Quantum Mechanical Studies of the Photodissociation of Nitrosoheme

1

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Quantum Mechanical Studies of the Photodissociation of Nitrosoheme Complexes Lek Chantranupong, Gilda H . Loew, and Ahmad Waleh Molecular Theory Laboratory, The Rockefeller University, Palo Alto, CA 94304

Initial steps in the photodissociation of nitrosyl ferrous and ferric heme complexes have been investigated by calculation of the ground and excited states and electronic spectra as a function of iron-ligand bond lengthening. The method used is an all valence semi empirical INDO-SCF-CI procedure with transition metal parameterization and the ability to calculate configura tion interaction for closed shell systems and, more recently, radical species. The results of these studies allow identification of a common photodissociating state involvingdπ->d*z2excitations for both systems, the same state found in previous studies of carbonyl and oxy ferrous heme complexes. This result is consistent with the observation of nearly equal initial photodissocia tion product for all four systems at a picosecond time scale. The low quantum yield at long time (~ 400 μs) observed for the ferrous nitrosyl complex is attributed to the population of charge transfer states with a minimum which allow trapping of the NO radical near the iron. Such states can be involved in both rapid geminate recombination or energy decay to the liganded state. The high quantum yield of the ferric nitrosyl complex is attributed to eventual photodissociation of ferric heme and NO radical on a triplet surface which does not allow population of the low energy charge transfer states also found to be present for these system. By contrast, the high quantum yield of the isoelectronic ferrous carbonyl heme complex is asso ciated with the absence of such charge transfer states. Photodissociation and recombination studies of diatomic molecules, O2, CO, and NO from d i f f e r e n t heme proteins (1-11) and liganded model heme complexes (5, 9, 12-14) have been very valuable i n understanding the dynamics of ligand binding and energy relaxation mechanisms. Much of the interest i n these studies stems p a r t i c u l a r l y from the need for a better understanding of the physiological function of 0097-6156/ 86/ 0321 -0002506.00/ 0 © 1986 A m e r i c a n C h e m i c a l Society

Gouterman et al.; Porphyrins ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

1.

CHANTRANUPONG ET AL.

Photodissociation of Nitrosoheme Complexes

3

hemoglobin which involves reversible binding of O2 at i t s heme active s i t e . Although photodissociation I t s e l f i s a nonbiological process, i t involves a series of b i o l o g i c a l l y s i g n i f i c a n t events which occur over d i f f e r e n t time scales, beginning with the metal-ligand bond breakage, leading to changes i n the e l e c t r o n i c and t e r t i a r y struc tures, and eventually to quaternary conformational changes i n the protein. It i s known that some of the early transient effects related to ligand photodissociation i n heme proteins evolve i n the pico- or subpicosecond range (14-27). Since i t i s u n l i k e l y that any spin, or t e r t i a r y - s t r u c t u r e changes would take place at such fast rates, these u l t r a f a s t events should be closely related to the primary heme-ligand bond breaking process i n i t i a t e d by the a c t i v a t i o n of the complex to a d i s s o c i a t i v e state by photon energy. I t , therefore, follows that the observed subpicosecond photodissociation properties for various ligands must be strongly dominated by the energy p r o f i l e of the excited e l e c t r o n i c states of their respective complexes during this process. Such a p r o f i l e w i l l determine the i n i t i a l d i s s o c i a t i v e y i e l d of the ligand which, i n turn, w i l l influence the subsequent events of photodissociation ranging from the evolution of the i n t e r mediate species to e l e c t r o n i c and conformational changes accompanying the ligand d i s s o c i a t i o n and/or recombination beyond the picosecond range (28-39). The primary requirement for the i n i t i a t i o n of the ligand bond breaking process i s the existence of photodlssociative states i n an appropriate energy range such that they can be populated by either direct e x c i t a t i o n into such states or through relaxation from states populated by higher energy e x c i t a t i o n s . The d i s s o c i a t i v e y i e l d , however, w i l l also depend on the existence or absence of competing mechanisms that may result i n the o v e r a l l deactivation of these d i s s o c i a t i v e states. In p a r t i c u l a r , low-lying excited states with paths for energy relaxation from higher-energy states and/or for energy decay to the ligand-bound ground state can e f f e c t i v e l y depopulate the photodlssociative states and thus reduce ligand d i s s o c i a t i o n y i e l d . It i s i n the context of these u l t r a f a s t processes that t h e o r e t i c a l investigation of the ground and excited state of complexes of various ligands with heme proteins can s i g n i f i cantly contribute to our understanding of their d i s s o c i a t i o n and recombination behaviors. We have previously reported the results of semiempirlcal INDOSCF-CI (40-43) studies of the energy p r o f i l e s of the excited states of closed-shell (S - 0) CO (44) and 0 (45) ferrous heme complexes as a function of iron-ligand distance. In these studies, the c r i t e r i o n of "decreasing excited state energy as a function of iron-ligand distance and no b a r r i e r to d i s s o c i a t i o n " was used to i d e n t i f y the photoactive states i n both complexes. These results were interpreted i n terms of a two-channel photodissociation mechanism i n oxyheme and a βingle-channel mechanism i n carbonylheme complex. It was shown that although the excited states corresponding to d d 2 were photodlssociative i n both carbonyl and oxyheme, photodissociation of oxyheme also proceeded through charge-transfer states below the Qband, which could also provide a competitive decay channel to the ground state. Consideration of the energies and l i f e t i m e s of various d i s s o c i a t i v e and decay channels then provided consistent explanations 2

