Optimal Size Matching and Minimal Distortion Energy: Implications for Natural Selection by the Macrocycle of the Iron Species in Heme

Abstract

In heme, a porphyrin macrocycle naturally selects an iron ionic species. It was found that this natural combination is directly related to the geometry sizes of both components. Three series of monostrapped nonplanar metalloporphyrins [M = FeIIICl, CoII, NiII] and their metal‐free counterparts were synthesized as model systems, their core sizes were compared, and density functional theory computations were used to calculate the molecular distortion energies. The results indicate that optimal size matching of both is to maintain minimal distortion energy of the macrocycle. This shows that the mutual selection process between the macrocycle and the metal ion is mainly based on the principle of minimum energy to tune the electronic structure of the central metal ion; this may be a common principle in natural tetrapyrroles containing metal species. The structural parameters of all the model compounds were directly obtained from their crystal structures.

Selective Excited-State Dynamics in a Unique Set of Rationally Designed Ni Porphyrins

“…As shown in Scheme , the length of the alkyl group was varied to include 4 C atoms (L4), 6 C atoms (L6), and 8 C atoms (L8). The synthetic details and characterization of permanently distorted Ni porphyrins (L4 and L6) have been reported previously . L8 was prepared in this work to further decrease the degree of distortion, and its synthesis is outlined in the Experimental Section.…”

“…Strapped nickel porphyrin L8-Ni was prepared according to previously published standard methods . 2,2′-[Octane-1,8-diylbis­(oxy)]­di­(5- tert -butyl)­benzaldehyde (4.66 g, 10 mmol) and bis­(2-pyrrolyl)-4-nitrophenyl-methane (5.34 g, 20 mmol) were added to a boiling propionic/butyric (2:1, v/v) acid mixture (200 mL) in small portions within 5 min.…”

“…Because the excitation pulse does not excite all of the molecules in the Xray beam path, the laser-on spectrum is actually a combination The synthetic details and characterization of permanently distorted Ni porphyrins (L4 and L6) have been reported previously. 33 L8 was prepared in this work to further decrease the degree of distortion, and its synthesis is outlined in the Experimental Section. The ruffling effect should be more pronounced as the chain length reduces from L8 to L4.…”

“…As a result, it is highly needed to have a permanently locked model system for which the distortion is known to be one of the fixed distortions such that the effects of the specific distortion on the chemistry and photochemical behavior can be unambiguously determined and distinguished from the effects of other types of distortions. In this work, we report the design and photophysical studies of a unique class of Ni porphyrins (Scheme ), in which the meso positions of the macrocycle were tethered by ethers that are attached to the t -butyl benzene substitutes . Using advanced time-resolved optical and X-ray absorption spectroscopy (XAS), we observed tunability in the ES relaxation pathway dependent on the degree of distortion and also determined the associated transient intermediate structure.…”

“…In this work, we report the design and photophysical studies of a unique class of Ni porphyrins (Scheme 1), in which the meso positions of the macrocycle were tethered by ethers that are attached to the t-butyl benzene substitutes. 33 Using advanced time-resolved optical and X-ray absorption spectroscopy (XAS), we observed tunability in the ES relaxation pathway dependent on the degree of distortion and also determined the associated transient intermediate structure.…”