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Theoretical Study on the Photosensitizer Mechanism of Phenalenone in Aqueous and Lipid Media César Espinoza, Ángel Trigos, and Manuel Eusebio Medina J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b03615 • Publication Date (Web): 18 Jul 2016 Downloaded from http://pubs.acs.org on July 25, 2016

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The Journal of Physical Chemistry

Theoretical Study on the Photosensitizer Mechanism of Phenalenone in Aqueous and Lipid Media César Espinoza,† Ángel Trigos,† Manuel E. Medina*,‡

†

Laboratorio de Alta Tecnología de Xalapa, Universidad Veracruzana, Calle Medicos 5, Col. Unidad del Bosque 91010, Xalapa, Veracruz, México.

‡

Centro de Investigaciones Biomédicas, Universidad Veracruzana, Dr. Luis Castelazo Ayala s/n, Col. Industrial las Animas 91190, Xalapa, Veracruz, México.

*

To whom correspondence should be addresed. E-mail: [email protected]

ACS Paragon Plus Environment

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KEYWORDS. Phenalenone, reaction mechanism, kinetics, rate constant, DFT.

ABSTRACT. The photosensitizer ability of phenalenone was studied in aqueous and lipid media through the single electron transfer reactions, employing the density functional theory. Although phenalenone is a well-known photosensitizer and is a widely used as an 1O2 reference sensitizer, little is known about the reaction mechanism involved. In this study we carried out a single electron transfer reaction between the basal, excited, oxidized and reduced state of phenalenone with oxygen molecules such as 3O2 and O2•‒. In aqueous media the photosensitizer capacity of phenalenone was measured through both type I and type II mechanisms. In lipid media the photosensitizer ability of phenalenone was attributed to the type II mechanism. The results indicated that the photosensitizer ability of phenalenone shows a heavy reliance on the media where the reaction occur whether this is an aqueous or lipid media. Finally, this study supports the idea about that electron transfer reactions can be used to study the photosensitizer ability of molecules.

INTRODUCTION. The oxidative stress is an unbalanced in the production and consumption of oxygen reactive species (ROS) and has been related with a large number of diseases such as cancer1-3, cardiovascular disorders4-6, atherosclerosis7-10 and several neurological disorders including Alzheimer’s and Parkinson’s diseases11-13. The ROS are oxidant molecules that can be neutral such as 1O2 singlet oxygen, ionic as O2•- anion superoxide or radicals such as hidroperoxyl •OOH and hidroxyl •OH. The ROS under oxidative stress conditions can cause damage to biological


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molecules such as lipids, aminoacids, proteins and DNA.14-19 In the generation of ROS, the mitochondrial metabolism and the oxidative phosphorylation cascade have been identified as key factors.20-23 The pro-oxidant molecules are any endobiotic or xenobiotic that can induces oxidative stress either by the generation of ROS or by inhibiting antioxidant systems.24 Photosensitizer dyes are pro-oxidant molecules that can carry out oxidation reactions through two reaction mechanisms; in the first (type I) free radical generation (O2•‒) was proposed and in the second the singlet oxygen molecule (1O2) was obtained (type II). The photosensitizer molecules that can form the 1

O2 molecule have an important application in the photodynamic therapy against the cancer, in

wastewater treatment, in blood sterilization, insecticides and herbicides.25,26 Phenalenone (PN, Figure 1) also called perinphtenone is an aromatic ketone that is a member of the phenalenone group that is widely distributed in nature; they are considered antimicrobial secondary metabolites and are termed phytoalexins in plants. It has also been reported that these compounds have been isolated from mushrooms.27,28 It was reported that the phenalenone derivatives shown leishmanicidal,29 antifungal,30 antioxidant,31 anti-HIV,32 antimicrobial33 and anticancer activity34. Phenalenone is widely used as an 1O2 reference sensitizer, since it is one of the known type II photosensitizers; it was also proposed that phenalenone is able to accept an electron and in the aqueous media it is converted into PN•−,35 it can also react with paraffin wax or n-heptane solution through the hydrogen transfer mechanism and the hydroxyperinaphthenyl radical formation was observed from 3PN.36 These results could mean that phenalenone is able to act as a type I photosensitizer. In a kinetic study of 3PN reactivity towards typical H- and electron-donors, it was characterized by means of nanosecond laser flash-photolysis and it was observed that the H-abstraction from tributylstannane occurs with a rate constant of 5 x 105 M-1


