N N
R'
R CuI
HO R'
R O
O Ph
H O2
N N
R'
R CuII
O R'
R O
N N
R'
R CuII
O R'
R O
O Ph
PhCHO H2O2
N N
R'
R CuII
O R'
R O
O OH PhCH2OH
(R' = tBu, SPh; R = tBu) PhCH2O– O2 (1 atm)
rate-determining step
Figure 12 Proposed mechanism for catalytic alcohol oxidation with concomitant evolution of H2O2using the system based on (19).86
S Cu O
O O Cu S
O
Cl2
RCH2OH
S Cu O
O O Cu S
O
H
H Cl2
–RCHO
S Cu O
O O Cu S
O
O H H
H R
S Cu O
O O Cu S
O
O H
H R
H
Cl2
Cl2 (24)
(tBu substituents not shown) + O2
– H2O2
rate-determining step
Figure 13 Proposed mechanism for the catalytic oxidation of primary alcohols using (24).92
N
O O
CuII NEt3
RCH2OH
N
O O
CuI NEt3 (H)
N
O O
CuII NEt3 O
O
(H) (H)
O2
(H) (H)
N
O O
CuII NEt3 O
H R
H
(H) N
O O
CuII NEt3 O
H R
(H) (28)
(29)
– RCHO – H2O2
Figure 14 Proposed mechanism for the catalytic oxidation of alcohols using (28).95
8.26.3.1 Protein Structure and Properties
The class I RNRs exist as homodimers, with the R1 protein being responsible for nucleotide reduction and the R2 protein housing the diiron-tyrosyl array. The tyrosyl radical in R2 (Y122 in theE. coli enzyme) has been characterized through EPR and ENDOR spectroscopy studies, which have led to a detailed description of its spin density distribution.102–104This distribution is similar to those deter- mined for other tyrosyl radicals in proteins (e.g., YD and YZ in photosystem II (see Chapter 8.20) despite differences in their environments and functions. Insight into the structure of the nearby diiron cluster was obtained primarily through studies of the met form, in which the tyrosyl radical is reduced but the metal atoms are at the FeIIIoxidation level. Combined Mo¨ssbauer,105,106resonance Raman,107 and magneticsusceptibility data108indicate an antiferromagnetically coupled (-oxo)(-carboxylato)- diiron (III) unit (J=108 cm1), which was further defined by X-ray crystallography for enzymes from several organisms (Figure 15).109–112 The structure of the diiron portion is similar to that identified for other nonheme diiron enzymes, as discussed in Chapter 8.13. Importantly, the oxygen atom of tyrosine Y122 is located 5.3 A˚ from one iron atom in theE. coli enzyme;109,110 a longer distance of7 A˚ with an intervening water molecule was identified in the enzymes from other organisms.111,112 These distances are presumed to be similar in the diiron-tyrosyl radical form, a notion corroborated by EPR relaxation and line broadening data that indicate magnetic exchange interactions between the diiron center and the radical.104,113This diiron-tyrosyl radical form is generated by the reaction of the diiron(II) site with O2,114 an intriguing process that involves unusual oxidized intermediates (including a FeIIIFeIVspecies) that have been the subject of intense study (see Chapter 8.13). The tyrosyl radical that is produced in this process then functions to generate a cysteinyl radical 35 A˚ distant in the R1 protein, where nucleotide reduction then ensues. The precise pathway by which this catalytically important R1 radical is created continues to be investigated.1,115,116
8.26.3.2 Model Complexes
A number of ligands comprising protected phenols, exemplified by (30)117and (31),118have been complexed to metal ions and oxidized in attempts to generate compounds with uncoordinated phenoxyl radicals. In most cases, UV-vis, IR, and/or EPR spectroscopic evidence was provided in support of phenoxyl radical formation, but in few cases were the compounds isolated as pure solids, and no X-ray structural data is available. A survey of these systems has been published.119
In efforts more directly aimed at modeling the diiron-tyrosyl radical array in RNR, the stable radical ligand (32) was prepared and its coordination chemistry probed.120,121 Reaction of (32) with [Fe2O(XDK)(MeOH)5(H2O)](NO3)2yielded a crystalline product formulated as (33) on the basis of elemental analysis, UV-vis, resonance Raman, magnetic susceptibility, and EPR data.
Notably, the presence of the (-oxo)(-carboxylato)diiron(III) unit and the phenoxyl radicals in (33) were indicated by features at 524 cm1and 1504 cm1in the Raman spectrum that are due to the FeOFe and7amodes, respectively. SQUID and EPR saturation-recovery data (g=2.00 signal) showed that the high spin FeIII ions are antiferromagnetically coupled (J=115 cm1),
Y122
N E238
N N
N Fe Fe
O O
O
O O O
O
O O
O O
O H118
D84 E115
E204 H241
Figure 15 View of the diiron-tyrosine unit in the met form of the class I RNR fromE. coli(pdb 1RIB).110
and that at temperatures less than 40 K the phenoxyl radicals exhibit Curie behavior. A comparison of the spin-lattice relaxation behavior of the radical EPR signal in the temperature range 6.7–115 K with that of a ZnII reference complex indicated little magnetic interaction between the radical and the diferric center, but enhanced relaxation of the diiron complex was observed at higher temperatures that was attributed to population of the S=1 excited state of the FeOFe unit.
Despite many important similarities between (33) and the diiron-tyrosyl radical array in RNR,
‘‘there is no significant exchange interaction due to spatial overlap between the phenoxyl radical and the diferriccenter’’ in (33),120consistent with a greater distance between these units (estimated to be13.5 A˚) than in the enzyme.
Me
O N N
N N
(32)
Fe O O N O2NO
N O
Fe O
ONO2
N
O N
N O
O O
O
N O
O O O
XDK N N
XDK =
= (32) (33)