounds, tellurium model compounds, a tin model compound, a selenium metabolite that is an excretory product, and a selenoprotein. ¹H-{⁷⁷Se}, ¹H-{¹¹⁹Sn},¹H-{¹²⁵Te}, and ¹H-{¹²³Te} heteronuclear multiple quantum coherence experiments were performed for the first time on these compounds. In all cases the use of indirect detection substantially increased the sensitivity of observing these nuclei. Coupling constants between 9.4 and 54.2 Hz were successful on selenium compounds and coupling constants between 14.3 and 102.5 Hz were successful on model tellurium compounds. The increase in sensitivity for the observation of selenium compounds was 68 which is close to the theoretical value of 73 and for the observation of tellurium the increase in sensitivity was 46 which is close to the theoretical value of 50.7. These large gains in sensitivity allowed the detection of selenium present in trimethylselenonium iodide at levels below 1 mM. This could conceivably allow this methodology to be adopted as a non-invasive method for the detection of this selenium metabolite and as a means of measuring selenium toxicity levels in blood or urine. Indirect ¹H-{⁷⁷Se} HMQC experiments were also successful on protein A from the glycine reductase complex of Clostridium sticklandii. In addition to enhancing the sensitivity of detecting selenium in this protein, these indirect experiments greatly simplify the spectrum so that only protons that are scalar coupled to selenium are seen in the NMR spectrum. Work on the tin compound had the same aim but it involves using tin as a filter and then performing a NOESY experiment. In the appendices it is shown how selenium NMR was used to identify the presence of a selenite ester in a long chain fatty acid. A lanthanide shift experiment aided the assignment of both the major and minor diastereomer of the selenite ester. The design of a triple resonance box is shown. Finally, electrochemical oxidation data on selected dithiins, thiatellurins, selenothiins, dithiiranes, and thiophene derivatives are presented.