Carnosine (Carnos), an histidine-containing dipeptide present in relatively high concentration in muscle and organs such as kidney and pancreas of many vertebrates, is capable of forming complexes with transition metals, such as Cu(II), Zn(II), etc [1-2]. In particular, the complexes with Co(II) ions bind molecular oxygen and may act as oxygen carries [3]. A Raman and IR study of Carnos and its complexes with Co(II) at different metal/ligand ratio (M/L) and pH 9 was carried out and some conclusions can be drawn on the structure of the species existing in these systems. The manner in which Carnos binds to the Co(II) ion depends on the relative concentration of the metal ion and the ligand present in the system. When M/L is ? 1, oxygen is bonded in the complex as a peroxide linkage between two chelated Co(II) ions and two binuclear peroxo complexes (monobridged and dibridged forms) were identified by the presence of two Raman peaks in the ?O-O stretching region (750-850 cm-1) [4]. The highest predominance of the peroxo species was found when M/L was 0.5; a further excess of Carnos slightly competes with oxygen for the coordination position of the Co(II) ion. Oxidation of peroxo to superoxo complexes takes place when no sufficient ligand is available (M/L 2) and shifts the ?O-O band to higher wavenumbers (? 1080 cm-1) [4]. In addition, under these experimental conditions a species not carrying oxygen, possibly a Co(III) chelate, is also formed. With respect to the coordination sites, each Co(II) ion of the binuclear dioxygenated complexes is bound to one oxygen atom and four nitrogens: N? and N? of two Carnos molecules, the peptide and the terminal amino nitrogen atoms. Conversely, when M/L is 2, in addition to the dioxygenated complexes, a complex is formed where the imidazole moiety of Carnos binds two different cobalt ions since both N? and N? nitrogens are deprotonated. In conclusion, both the Raman and IR spectra present marker bands useful for the identification of the predominant complex existing in the Co(II)-Carnos systems. In particular, the histidyl and oxygen Raman marker bands found in this study may be useful in investigating metal-histidine interactions and bonding of O2 to metal centres in oxygen-carrying proteins.
A vibrational investigation of the Co(II)-Carnosine oxygenated complexes at basic pH
A Torreggiani;
2001
Abstract
Carnosine (Carnos), an histidine-containing dipeptide present in relatively high concentration in muscle and organs such as kidney and pancreas of many vertebrates, is capable of forming complexes with transition metals, such as Cu(II), Zn(II), etc [1-2]. In particular, the complexes with Co(II) ions bind molecular oxygen and may act as oxygen carries [3]. A Raman and IR study of Carnos and its complexes with Co(II) at different metal/ligand ratio (M/L) and pH 9 was carried out and some conclusions can be drawn on the structure of the species existing in these systems. The manner in which Carnos binds to the Co(II) ion depends on the relative concentration of the metal ion and the ligand present in the system. When M/L is ? 1, oxygen is bonded in the complex as a peroxide linkage between two chelated Co(II) ions and two binuclear peroxo complexes (monobridged and dibridged forms) were identified by the presence of two Raman peaks in the ?O-O stretching region (750-850 cm-1) [4]. The highest predominance of the peroxo species was found when M/L was 0.5; a further excess of Carnos slightly competes with oxygen for the coordination position of the Co(II) ion. Oxidation of peroxo to superoxo complexes takes place when no sufficient ligand is available (M/L 2) and shifts the ?O-O band to higher wavenumbers (? 1080 cm-1) [4]. In addition, under these experimental conditions a species not carrying oxygen, possibly a Co(III) chelate, is also formed. With respect to the coordination sites, each Co(II) ion of the binuclear dioxygenated complexes is bound to one oxygen atom and four nitrogens: N? and N? of two Carnos molecules, the peptide and the terminal amino nitrogen atoms. Conversely, when M/L is 2, in addition to the dioxygenated complexes, a complex is formed where the imidazole moiety of Carnos binds two different cobalt ions since both N? and N? nitrogens are deprotonated. In conclusion, both the Raman and IR spectra present marker bands useful for the identification of the predominant complex existing in the Co(II)-Carnos systems. In particular, the histidyl and oxygen Raman marker bands found in this study may be useful in investigating metal-histidine interactions and bonding of O2 to metal centres in oxygen-carrying proteins.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
