Multiheme proteins: effect of heme–heme interactions

Abstract

The family of multiheme proteins constitutes one of the fascinating molecular machineries designed by Nature to execute a large variety of functions. A high level of conservation among the structural arrangement of heme units is evident among various multiheme cytochromes. The relative arrangement of the heme centers and the intermacrocyclic interactions therein have been found to exhibit a major role in functional properties of such a widely distributed family. The existence of more than one heme center provides an effective and efficient tool to modulate various structures and properties that are needed for its function. This Frontier overviews a brief account of our on-going efforts to examine some of the design principles in which the inter-heme distance and their relative orientations are appropriately chosen to elucidate, at the molecular level, the effects of heme-heme interactions and electronic communication in the synthetic dihemes.

Intermacrocyclic Interactions upon Stepwise Oxidations in a Monometallic Porphyrin Dimer: Ring versus Metal‐Center Oxidations and Effect of Counter Anions

“…A tightly associated counterion (observed in the X‐ray structure of the complex) will have a significant steric effect as a consequence of its close proximity which results in a large distortion on the ring. The present investigation clearly demonstrates the effect of large counter ions and are consistent with previous studies [7c,d, 19] …”

“…This suggests that the PF 6 counter ion in mono ‐1⋅ (PF 6 ) 2 remains as a tightly associated ion‐pair as observed earlier [9c,d] and, thus, behaving differently from the free ion. The present investigation clearly display the role of counter ions and are consistent with our previous studies [7c,d, 9c,d] …”

“…Counter ion such as SbF 6 forms strong H‐bonding interaction with NH of the porphyrin core and causes a significant distortion in the porphyrin core (as observed in the X‐ray structure). It is also possible that the solid‐state interactions can be retained in solution [7c,d, 9c,d, 19] . In support of such a proposal, 19 F NMR has been performed for mono ‐1⋅ (PF 6 ) 2 and mono ‐1⋅ (SbF 6 ) 2 .…”

“…Two interactions mostly applicable here are: (i) H‐bonding interaction between the porphyrinic NH and electronegative atom X of counter anions and (ii) ion‐pair interaction between counter anion and porphyrin π‐cation radical. These interactions distort the porphyrin ring while the degree of such distortion depends on the nature of counter ions [7c,d, 19] which is also reflected in the absorption spectra [20] . As observed in the X‐ray structure of mono ‐1⋅ (SbF 6 ) 2 , SbF 6 ion is engaged in H‐bonding interaction and also polarize the porphyrin ring (ion‐pair interaction) and thereby red‐shifted the Soret band with increase in intensity [20] .…”

Controlling the Photophysics of Aromatic Guests Using a Cyclic Porphyrin Dimer: Synthesis, Structure, and Encapsulation‐Mediated “ON‐OFF” Switch

“…As a consequence, various properties of the isolated molecules are altered, which lead towards potential application in different areas such as, catalysis, selective recognition, ion transport etc . [1c], In this regard, the design of artificial container incorporating porphyrin dimer has enticed a great deal of attention owing to its cleft‐like arrangement, which can bind and activate a variety of substrates . Such bisporphyrinic containers usually provide a way to encapsulate different guests in their cavity, which eventually modulate the molecular properties of the encapsulated guests.…”