n

2


4

PORPHYRINS:

E X C I T E D STATES A N D D Y N A M I C S

for the differences i n the observed picosecond photodissociation properties of the two ligands. The INDO program used i n the closed-shell oxy- and carbonylheme studies has recently been modified to include a Restricted HartreeFock (RHF) and a generalized configuration interaction (CI) treatment for c a l c u l a t i o n of the excited states of open-shell systems with unpaired electrons (46). This new c a p a b i l i t y allows us to calculate the excited state p r o f i l e s and examine the photodissociation propert i e s of heme complexes with a variety of ligands and metals i n d i f f e r e n t spin and oxidation states. Such studies are p a r t i c u l a r l y s i g n i f i c a n t as changes i n both ligand and metal can a l t e r the heme electronic structure and thus the photophysics of the primary events of ligand photodissociation. Hoffman and Gibson (5_) have shown a good c o r r e l a t i o n between quantum y i e l d of photodissociation and formal electronic configurations for a number of metals and ligands, which lead them to an o v e r a l l stereo-electronic c l a s s i f i c a t i o n of these complexes. In p a r t i c u l a r , they have observed that the Isoelectronic systems, [Fe(II)+C0], [Mn(II)+N0], and [Fe(III)+NO] a l l display ligand d i s s o c i a t i o n under photolysis with high quantum y i e l d . The f l a s h photolysis of NO and k i n e t i c s of i t s recombination i n d i f f e r e n t ferriheme proteins have also been studied i n d e t a i l (6-8). In contrast to ferriheme complexes, ferrous nitrosylhemoglobin (HbNO) shows a n e g l i g i b l y small quantum y i e l d i n microsecond f l a s h photolysis studies (4). However, recent picosecond experiments (24, 27) on HbNO show that NO ligand, s i m i l a r to CO and 0 , i s e f f i c i e n t l y photolyzed i n this time range, although i t s behavior after photod i s s o c i a t i o n i s s t r i k i n g l y d i f f e r e n t . In p a r t i c u l a r , the i n i t i a l photoproducts of HbNO have a l i f e t i m e of 17 ps and the s t a r t i n g material i s recovered within 400 ps. This has been interpreted i n terms of fast geminate recombination of NO (27). In this paper, we present the results of the calculations of the excited-state energies and characters for n i t r o s y l ferrous and f e r r i c heme complexes as a function of i r o n - n i t r o s y l distance. The calculated energy p r o f i l e s are then used to i d e n t i f y the photoactive states for each complex. Similar calculations are also underway to characterize the photoactive states of Mn(II)N0 complexes (47). 2

Method The calculations were carried out by using a seraiemplrical INDO (intermediate neglect of d i f f e r e n t i a l overlap) program (40-43) which allows for the treatment of t r a n s i t i o n metal complexes and inclusion of extensive configuration i n t e r a c t i o n s . The geometry of both n i t r o s y l f e r r i c and ferrous heme complexes were obtained from an Xray c r y s t a l structure of 6-coordinate FeTPP(NMeIm)N0 (48) with end-on NO ligand. This geometry was held constant except for variations of the Fe-NO distance along the axis normal to the plane of porphyrin. While an X-ray structure has been reported for a 5-coordinate f e r r i c heme NO complex (49), with a linear NO geometry, the bent geometry of the 6-coordinate ferrous complex should more closely resemble that i n the intact protein. The ground- and excited-state electron d i s t r i b u t i o n and energies, i n each case, were calculated at f i v e d i f f e r e n t i r o n n i t r o s y l distances corresponding to successive displacement of NO i n


1.

