Role of α-Helix Conformation Cooperating with NH···S Hydrogen Bond in the Active Site of Cytochrome P-450 and Chloroperoxidase: Synthesis and Properties of [MIII(OEP)(Cys-Helical Peptide)] (M = Fe and Ga)

Heme-thiolate protein model complexes, [Fe III (OEP)(Ac-L c XAF-LLLLL-ALFL-OMe)] {OEP = octhaethylporphinato, X = Leu ( 1 ) and Pro ( 2 )}, having α-helical structure were synthesized and characterized as P-450 and CPO models. The stable ligation of cysteinyl thiolate at the axial position of model complexes was established using MCD, UV−visible, 1 H NMR, and ESI-MS. These results indicate that 1 and 2 possess high-spin 5-coordinated Fe(III) ions. The helical contents were examined by CD measurements in THF which revealed the contents of 1 and [Ga III (OEP)(Ac-L c LAF-LLLLL-ALFL-OMe)] ( 3 ) to be both 49%, though the thiolate anion (Et 4 N){Ac-LC(S - )LAF-LLLLL-ALFL-OMe} ( 5 ) has the value of 57%. Thus, the α-helical NH···O C hydrogen bond network is intercepted by the NH···S hydrogen bond between NH(Leu3) and S γ (Cys2). However, the helical percentages of 2 (59%) and [Ga III (OEP)(Ac-L c PAF-LLLLL-ALFL-OMe)] ( 4 ) (57%) are similar to that of (Et 4 N){Ac-LC(S - )LAF-LLLLL-ALFL-OMe} ( 6 ) (52%). The NMR analysis of the solution structure of the diamagnetic Ga(III) complex 4 indicates the distances between CysS γ and NH of the third residue to be 2.37 Å, which is shorter than those of [Ga III (OEP)(Z- Cys -Pro-Ala-Leu-OMe)] (2.58 Å) and [Ga III (OEP)(Z- Cys -Pro-Leu-OMe)] (3.00 Å). These results show that the NH···S hydrogen bond of 4 is shortened by α-helix dipole and then the α-helix is stabilized. The redox potentials of Fe III /Fe II for 1 and 2 are −0.54 and −0.55 V (vs SCE in CH 2 Cl 2 ), respectively. The values are more positively shifted from those of [Fe III (OEP)(Z- Cys -Leu-Gly-OMe)] (−0.61 V vs SCE) and [Fe III (OEP)(Z- Cys -Pro-Leu-OMe)] (−0.68 V vs SCE). Complex 1 indicates the positive shift of only 70 mV because the Cys-Leu-Ala fragment on the N terminus breaks the α-helical conformation. In contrast, complex 2 has larger positive shift (130 mV) by the α-helical conformation cooperating with the NH···S hydrogen bond. These data suggest that the redox reactions are regulated by the cooperating effect of the α-helix and the NH···S hydrogen bonds in the native enzymes.