Koji KANO and Hiroaki KITAGISHI (Doshisha University, Kyoto, Japan) Cyclodextrin Dimers as Simple Myoglobin Models in Aqueous Solution

Carrier of Diatomic Molecules

 Myoglobin (Mb) is an oxygen-storage hemoprotein.  Oxygen bound to Mb is stabilized by two His residues.  Heme center is surrounded by a hydrophobic wall of the protein. His 64 (distal His) His 93 (proximal His)

A picket-fence Por prepared by Prof. Collman’s group Collman, J. P.; Boulatov, R.; Sunderland, C. J.; Fu. L. Chem. Rev. 2004, 104, Jameson, G. B.; Rodley, G. A.; Robinson, W. T.; Gagne, R. R.; Reed, C. A.; Collman, J. P. Inorg. Chem. 1978, 17,

 Many artificial dioxygen receptors have been prepared.  These model compounds can bind dioxygen only in absolute organic solvents such as toluene.  Dioxygen adducts are easily autoxidized in the presence of a trace amount of water.  Many artificial dioxygen receptors have been prepared.  These model compounds can bind dioxygen only in absolute organic solvents such as toluene.  Dioxygen adducts are easily autoxidized in the presence of a trace amount of water.

Jiang, D.-L.; Aida, T. Chem. Commun. 1996, Zingg, A.; Felber, B.; Gramlich, V.; Fu, L.; Collman, J. P.; Diederich, F. Helv. Chim. Acta 2002, 85, Dendrimers as Mb models

 Difficulty in preparation of five coordinate Fe(II)Por Why is modeling of the Mb or Hb functions so difficult?

 Easy oxidative dimerization of O 2 -Fe(II)Por yielding a  -oxo-dimer of Fe(III)Por Why is modeling of the Mb or Hb functions so difficult?

 Direct oxidation of Fe(II)Por to Fe(III)Por with O 2 Why is modeling of the Mb or Hb functions so difficult?

 Water-promoted autoxidation of O 2 -Fe(II)Por Why is modeling of the Mb or Hb functions so difficult?

The main reason why modeling in aqueous solution is so difficult.

Chem. Lett. 1996, J. Am. Chem. Soc. 2002, 124, Chem. Lett. 1996, J. Am. Chem. Soc. 2002, 124, pK a 4.8 pK a 0.4

Inorg. Chem. 2006, 45, J. Am. Chem. Soc. 2008, 130,

Synthetic route of Py2CD and Py3CD Py2CD Py3CD

Experimental procedures for examining O 2 and CO binding of Fe(II)PCD in an aqueous solution at pH 7.0.

UV-vis spectra of Fe(II)PCD, O 2 -Fe(II)PCD and CO-Fe(II)PCD in phosphate buffer at pH 7.0 and 3 o C.

O 2 affinity at 25 o C and pH 7.0 P 1/2 : hemoCD 17 Torr Fe(II)PCD 176 Torr P 1/2 : Mb (sperm whale) 0.29 Torr Hb (human R) 0.17 Hb (human T) 26 Model systems in organic solvents 0.1 ~ 2150 O 2 affinity at 25 o C and pH 7.0 P 1/2 : hemoCD 17 Torr Fe(II)PCD 176 Torr P 1/2 : Mb (sperm whale) 0.29 Torr Hb (human R) 0.17 Hb (human T) 26 Model systems in organic solvents 0.1 ~ 2150

CO affinity at 25 o C and pH 7.0 P 1/2 : hemoCD 1.5 x Torr Fe(II)PCD Torr P 1/2 : Mb (sperm whale) 0.02 Torr Hb (human R) CO affinity at 25 o C and pH 7.0 P 1/2 : hemoCD 1.5 x Torr Fe(II)PCD Torr P 1/2 : Mb (sperm whale) 0.02 Torr Hb (human R) 0.013

A conformation of Fe(II)PCD is similar to that of Mb or Hb. A conformation of hemoCD is similar to that of leghemoglobin.