C H A N T R A N U P O N G ET AL.


5

0.2-Â steps from an i n i t i a l bonding distance of 1.743 Â. Excited states of the closed-shell f e r r i c complex were calculated by performing single excitation CI calculations using a toal of 207 configurations as described previously (44, 45). For the open-shell ferrous complex, the Self Consistent F i e l d (SCF) calculations were performed by using a generalized open-shell operator (46), and the CI calculations were obtained from a Rumer diagram technique (50-54). At each iron-ligand distance, the energies of doublet excited states were calculated by using a t o t a l of 200 configurations corresponding to single excitations spanning an active space of 10 highest doubly occupied, 1 singly occupied, and 9 v i r t u a l molecular o r b i t a l s of the reference ground state. Results The n i t r o s y l f e r r i c heme complex presented a computational problem at large Fe-NO distances due to the o v e r a l l closed-shell nature of the liganded complex and the r a d i c a l nature of the dissociated heme and ligand. The "frozen" (S = 0) spin state imposed on this complex resulted i n SCF convergence on a Fe(II)-N0 ground state at Fe-NO • 2.143 Â and larger. However, at smaller Fe-NO distances, the correct Fe(III) oxidation state of the complex corresponding to a spin paired b i r a d i c a l singlet was obtained. These results indicated that the i d e n t i f i c a t i o n of the photoactive states of the f e r r i c complex should be determined from the energy p r o f i l e calculations at Fe-NO distances not too f a r from the equilibrium bonding distance. No such computational problems were encountered i n the n i t r o s y l ferrous heme calculations, since a doublet spin state of the complex i s consistent with the r a d i c a l nature of the NO ligand at a l l Fe-NO distances. Table I shows the atomic o r b i t a l composition and energies of the ground-state molecular o r b i t a l s of the n i t r o s y l f e r r i c heme complex at three Fe-NO distances of 1.743 (bonding), 1.943, and 2.143 A. Table II shows the same results for the n i t r o s y l ferrous heme complex at Fe-NO = 1.743 and 2.143 A. The singly occupied o r b i t a l (79) has primarily NO % and σ character with both à and d 2 contributions. Although this o r b i t a l i s deeply buried, as can be seen from i t s energy, i t i s shown between the doubly occupied and v i r t u a l o r b i t a l s for convenience. For comparison, the ground-state molecular o r b i t a l s of carbonyl-(44) and oxyheme (45) complexes are shown i n Table I I I . Comparison of the molecular o r b i t a l energies of the ligand-bound f e r r i c and ferrous complexes i n Tables I and II shows that, aside from a general energy s h j f t of about 0.12 au, the r e l a t i v e energies of the porphyrin π and π o r b i t a l s are l i t t l e affected by the change i n the oxidation state of iron. However, both iron and NO ligand o r b i t a l s are s i g n i f i c a n t l y affected by this change. The energies of the iron d and d o r b i t a l are, respectively, 0.14 and 0.18 au lower i n the f e r r i c than In ferrous complex. In the f e r r i c complex, the two NO π o r b i t a l s are the lowest empty orjjitals and have nearly equal energies below those of porphyrin eg o r b i t a l s . In the ferrous complex, however, these o r b i t a l s are s p l i t with the singly occupied o r b i t a l becoming deeply ^buried and the v i r t u a l o r b i t a l energy increasing above the e π o r b i t a l s . The main effect or lengthening the Fe-NO distance i n n i t r o s y l f e r r i c heme i s an increase i n the d^ contribution to v i r t u a l o r b i t a l s +

%

%

z

a


PORPHYRINS: E X C I T E D STATES A N D DYNAMICS

6

Table I. Ground State O r b i t a l Description of N i t r o s y l F e r r i c Heme at Fe-NO Distances of 1.743 A, 1.943 A, and 2.143 A. Fe(III)-NO - 1.743 A Orb. Energy No. (au)