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s-1. According to this study, the 3PN also reacts via photoinduced electron transfer with 1,4diazabicyclo[2.2.2]octane (DABCO) with a rate constant close to the diffusional control limit where the spectrum of the solvated free radical anion of phenalenone has a maximum value at 440 nm in acetonitrile.37 The mechanism for populating the triplet state of PN responsible for this reaction was studied. In the initial population of the S2 excited state of (π–π*) character, the system undergoes an internal conversion to the 1(n–π*) state, then relaxes to the minimum, but rapidly populates the triplet manifold through a very efficient intersystem crossing to the 3(π–π*) state. Radiationless deactivation processes are ruled out on the basis of the high-energy barriers found for the crossings between the excited states and the ground state.38 Thus, the main goal of the present work was to perform a detailedx study on the mechanisms of reactions involved in the pro-oxidant property through the photosensitizer ability of phenalenone and to provide kinetic data on such processes. For that purpose, we have modeled the reactions between neutral, excited, oxidized and reduced phenalenone with 3O2 and O2•‒, in polar and nonpolar environments. The single electron transfer was the mechanism considered in this study. Thermodynamic and kinetic data are provided, as well as the contributions of the different reaction mechanisms to the pro-oxidant property through the photosensitizer ability of phenalenone. All this new information is expected to contribute towards a better understanding of the pro-oxidant role through the photosensitizer ability of phenalenone.

THEORETICAL CALCULATIONS. All the electronic calculations were performed using the Gaussian 09 package program.39 Geometry optimization and frequency calculations were carried out employing the M05-2X40


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functional in conjunction with the 6-311++G(d,p) basis set, employing the continuum solvation model density (SMD)41 with pentylethanoate and water as solvent in order to mimic a biological lipid and aqueous environment. The M06-2X functional has been recommended for kinetic calculations by its developers and it has been successfully used by independent authors for that purpose.42-47 It is among the best performing functionals for calculating reaction energies involving free radicals and for kinetic calculations in solutions.48,49 The SMD is a solvation model that can be applied to any charged or uncharged solute in a liquid medium for which few key descriptors are known.41 First singlet excited-state (1PN*) geometry optimizations and frequencies calculation were performed by TD-DFT calculations, including solvent effects, at the TD-M06-2X/6-311++G(d,p) level. The rate constants (k) were calculated using the conventional transition state theory (TST),50-52 according to: =

∆ / (1) ℎ

where kB and h are the Boltzmann and Planck constants, T is the temperature, ∆G≠ is the Gibbs free energy of activation and R is the gas constant. In this study we report: 1) the Gibbs free energy of reaction (∆ ), that was obtained from

reactants and products of each electron transfer reaction and 2) the Gibbs free energy of activation (∆ ), that was calculated according to the following.

All the reactions considered in this study involved a single electron transfer mechanism (SET) and to calculated the barrier of reaction the Marcus theory was employed.53-55 The barrier of SET ) was defined from two thermodynamic parameters, the Gibbs free energy of reaction (∆

reaction (∆ ) and the nuclear reorganization (λ).


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∆

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Δ = 1 + (2) 4

The nuclear reorganization (λ) was calculated according to the following: = Δ − Δ (3)

Where ∆ESET was the difference of no adiabatic energy between reactants and vertical product. This approximation is similar to that proposed by Nelsen and col.56 for a great number of intramolecular electron exchange reactions. Some of the calculated rate constant (k) values are close to, or within, the diffusion-limit regime. Accordingly, the apparent rate constant (kapp) cannot be directly obtained from TST calculations. In the present study the Collins-Kimball theory57 is used for that purpose: "## =

$ (4) $ +

where k is the thermal rate constant, obtained from TST calculations, and kD is the steady-state Smoluchowski58 rate constant for an irreversible bimolecular diffusion-controlled reaction: $ = 4%&'( )( (5) where R denotes the reaction distance, NA is the Avogadro number, and DAB is the mutual diffusion coefficient of the reactants A and B. DAB has been calculated from DA and DB according to reference,59 and DA and DB have been estimated from the Stokes–Einstein approach:60,61 '=

(6) 6%,-

where kB is the Boltzmann constant, T is the temperature, η denotes the viscosity of the solvent, in our case water (η = 8.91 x 10-4 Pa s) and pentylethanoate (η = 8.62 x 10-4 Pa s); and ɑ is the radius of the solute.


