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, [FeIII(OEP)(Ac-LcXAF-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, 1H 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 [GaIII(OEP)(Ac-LcLAF-LLLLL-ALFL-OMe)] (3) to be both 49%, though the thiolate anion (Et4N){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 Sγ(Cys2). However, the helical percentages of 2 (59%) and [GaIII(OEP)(Ac-LcPAF-LLLLL-ALFL-OMe)] (4) (57%) are similar to that of (Et4N){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 [GaIII(OEP)(Z-Cys-Pro-Ala-Leu-OMe)] (2.58 Å) and [GaIII(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 FeIII/FeII for 1 and 2 are −0.54 and −0.55 V (vs SCE in CH2Cl2), respectively. The values are more positively shifted from those of [FeIII(OEP)(Z-Cys-Leu-Gly-OMe)] (−0.61 V vs SCE) and [FeIII(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.