Character

92

-0.0017

(54% Imida, 18% Pora, 10% Fe a ) *

91

-0.0097

(100% a

90

-0.0229

(98% e ) *

89

-0.0235

(96% e ) *

88

-0.0404

(85% Imid*, 14% Imida)*

87

-0.0522

(99% b

86

-0.0787

(85% Imidii, 15% Imida)*

85

-0.0878

(100% b

84

-0.1026

(53% d 2_y2, 13% d 2, 16% Pora, 1% N0a)*

83

-0.1029

(50% d 2,15% d 2_ 2, 9% Pora,8% Ν0σ,3% ΝΟπ)*

82

-0.1527

(97% e ) *

81

-0.1533

(97% e ) *

80

-0.1719

(87% ΝΟπ, 9% d ^ *

79

-0.2111

(32% ΝΟπ, 37% d ^ 14% NOa,

l u

)*

g

g

l u

)*

2 u

)*

x

z

z

x

y

g

g

10% ΡοΓπ, 3% d

2 z

)*

78

-0.3444

100% a

77

-0.3599

84% a

76

-0.4310

57% e , 15% Ιπιΐάπ, 6% ΝΟπ,

2 u

l u

, 5% ΝΟπ, 4% d,^

g

8% d

2% d

%9

2 z

, 2% ΝΟσ

75

-0.4322

60% Ιιηΐάπ, 20% ΡοΓπ, 9% Imida, 2% dπ

74

-0.4347

74% e , 13% d ^ 6% Ιιηΐάπ

73

-0.44.70

94% b

72

-0.4484

93% a

2 u

71

-0.4619

96% e

g

70

-0.4628

98% e

g

69

-0.4891

33% d

g

l u

%9

7% d

x y

22% Pom,

, 1% d

12% ΝΟπ, 8% Ν0α,

2 z

, 23% d

68

-0.5010

30% Ιϋ^Λίπ, 27% d

67

-0.5030

57% d . 14% d , 9% lm±dn xy* π' v

x y

%9

9

3% ΝΟπ 3% ΝΟπ


1.

C H A N T R A N U P O N G ET A L .

Table I.


Continued Fe(III)-NO - 1.943 A

Orb.

Energy

No.

(au)

92

-0.0002

Character (75% Pora, 7% Imida, 6% Fea)*

91

-0.0095

(100% a

90

-0.0218

(97% e ) *

89

-0.0224

(97% e ) *

l u

)*

g

g

88

-0.0355

(85% Imidic, 14% Imida)*

87

-0.0490

(94% b

l u

, 2% (^2^2)*

86

-0.0640

(69% d 2_ 2, 15% Pora)*

85

-0.0716

(72% d 2, 6% NOa, 6% Imida, 2% ΝΟπ)*

84

-0.0732

(82% Irnidu, 15% Imida, 2% d 2 ) *

x

y

z

z

83

-0.0880

(100% b ^ ) *

82

-0.1471

(55% ΝΟπ, 40% e ) *

81

-0.1498

(93% e ) *

80

-0.1518

(54% e , 39% ΝΟπ, 3% d )*

79

-0.2215

(35% Pom, 28% ΝΟπ, 15% NOa,

g

g

%

g

17% d^, 2% d 78

-0.3454

100% a

77

-0.3624

61% a

76

-0.4177

2 u

2

)*

z

l u

, 18% ΝΟπ, 7% ΝΟα, 8% d

%

54% e , 28 Xd

%9

2%

9% ΝΟπ, 4% ΝΟα,

d}

75

-0.4233

69% e , 29% d^ g' π

74

-0.4274

81% lm±d% 9% Imida

73

-0.4431

95% b

l u

72

-0.4457

92% a

2 u

71

-0.4602

93% e , 2% d

e

$

, 1% dn

&

Y V

8

xy

70

-0.4611

96% e , 1% d ^

69

-0.4624

89% d

68

-0.4753

38% e , 37% d^, 7% Ιιηΐάπ, 5% ΝΟπ

67

-0.4810

55% d ^ 21% e , 13% Ι π ^ π , 3% ΝΟπ

g

x y

g

g

Continued on next page


7

P O R P H Y R I N S : E X C I T E D STATES A N D D Y N A M I C S

8

Table I.

Continued Fe(III)-NO = 2.143 A

Orb. No.