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The methodology employed in the present investigation is in line with the quantum mechanicsbased test for overall free radical scavenging activity (QM-ORSA), which has been validated by comparison with experimental results and its uncertainties have been proven to be no larger than those arising from experiments.62

RESULTS AND DISCUSSION The study of a pro-oxidant role through the photosensitizer ability of phenalenone was carried out in aqueous and lipid media. In Table 1 were shown the results of excitation energy, the oscillator strength, the most important contribution to excitation and the λmax experimental. The results indicated that the most important contribution in the excited state of phenalenone was attributed to HOMO→LUMO in both medias.

Table 1. Excitation energy (nm y eV), oscillator strength (f), excitation contribution (%) and experimental λmax (nm), at 298.15 K. Media

λmax

eV

f

% contribution

Exp. λmax

Aqueous

353.21

3.5102

0.2892

94.41 (H → L)

355ª,42

Lipid

348.55

3.5571

0.3003

95.55 (H → L)

353b,19

a

in aqueous media; b in n-heptane.

To carry out the study of the photosensitizer ability of phenalenone in aqueous and lipid media the presence of 3O2 and O2•‒ were considered because these species are abundant in the organism. The single electron transfer SET mechanism was considered between the phenalenone in its basal (1PN), excited (1PN* and 3PN), oxidized (1PN•+) and reduced (1PN•‒) forms with the oxygen (3O2) and radical anion superoxide (O2•‒) molecules.


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To carry out the study of the photosensitizer ability of phenalenone the following reactions were considered: 1)

1

PN + 3O2 → PN•+ + O2•‒

2)

1

PN* + 3O2 → PN•+ + O2•‒

3)

3

PN + 3O2 → PN•+ + O2•‒

4)

1

PN + O2•‒ → PN•‒ + 1O2

5)

1

PN + O2•‒ → PN•‒ + 3O2

6)

1

PN* + O2•‒ → PN•‒ + 1O2

7)

1

PN* + O2•‒ → PN•‒ + 3O2

8)

3

PN + O2•‒ → PN•‒ + 1O2

9)

3

PN + O2•‒ → PN•‒ + 3O2

10) 1PN + 1PN → PN•+ + PN•‒ 11) 1PN + 1PN* → PN•+ + PN•‒ 12) 1PN + 3PN → PN•+ + PN•‒ 13) 1PN* + 1PN* → PN•+ + PN•‒ 14) 1PN* + 3PN → PN•+ + PN•‒ 15) 3PN + 3PN → PN•+ + PN•‒ 16) PN•+ + O2•‒ → 1PN + 1O2 17) PN•+ + O2•‒ → 1PN + 3O2 18) PN•+ + O2•‒ → 3PN + 1O2 19) PN•+ + O2•‒ → 3PN + 3O2 20) PN•‒ + 3O2 → 1PN + O2•‒ 21) PN•‒ + 3O2 → 3PN + O2•‒


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Among the reactions proposed, those reactions that yielded O2•‒ molecules are considered as reactions that contribute to type I mechanism from photosensitizer ability of phenalenone; while those reactions that yielded 1O2 molecules are considered reactions that contribute to type II mechanism from photosensitizer ability of phenalenone. The geometric parameters of 1PN, 1PN*, 3PN, PN•+ and PN•‒ are shown in Table 2 and in Figure 1 was shown the atomic numbering. In this, it can be observed that the 1PN*, 3PN and PN•‒ shown similar parameters in all bond distances considered for both medias, lipid and aqueous, among these can be observed the increase of the O1-C2 bond distance; while the geometric parameters for PN•+ shown a different behavior in both medias and among which can be observed the decrease of the O1-C2 bond distance.

