Insight into the mechanism of CO-release from trypto-CORM using ultra-fast spectroscopy and computational chemistry

Photolysis of trypto-CORM, fac-[Mn(tryp)(CO)₃(NCMe)] (tryp = tryptophanate) at 400 nm results in controlled CO-release which may be utilised to inhibit the growth of Escherichia coli (E. coli). An investigation into the fundamental processes which underpin the CO-release event is described. Time-dependent density functional theory (TD-DFT) indicates that irradiation at 400 nm results in LMCT from the indole group of the amino acid to orbitals based on the metal as well as the carbonyl and NCMe ligands. Ultra-fast time-resolved infra-red spectroscopy (TRIR) demonstrates that in NCMe solution, photolysis (400 nm) results in loss of CO in under 3 ps with the sequential generation of three new states with two carbonyl ligands and a coordinated tryptophanate. The first species is assigned to vibrationally hot ³[Mn(tryp)(CO)₂(NCMe)] which undergoes cooling to give the complex in its v = 0 state. This triplet state then undergoes solvation (τ≈ 20 ps) with a concomitant change in spin to give [Mn(tryp)(CO)₂(NCMe)₂] which persists for the remainder of the experiment (800 μs). These data indicate that following the initial photochemically induced loss of CO, any thermal CO loss is much slower. Related experiments with trypto-CORM in a mixture of DMSO and D₂O gave analogous data, indicating that this process also occurs in the medium used for the evaluation of biological properties.