Energy (au)

Character

92

-0.0005

(79% Pora, 4% Fea)*

91

-0.0116

(100%

a

l u

)*

90

-0.0226

(96%

89

-0.0233

(95%

88

-0.0312

(84% Ι π ^ π ,

87

-0.0373

(67% d 2

ν * ν

14% Imida)

2 , 11% Pora)

χ

y

86

-0.0489

(55% b ,

85

-0.0494

(48% d 2, 29% b

84

-0.0684

(85% Ι π ^ π ,

83

-0.0880

(100%

82

-0.1232

(97% ΝΟπ)*

81

-0.1465

(97%

80

-0.1471

(97%

79

-0.2215

(53% ΡοΓπ, 24% ΝΟπ, 14% ΝΟα,

78

-0.3446

68% a ,

18% ΝΟπ, 9% ΝΟα, 2% d^

77

-0.3614

76% a ,

13% ΝΟπ, 5% ΝΟα

-0.4078

41% e , 49% ά

75

-0.4120

48%

74

-0.4234

82% Ι π ^ π ,

73

-0.4349

96%

d

72

-0.4437

96%

b

lu

71

-0.4464

92%

a

2u

70

-0.4591

93%

69

-0.4601

96%

68

-0.4666

48% e ,

67

-0.4686

56% e g», 37% d> , 15 NOic

l u

30% d 2, 2% d 2_ 2) z

z

x

l u

y

, 2% d 2_ 2) x

y

15% Imida)

b )* 2 u

ν *

6% d^)*

76

l u

2 u

%9

g

v

4

%

d

z

*'

13% Imida

xy

3

V e

4

2% ΝΟπ, 1% d 2

%

d

*

8

g

12% Imidii π


1.



Table I I .

Ground State O r b i t a l Description of N i t r o s y l

Ferrous Heme at Fe-NO Distances of 1.743 Â and 2.143 Â Fe(II)-NO = 1.743 Orb. No.

Energy (au)

Γ

Character

92

0.1013

(94% Imida)

91

0.0999

(99%

90

0.0913

(96% e ) *

89

0.0906

(95% e ) *

88

0.0735

(43%

d 2_ 2, 24% b , 9% d 2, 7% Pora)

87

0.0711

(58%

d 2, 5% d 2_ 2, 5% N0a, 5% Imida)

86

0.0672

(62%

b

85

0.0656

(82%

Irnidit, 14% Imida)*

84

0.0291

(84%

Irnidu, 15% Imida)*

83

0.0226

(100%

82

-0.0114

(84%

81

-0.0373

(97% e ) *

80

-0.0380

(97% e ) *

79

-0.3285

62% ΝΟπ, 26% ΝΟα, 8% d,,, 3% d^2

78

-0.2281

100% a

77

-0.2390

94% a

76

-0.2925

43%

2u e , 48% d

75

-0.3012

54%

eg». 40% dπ

74

-0.3250

41%

d

73

-0.3253

55%

d

72

-0.3311

70%

b , 21% Ι π ^ π

83%

a

90%

e , 3% d

71

-0.3320

a

l u

)*

g

g

x

y

l u

z

x

l u

b

z

y

, 23% d 2 _ 2 ) * x

2 u

y

)* d )*

ΝΟπ, 12%

%

g

g

l u

6% ΝΟπ

n9

g

w

x y

, 21% b

x y

, 21% Imidπ, 15% b

l u

, 29% Ιπιΐάπ l u

l u

2 u

, 9% Imidπ

70

-0.3489

69

-0.3498

97%

e

68

-0.3517

52%

e , 27% d , 7% Ι π ^ π , 6% ΝΟπ

67

-0.3599

40%

e , 47% ά ,

g

%

g

g

g

1l

π

6% ΝΟπ Continued on next page


9

PORPHYRINS: E X C I T E D STATES A N D DYNAMICS

Table I I .

Continued Fe(II)-NO - 2.143 A

Orb. No.

Energy (au)