Table 2. Bond distances of the optimized geometries of phenalenone derivatives in aqueous and lipid media. Aqueous media Bond

1

PN

1

PN*

3

Lipid media

PN

PN•+

PN•‒

1

PN

1

PN*

3

PN

PN•+

PN•‒

O1-C2

1.234

1.261

1.262

1.224

1.284

1.221

1.242 1.254 1.218 1.257

C2-C3

1.463

1.437

1.432

1.471

1.426

1.472

1.447 1.432 1.474 1.438

C3-C4

1.347

1.387

1.390

1.361

1.376

1.344

1.385 1.394 1.361 1.373

C4-C5

1.450

1.403

1.392

1.424

1.422

1.454

1.404 1.389 1.425 1.423

C5-C6

1.381

1.435

1.453

1.419

1.410

1.379

1.433 1.454 1.418 1.410

C6-C7

1.411

1.368

1.360

1.377

1.392

1.412

1.368 1.360 1.378 1.391

C7-C8

1.372

1.420

1.421

1.405

1.389

1.371

1.422 1.420 1.405 1.389

C8-C9

1.421

1.399

1.417

1.411

1.417

1.421

1.396 1.417 1.410 1.418

C9-C10

1.415

1.418

1.404

1.408

1.418

1.416

1.422 1.404 1.409 1.417

C10-C11 1.376

1.389

1.399

1.388

1.386

1.375

1.388 1.400 1.388 1.388

C11-C12 1.406

1.388

1.380

1.392

1.394

1.407

1.389 1.379 1.392 1.392

C12-C13 1.383

1.418

1.418

1.398

1.408

1.380

1.418 1.419 1.398 1.406


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C13-C2

1.483

1.465

1.462

1.482

1.457

1.491

1.472 1.465 1.486 1.469

C9-C14

1.417

1.431

1.430

1.416

1.431

1.417

1.433 1.431 1.417 1.433

All bond distances are in angstroms.

Thermodynamic study The study of the photosensitizer ability of phenalenone in aqueous and lipid media started with a thermodynamic analysis, the results are shown in Table 3. In aqueous media the feasible reactions were: 2, 6, 7, 9, 11, 13-17, 19 y 20; in reactions 2 and 20 was generated the O2•‒ molecule, while in reactions 6 and 16 the 1O2 molecule was yielded; in reactions 7, 9, 17 and 19 the O2•‒ molecule was consumed and 3O2 was yielded; in reactions 11 and 13-15 the electron transfer between phenalenone molecules occurred and the PN•+ and PN•‒ were obtained as products; in reaction 20 the O2•‒ molecule was yielded. The feasible reactions in the lipid media were: 5-9 and 13-19; the reactions 6, 8, 16 and 18 generated 1O2 as a product, while in reactions 5, 7, 9, 17 and 19 the O2•‒ was consumed and 3O2 was yielded as a product; in reactions 13-15 the electron transfer between phenalenone molecules occurred and phenalenone was yielded as cation and anion radicals. According to the thermodynamic results in aqueous and lipid media, the kinetic analysis should take place in the exergonic reactions; therefore, in aqueous media the kinetic study are going to perform in the reactions: 2, 6, 7, 9, 11, 13-17, 19 and 20; while in lipid media the kinetic study should take place in the reactions: 5-9 and 13-19.

Table 3. Gibbs free energy (∆G, kcal/mol) of reaction for the reaction mechanism study of the phenalenone photosensitizer activity, at 298.15 K. Entry