Cage Effects The Fe center of FeTPPS is completely capped with the two CD cavities. O 2 as well as CO released from the Fe(II) center cannot slip out of the cleft of CD capsule because of its hydrophobic nature. Released O 2 or CO rebinds to the Fe(II) center. Fe(II)PCD

Picket-fence porphyrin

Oxy-hemoCD

Reductive nitrosylation of Fe(III)PCD and oxidation of (NO)Fe(II)PCD

Nitric Oxide NO is biosynthesized from arginine and dioxygen by nitric oxide synthases (NOS). NO causes relaxation of smooth muscle to control blood pressure. NO stimulates the soluble guanylate cyclase leading to subsequent formation of cyclic GMP. Macrophages generate NO to kill antigen. etc.

Nitrosylation of Fe(II)PCD max 420 nm

max 401 nm Reductive nitrosylation of Fe(III)P(TMe-  -CD) 2 complex

No reductive nitrosylation occurs in the absence of cyclodextrin.

max = 401 nm max = 420 nm No reductive nitrosylation

This mechanism has been well established. Oxy-Mb regulates NO in biological system. The mechanism has not been clarified. NO inhibits the activities of proteins such as cyt P450, cyt c oxidase, nitrile hydrase, and catalase.

 (NO)Fe(II)PCD is gradually oxidized to Fe(III)PCD and NO 3 - in an aerobic aqueous solution at pH 7.0,  (NO)Fe(II)P(TMe-  -CD) 2 is not oxidized at all.

In the case without cyclodextrin  In the absence of CD, reductive nitrosylation cannot be applied.  (NO)Fe(II)TPPS can be prepared from nitrosylation of Fe(II)TPPS in a glove box.  (NO)Fe(II)TPPS is very unstable in an aerobic aqueous solution. Ring- opening reaction of FeTPPS may occur.

t 1/2 = 6.3 h t 1/2 = ∞ No oxidation occurs. Decomposition of the porphyrin ring occurs.

k max : the maximum reaction rate constant for autoxidation of (NO)Fe(II)PCD k max = 5.1 x s -1 k off : the reaction rate constant for the dissociation of NO from (NO)Fe(II)PCD k off = 5.6 x s -1

Mechanism for oxidation of (NO)Fe(II)PCD with dioxygen Rate-determining step

Eyring plot for autoxidation of (NO)Fe(II)PCD.  H ‡ = 98.9 kJ mol -1  S ‡ = 0.17 J mol -1 K -1  A large activation enthalpy change reflects the endothermic dissociation of the NO-Fe(II) bond.  Since activation entropy change is almost zero, no bimolecular reaction participates in the rate- determining step. The thermodynamic parameters support the reaction mechanism proposed herein.

Oxidation of (NO)Mn(II)PCD by O 2

Autoxidation of (NO)Mn(II)PCD (NO)Mn(II) Mn(III) Zero-order kinetics was observed for the aut- oxidation of (NO)Mn(II)PCD. t 1/2 ~ 35 h

If the equilibrium A ⇌ B exclusively shifts to A and a very small amount of B existing in the system relatively rapidly reacts to yield a final product, such a reaction obeys zero-order kinetics. very slow

The rate of autoxidation of Mn(II)PCD is much faster than that of (NO)Mn(II)PCD.

Autoxidation of (NO)Mn(II)P(TMe-  -CD) 2 (NO)Mn(II) Mn(III) Mn(II)

Mechanism for Oxidation of (NO)Mn(II)PCD with Dioxygen

Mechanism for oxidation of (NO)Fe(II)PCD with dioxygen

 HemoCD and Fe(II)PCD are good carriers of simple diatomic molecules such as O 2, CO, and NO in aqueous solution.  HemoCD shows the extremely high CO affinity that might be used for medicinal purposes.  Fe(II)PCD is a good model for studying interactions of NO with hemoproteins.  The mechanism for oxidation of (NO)Fe(II)Por by O 2 was clarified in the present study.  HemoCD and Fe(II)PCD are good carriers of simple diatomic molecules such as O 2, CO, and NO in aqueous solution.  HemoCD shows the extremely high CO affinity that might be used for medicinal purposes.  Fe(II)PCD is a good model for studying interactions of NO with hemoproteins.  The mechanism for oxidation of (NO)Fe(II)Por by O 2 was clarified in the present study. Summary