The photochemistry and photobiology of vitamin B12

Abstract

Biologically active derivatives of vitamin B are organometallic cobalt-corrinoid complexes crucial for the healthy function of humans, animals and microbes. Their role as cofactors to numerous, thermally-driven metabolic enzymes is well described, and varies depending on the nature of the upper axial ligand. This ligand also significantly influences the photophysics and photochemistry of B. In this Perspective I will discuss the various aspects of B photochemistry, from its dynamic spin chemistry to the considerable potential it has for biotechnology applications. Although for many years this photochemistry was thought to have no biological function, in recent years it has become apparent that B photochemistry at very least has a role in light-dependent bacterial transcriptional regulation. I will provide an overview of what has been reported about B photobiology to date, with particular emphasis on the mechanism of the transcriptional repressor, CarH, the subject of my Young Investigator Award Plenary Lecture at the European Society for Photobiology Congress 2015 in Aveiro, Portugal.

The Elusive 5′-Deoxyadenosyl Radical: Captured and Characterized by Electron Paramagnetic Resonance and Electron Nuclear Double Resonance Spectroscopies

“…4–6 Important insights into its behavior have been attained through kinetic and spectroscopic investigations in which the Co–C5′ bond of B 12 is photolytically cleaved, after which the 5′-dAdo· radical can either reform that bond or proceed to react with substrate. 7,8 However, strenuous efforts over many years 9 to trap and fully characterize the 5′-dAdo· formed from B 12 have been unavailing: the radical has simply been too reactive to trap.…”

Interplay between chromophore binding and domain assembly by the B12-dependent photoreceptor protein, CarH

“…Although the photochemistry of B 12 species has been studied for decades, its role in photobiology is only now becoming apparent. 4 The discovery of the bacterial transcriptional regulator, CarH, 5,6 has brought forth a new area of photobiology based on B 12 as a photoactive chromophore. 7 AdoCbl photochemistry in CarH results in transcriptional activation in bacteria, which leads to the biosynthesis of carotenoids in response to photo-oxidative stress.…”

“…7 AdoCbl photochemistry in CarH results in transcriptional activation in bacteria, which leads to the biosynthesis of carotenoids in response to photo-oxidative stress. Mechanistic details about its function are now beginning to emerge 4,[6][7][8][9][10][11] and it is already showing great promise and versatility as the basis of photoactivated, biomolecular tools. [12][13][14][15][16] The size and structural complexity of AdoCbl reect a long and expensive biosynthetic pathway 17 and mean that its uptake 18 and subsequent binding to its dependent enzymes 19 are oen tightly regulated.…”

Ultrafast Excited State Dynamics and Fluorescence from Vitamin B12 and Organometallic [Co]–C≡C–R Cobalamins

“… 8 The transitions between these states have been the subject of many different experimental and theoretical studies. 8 − 10 …”

“… 19 − 21 These Cbls are also light sensitive and undergo photoinduced homolysis of their Co–C bond. 8 − 10 The unique Co–C bond of organometallic Cbls also provides space for chemical manipulation of both their thermal and photochemical reactivity. 13 , 22 − 24 Strong Co–C bonds and their photochemical stability are exploited in a range of alkynyl cobalamins 23 , 25 , 26 designed as potential antivitamins B 12 .…”