Reaction

Aqueous

Lipid


10

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1

1

PN + 3O2 → PN•+ + O2•-

54.65

92.73

2

1

PN* + 3O2 → PN•+ + O2•-

-11.61

21.92

3

3

PN + 3O2 → PN•+ + O2•-

9.99

48.19

4

1

PN + O2•- → PN•- + 1O2

46.54

31.24

5

1

PN + O2•- → PN•- + 3O2

9.79

-6.64

6

1

PN* + O2•- → PN•- + 1O2

-19.72

-39.57

7

1

PN* + O2•- → PN•- + 3O2

-56.46

-77.45

8

3

PN + O2•- → PN•- + 1O2

1.87

-13.30

9

3

PN + O2•- → PN•- + 3O2

-34.87

-51.18

10

1

PN + 1PN → PN•+ + PN•-

64.45

86.09

11

1

-1.81

15.28

12

1

19.78

41.55

13

1

-68.07

-55.53

14

1

-46.48

-29.26

15

3

16 17 18 19 20 21

1

•+

•-

PN + PN* → PN + PN 3

•+

•-

PN + PN → PN + PN 1

•+

•-

PN* + PN* → PN + PN 3

•+

•-

PN* + PN → PN + PN 3

•+

•-

PN + PN → PN + PN

-24.88

-3.00

•+

•-

1

1

-17.91

-54.85

•+

•-

1

3

-54.65

-92.73

•+

•-

3

1

26.76

-10.30

•+

•-

3

3

PN + O2 → PN + O2 PN + O2 → PN + O2 PN + O2 → PN + O2 PN + O2 → PN + O2

-9.99

-48.19

•-

3

1

•-

-9.79

6.64

•-

3

3

•-

34.87

51.18

PN + O2 → PN + O2 PN + O2 → PN + O2

Kinetic study The kinetic study concerning the photosensitizer capacity of phenalenone in aqueous media is shown in Table 4. The results shown that reactions 2, 6, 8, 11, 16, 19 and 20 have a small Gibbs free energy of activation and therefore, they have a rate constant calculated that is limited by the diffusion rate; the results also shown, that the reaction 9 produces a Gibbs free energy of activation of 6.58 kcal/mol and a reaction rate constant of 9.25 x 107 M-1 s-1; the other reactions shown a rate constant of reaction too slow to be observed.


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Table 4. Gibbs free energy of activation (∆G≠, kcal/mol), lambda (λ, kcal/mol) and rate constant of diffusion and apparent (kD y kapp, M-1 s-1) of the photosensitizer capacity of phenalenone in aqueous media, at 298.15 K. Entry

Reaction

∆G≠

λ

kD

kapp

2

1

PN* + 3O2 → PN•+ + O2•-

0.18

14.84

7.89 x 109

7.88 x 109

6

1

PN* + O2•- → PN•- + 1O2

0.40

14.84

7.70 x 109

7.68 x 109

7

1

PN* + O2•- → PN•- + 3O2

28.16

15.15

7.70 x 109

1.42 x 10-8

8

3

PN + O2•- → PN•- + 1O2

4.67

14.69

7.85 x 109

1.81 x 109

9

3

PN + O2•- → PN•- + 3O2

6.58

15.00

7.85 x 109

9.25 x 107

11

1

PN + 1PN* → PN•+ + PN•-

0.74

6.04

7.41 x 109

7.38 x 109

13

1

PN* + 1PN* → PN•+ + PN•-

271.93

3.80

7.41 x 109

2.91 x 10-187

14

1

PN* + 3PN → PN•+ + PN•-

134.56

3.44

7.43 x 109

1.44 x 10-86

15

3

PN + 3PN → PN•+ + PN•-

35.41

3.29

7.41 x 109

6.84 x 10-14

16

PN•+ + O2•- → 1PN + 1O2

0.20

17.34

7.80 x 109

7.79 x 109

17

PN•+ + O2•- → 1PN + 3O2

22.86

17.66

7.80 x 109

1.09 x 10-4

19

PN•+ + O2•- → 3PN + 3O2

0.90

18.02

7.80 x 109

7.76 x 109

20

PN•- + 3O2 → 1PN + O2•-

0.36

18.11

7.96 x 109

7.94 x 109

The most important reactions of the photosensitizer ability of phenalenone in aqueous media are shown in Scheme 1. According to the above the 1PN* was consumed through three routes of reaction that competing with each other; in the first one the 1PN* evolved to 3PN, in the second reaction was carried out the oxidation of 1PN* to PN•+ occurred and the oxygen molecule was reduced to O2•‒; in the third reaction routes the 1PN* was reduced to PN•‒ and the O2•‒ molecule was oxidized to 1O2. The 3PN was reduced to PN•‒ in the presence of O2•‒ and 1O2 resulted as a product; the PN•‒ molecule can be oxidized to regenerate 1PN in the presence of 3O2, to obtain O2•‒ as a product.


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The PN•+ can be reduced by O2•‒ through two reactions that compete with each other, in the first reaction the 3PN and 3O2 were generated as products, while the second reaction yielded the 1PN and 1O2 and in the last reaction the phenalenone was regenerated. The above results shown that the photosensitizer ability of phenalenone in aqueous media were attributed to type I and II mechanisms; the reactions involved in the mechanism type I were: 2) 1PN* + 3O2 → PN•+ + O2•‒ 20) PN•‒ + 3O2 → 1PN + O2•‒ while the type II mechanism involved the following reactions: 6) 1PN* + O2•‒ → PN•‒ + 1O2 8) 3PN + O2•‒ → PN•‒ + 1O2 16) PN•+ + O2•‒ → 1PN + 1O2 so that, a reaction rate for each mechanism (ktype I and ktype II) can be calculated as well as an overall reaction rate (koverall), according to the following equations: ./#0 1 = +

