Development of Variable Magnetic Field Instrumentation for Transient Absorption Measurements from Femtoseconds to Seconds for Chemical and Biochemical Studies

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Abstract

Cobalamin, also known as vitamin B12 or adenosylcobalamin (AdoCbl) is the largest and the most complex of all types of vitamins and the only one containing cobalt. It plays a key role in normal functioning of the brain and nervous system and is necessary in formation of blood. All biologically active forms of B12 have an unusually labile C-Co to the upper axial ligand, which has bond dissociation energy of 31 kcal/mol (for 5’-deoxyadenosylcobalamin, AdoCbl). Absorption of light below 610 nm will induce homolysis of the C-Co bond to produce a spin-correlated, geminate radical pair (RP). Calculations based on the average hyperfine couplings in the AdoCH2⋅CblII RP suggest that the spin state mixing for a separated pair is likely to be rapid (a period of ~430 ps). One might also expect fast relaxation processes, due to the presence of heavy transition metal, although none were needed to model the saturation of the Zeeman effect by magnetic fields in AdoCbl. To more fully investigate these phenomena we have developed novel transient absorption (TA) technique that allowed investigations of magnetic perturbation of the cobalamin photoresponse from femtoseconds to seconds. To provide a wide range of magnetic field (MF) strength values, powerful neodymium disc magnets as well as Halbach array were employed. Data of both, computer simulations and measurements is presented. A previously published MFE in a Flavin Adenine Dinucleotide (FAD) intramolecular electron transfer reaction was chosen as a biological test system. The field-dependence was not only reproduced, but also extended by measurements covering a range of MFs up to 1 T. A newly developed technique was used for TA MFE studies with both free AdoCbl and AdoCbl-dependent ethanolamine ammonia lyase (EAL) on ultrafast timescale. The new optical setup allowed for MF induced changed to be measured directly. By employing lock-in amplifier even very low signals, which in standard techniques are usually overwhelmed by ever present noise, could be detected. Technical drawings, designs of magnet holder, double capillary flow cell and the optical table layout are all discussed. Data from all experimental chemical systems is presented.

Keyword(s)

magnetic field effects; radical pair mechanism; radicals; transient absorption; ultrafast

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