Character

0.1057

(95% Imida)*

91

0.1020

(100% a

90

0.0938

(96% e ) *

89

0.0931

(93% e ) *

88

0.0878

(68% d 2. 2, 12% Pora)*

87

0.0749

(78% d 2, 7% Imida)*

86 85

0.0718

(88% b

0.0704

(80% I m i d ^

84

0.0340

(84% ImidTi, 15% Imida)*

83

0.0247

(100% b ^ ) *

82

-0.0346

(97% e ) *

81

-0.0353

(97% e ) *

80

-0.0451

(89% ΝΟπ, 5% ΝΟα, 2%

79

-0.3725

69% ΝΟπ, 26% ΝΟα

78

-0.2257

100% a

77

-0.2375

94% a

76

-0.2883

51% d

n9

38% e

g

75

-0.2912

54% d

n9

41% e

g

74

-0.3087

96% d

73

-0.3208

58% Im±dii

72

-0.3278

72% b

71

-0.3309

93% a

70

-0.3465

83% e , 7%

69

-0.3475

98% e

68

-0.3482

63% e , 26% d

67

-0.3518

55% e , 41% d

92

l u

)*

g

g

x

y

2

l u

, 3% Irnidn, 2% d 2 _ 2 ) * x

y

13% Imida)*

g

g

d )* %

l u

2 u

x y

9

l u

29% b ^ , 9% Imida

, 22% Imidπ

2 u

g

d

n

g

g

g

n9

6% Imidπ

%




Table I I I . Ground State O r b i t a l Description of Ligand-bound Carbonyl and Oxyheme Complexes Carbonylheme (Fe-CO » 1.77 A) Orb. No.

Character

92

(90% Imida, 3% d 2 ) *

91

(100% a ) *

90

(59% d 2_ 2, 37% Pora)*

89

(93% e , 3% COn, 2% d 2 ) *

88

(88% e , 5% d 2,

2

l u

x

y

g

2

g

3% COTC)*

2

87

(43% d 2, 21% Imida, 14% Pont, 7% COa)*

86

(65% COTI, 21% Imidu, 7% d^)*

85

(55% COit, 37% b

84

(65% b

83

(79% Imidu, 15% COit, 1% d )*

82

(100% b ) *

2

, 5% d )* %

l u

, 30% COit, 3% d ^ *

l u

%

2 u

81

(99% Imidit, 1% CO*)*

80

(98% e , 2% d^)*

79

(98% e , 2% d ^ *

78

100% a

g

g

l u

77

97% a

76

62% d^, 31% e , 4% COn

75

62% d

74

96% d

73

70% Imidit, 30% b

72

93% a

2 u

g

%y

33% e , 4% CO* g

v v

l u

2 u

, 3% COn , 22% Irnidu

71

78% b

l u

70

98% e

g

69

99% e

g

68

68% e , 17% d , 15% Imidic, 4% COii

67

70% e , 23% d , 6% 00π

g

%

%

Continued on next page


P O R P H Y R I N S : E X C I T E D STATES A N D D Y N A M I C S

Table I I I .

Continued Oxyheme (Fe-0

Orb. No.

s

1-75 Â)

2

Character

92

(99%

91

(85% Imida, 12% Porn)*

90

(99% e ) *

89

(88% e ,10% Imida)*

a

l u

)*

g

e

*

88 (100%

*

b >* lu

87

(55% d 2, 13% Pora, 13% Imida, 7% 0 a , 4% d ^ . ^ )

86

(58% d 2_ 2, 34% Pora, 4% d 2, 1% 0 a ) *

85

(98% Imidic, 1% d 2 ) *

84

(100% b

83

(99% Imidic)*

82

(82% e , 12% d

81

(99% e , 1%

80

(43% 0 π , 36% d ,

79

100% a

78

99% a

77

47% 0 π , 32% ά , 21% e

z

2

x

y

z

2

z

2 u

)*

5% 0 π ) *

%9

g

2

dj*

g

%

2

18% e ) * g

l u

2 u

2

π

g

76

43% e , 34% d^, 12% 0 tc, 7% 0 σ

75

36% Porn, 32% 0 ic, 27% 0 σ , 1% ά

g

2

2

2

%

2

74

82% b

l u

, 18% Imidic

73

87% a

2 u

, 5% 0 ic, 3% 0 σ, 1% d

72

69% Imidic, 28% Poric, 1% 0 ic, 1% 0 σ , 1% d

71

90% e , 3% 0 π , 3% d

70

98% e

69

79% e , 7% d

2

2

x y

2

g

%

2

n

2

g

g

68

87% d

67

48% d

x y

%9

6% 0 ic, 3% d ^ - ^

%9

2

, 6% Pora, 1% 0 ic 2

27% e , 15% Imidic, 7% 0 a, 3% 0 ic g

2

2


1.



