Ambient Temperature MCD of Metalloproteins

Approximately one in three proteins contains metal, probably the best known example being hemoglobin which binds the transition metal iron as a heme and is responsible for the red colour of blood. Generally a protein will contain one or more transition metals when its task is to pass electrons between other proteins or when its substrate is a small inorganic molecule. One example of a group of these small inorganic molecules is provided by the chemicals which are interconverted in the global nitrogen cycle. These includes the well known series of nitrate, nitrite, nitric oxide, nitrous oxide, nitrogen and ammonia. The dioxygen molecule which we breath in to live is another example. We may be interested in proteins extracted from soil bacteria which interconvert chemicals of the nitrogen cycle or we may study the cytochrome oxidase protein which binds the oxygen we breath and reduces it to water using electrons from our food. But in any case where a transition metal is involved we need specialised techniques to investigate how they work. One method to probe the nature of a metal site in a protein is to measure the absorption of light using electronic absorption spectroscopy. Obviously we can do this because the metals make the protein coloured. But the metals can also make the protein magnetic. If they are coloured AND magnetic then this allows us to use a technique called Magnetic Circular Dichroism spectroscopy, discovered by the British chemist Michael Faraday in 1845. This is similar to absorption spectroscopy but uses special polarised light and a strong magnetic field. In order to produce the strong magnetic field we use a superconducting magnet.

The strong magnetic field required for MCD spectroscopy is currently provided by two different Oxford Instruments SpectroMag type systems. In the 'LT' magnet, a 5Tesla split-coil SM4 system, the helium serves to cool the superconducting coils AND cool the sample down to temperatures of ~1.6K. The second, a 6Tesla 'RT' magnet, has a room temperature externally accessible bore in which the sample is placed for ambient temperature measurements. In this case, the liquid helium is used only to cool the coils. The complementary use of both systems is necessary for a complete investigation of metalloprotein samples and the RT magnet is also essential for the combined MCD/electrochemistry experiments (MOTTLE). Our current RT magnet is over twenty years old and has recently failed. We are therefore seeking funds for a replacement which will be of a slightly modified design in order to take account of more recently developed applications such as MOTTLE.