Linkage isomers binding

The binding mode of uracils and thymines in neutral and deprotonated forms has been reviewed up to 1987 [13]. They coordinate hard, and relatively few soft metal ions, through 0(4) (preferentially) and 0(2). Uracil (thymine) behaves as a weak dibasic acid in alkaline media with the more basic site N(3) at pKa 9.69 (10.16), as compared to N(l) at pKa 14.2. At high pH the monoanions of uracil and thymine bind the metal ions preferentially via N(l). However, the N(3) linkage isomer of the Ptn complex has also been obtained [24]. The relatively few examples of complexes with soft metal ions, containing monodentate uracilate anions, are due to the high tendency of the ligand to bind additional metal ions to form polynuclear species [13]. 

As part of the work on model heme FeNO complexes, mechanistic studies on the reversible binding of nitric oxide to metmyoglobin and water soluble Fe, Co and Fe porphyrin complexes in aqueous solution, ligand-promoted rapid NO or NO2 dissociation from Fe porphyrins, reductive nitrosylation of water-soluble iron porphyrins, activation of nitrite ions to carry out O-atom transfer by Fe porphyrins, demonstration of the role of scission of the proximal histidine-iron bond in the activation of soluble guanylyl cyclase through metalloporphyrin substitution studies, reactions of peroxynitrite with iron porphyrins, and the first observation of photoinduced nitrosyl linkage isomers of FeNO heme complexes have been reported. 

In addition to the most commonly found end-on M-NO binding mode, we consider the existence of stable linkage isomers generated through reversible photoinduced (and eventually thermal) transformations.15 Photoreactivity may also lead to the irreversible delivery of NO from MNO " complexes.16... 

It is the nature of the histidine binding to Cu that has intrigued so many chemists and involved so many spectroscopic techniques CD, ESR, IR, H and C NMR and UV/visible. The general consensus of these studies is that the protonated form has the structure (9) and that the neutral species is a mixture of linkage isomers in facile equilibrium with each other ( heme 1). The enigma is whether or not axial coordination (pseudo-tridentate behaviour) occurs in aqueous solution. If it does occur then, for L-His, franj-CuN4 and ciy-CuN30 would be less likely to form. The solid-state structures are of little help for when His is tridentate, as in [Cu(L-HisO)(L-ThrO)] and [Cu(l-... 

Since nitro can bind as NO, or ONO, two linkage isomers are possible, as well as one ionization isomer ... 

Once again, the trans cr-bonding alkyl or aryl ligand exerts a powerful influence on the M-N-O bond angle. Studies on transient intermediates during photolysis of Ru-NO nitrosyls have revealed different modes of binding (and dissociation) of coordinated NO at the ruthenium centers. Metastable NO linkage isomers have been observed for MNO (M = Fe, Ru, Os) and for MNO ° complexes of Ni, as well as for FeNO iron nitrosyl porphyrins [208-212]. 

Certain ligands can bind to metal ions in more than one way. One example of such a ligand is nitro, — N02, and nitrito, —ONO .The donor atom is written first in the chemical formula. An example of linkage isomers is shown on page 1001. 

---