13

83 and 84 resulting In an appreciable net increase i n thejr energies as well as changes i n their r e l a t i v e positions. The NO π o r b i t a l s are also affected by the iron-ligand bond lengthening, which results i n their stronger mixing with the porphyrin e π and a π orbitals. The effect of bond lengthening i n the ferrous complex on the iron d o r b i t a l s i s primarily an increased l o c a l i z a t i o n with smaller changes i n their o r b i t a l energies and no change i n t h e i r order. The NO π o r b i t a l s , on the other hand, are appreciably lowered i n energy accompanied by a decrease i n the d and d^ contributions to these orbitals. Simplified, scaled, diagramatic representations of the excited state energies of n i t r o s y l f e r r i c and ferrous heme complexes at successively increasing Fe-NO distances are shown i n Figures 1 and 2, respectively. The excited states of the f e r r i c complex are shown only for Fe-NO distances up to 2.143 Â because of the necessity of relaxing the "frozen" spin approximation beyond this distance. Each excited state calculated at each Fe-NO distance i s represented by a horizontal l i n e . However, only the Soret and Q-band, and the states that show large variations i n energy with iron-ligand distance or correspon* to important charge-transfer states are i d e n t i f i e d by character. The percentages shown to the r i g h j of some of the excited states specify the contribution of the d •> d 2 type transitions to these states. The atomic o r b i t a l notations used to specify important transitions i n Figures 1 and 2 represent only the main character and not the entire composition of the ralecular o r b i t a l s involved. The remaining o r b i t a l characters can be obtained by examining Tables I and II which contain a complete description of the same o r b i t a l s . Examination of Figures 1 and 2 stjows that only the states corresponding to d ·* d 2 and d > d 2 transitions display a d i s s o c i a t i v e p r o f i l e according to the c r i t e r i o n of "decreasing energy with increasing iron-ligand distancg and no barrier to d i s s o c i a t i o n . " Since the contributions from d^ + d 2 t r a n s i t i o n are distributed i n excited states over the entire energy range between the Q and the Soret band i n both ferrous and f e r r i c complexes, the c o l l e c t i v e behavior of a l l of these states are indicated by v e r t i c a l bars and s o l i d arrows. In common with oxy and carbonyl ferrous heme complexes, as shown i n previous studies (44, 45), neither the Soret or the Q band show any d i s s o c i a t i v e behavior. An important aspect of the results i s the s i g n i f i c a n t l y d i f f e rent behavior obtained for the charge-transfer states i n n i t r o s y l ferrous and f e r r i c complexes. For the n i t r o s y l f e r r i c heme, a number of such states primarily a^ , a NO π , (NO π, d^) calculated at energies below the Q-band are similar to those calculated for oxy ferrous heme (45). However, while these states display dissociate p r o f i l e s i n case of 0 , no such pattern can be recognized for the NO ligand. The behavior of the a , a NO π, NO % charge-transfer states i n the n i t r s y l ferrous heme complex i s very d i f f e r e n t . The energies of these states (Figure 2) are higher, i n the range between the Q and Soret band. More i n t e r e s t i n g l y , these state display a minimum corresponding to a 0.4 Â increase i n the Fe-NO distance. The minimum persists when the NO ligand i s rotated 45 degrees about the Fe-N axis and the CI calculations are repeated. We have determined that this behavior i s not due to convergence errors. It i s also 2 u

a

a

%

%

z

z

z

z

u

2 u

2

l u

2 u


PORPHYRINS: EXCITED STATES A N D DYNAMICS

14

Fe(III)N0

.-1

π-»· π (soret) d

30,000 H

-*• d*2 xy ζ (66%)

25 000-T400 r

W

V

20,000-4-500

π* π

(Q)

-(14%) -(35%)

h-600 15,000H L700 I

(12%) I

(38%)

.(22%)

(29%)

h800 a

lvT

^

ιο,οοοΗ -(21%) a

V

2u

(38%)

(10%) 5,000'

(37%)

.(18%)

a

lu ^

( Ν Ο

π '

V

1.743 A

1.943 A

2.143 A

Figure 1. Simplified, scaled, diagramatic representation of the excited-state energies of n i t r o s y l f e r r i c heme complex at i r o n nitrogen distances of 1.743, 1.943 and 2.143 A showing c o r r e l a tion between similar photodissociating states. The π π t r a n s i t i o n of the Soret and Q bands are also i d e n t i f i e d .


1.

15


C H A N T R A N U f o N G ET A L .

Fe(II)N0

nm π-*π* (soret)

30,000

(40%) (22%)

v

w

(19%) (23%)

*• d ο 25,000 Τ 400 ^31%)

|^i^(36%) m

Quantum Mechanical Studies of the Photodissociation of Nitrosoheme

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