./#0 11 = 2 + 3 + 42

5607"88 = ./#0 1 + ./#0 11

(7) (8) (9)

The results of the reaction rate obtained in the photosensitizer capacity of phenalenone in aqueous media, are shown in Table 5. Type I and II mechanisms have reaction rates of 1.58 x 1010 and 1.73 x 1010 M-1 s-1, respectively; therefore, the ratios of both mechanisms are near to 50%; hence, there is no mentionable difference by either of both mechanisms in the photosensitizer ability. The overall reaction rate in the photosensitizer capacity of phenalenone in aqueous media was of 3.31 x 1010 M-1 s-1.


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Table 5. Reaction rate of type I, II mechanisms, overall (k, M-1 s-1) and branching ratios (Г, %) of the photosensitizer capacity of phenalenone in aqueous media, at 298.15 K. path

k

Г

type I

1.58 x 1010

47.8

type II

1.73 x 1010

52.2

overall

3.31 x 1010

The results of the kinetic study concerning the photosensitizer capacity of phenalenone in lipid media are shown in Table 6. The results shown a small Gibbs free energy of activation for the reactions 5, 8, 15 and 18 and these reactions shown a reaction rate constant limited by the diffusion rate; reactions 6 and 19 shown a Gibbs free energy of activation of ~11 kcal/mol and they show a slow reaction rate constant; while the other reactions have a Gibbs free energy of activation much higher and shown a reaction rate constant too slow to be observed.

Table 6. Gibbs free energy of activation (∆G≠, kcal/mol), lambda (λ, kcal/mol) and rate constant of diffusion and apparent (kD y kapp, M-1 s-1) of the photosensitizer capacity of phenalenone in lipid media, at 298.15 K. Entry

Reaction

∆G≠

λ

kD

kapp

5

1

PN + O2•- → PN•- + 3O2

1.97

18.81

8.23 x 109

7.94 x 109

6

1

PN* + O2•- → PN•- + 1O2

10.58

14.66

8.23 x 109

1.09 x 105

7

1

PN* + O2•- → PN•- + 3O2

64.61

15.06

8.23 x 109

2.71 x 10-35

8

3

PN + O2•- → PN•- + 1O2

0.04

14.86

8.21 x 109

8.20 x 109

9

3

PN + O2•- → PN•- + 3O2

21.15

15.26

8.21 x 109

1.96 x 10-3

13

1

PN* + 1PN* → PN•+ + PN•-

268.95

2.60

6.07 x 109

4.51 x 10-185

14

1

PN* + 3PN → PN•+ + PN•-

62.51

2.80

7.66 x 109

9.36 x 10-34

15

3

PN + 3PN → PN•+ + PN•-

0.02

3.48

7.66 x 109

7.65 x 109


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16

PN•+ + O2•- → 1PN + 1O2

28.48

14.38

8.21 x 109

8.24 x 10-9

17

PN•+ + O2•- → 1PN + 3O2

102.79

14.78

8.21 x 109

2.79 x 10-63

18

PN•+ + O2•- → 3PN + 1O2

0.87

18.26

8.21 x 109

8.16 x 109

19

PN•+ + O2•- → 3PN + 3O2

11.67

18.67

8.21 x 109

1.72 x 104

In Scheme 2, the most important reactions in the photosensitizer capacity of phenalenone in lipid media are shown. According to the kinetics results, the 1PN* evolve to 3PN; the 3PN can react through two routes of reaction, in the first reaction 3PN was reduced to PN•‒ with O2•‒ and the 1

O2 molecule was yielded as a product and in the second reaction two molecules of 3PN react to

form PN•+ and PN•‒ molecules; PN•+ can regenerate 3PN in the presence of the O2•‒ molecule and yield 1O2 as a product; finally, the 1PN can be reduced to PN•‒ in the presence of O2•‒ and 3O2 was yielded as a product. Accordign to the above, type II mechanism is the most important route to consider in the photosensitizer capacity of phenalenone; therefore, the phenalenone is an excellent photosensitizer for obtaining an 1O2 molecule in lipid media. According to the above results, the photosensitizer ability of phenalenone in lipid media occurred through type II mechanism; the reactions involved with this mechanism were: 8) 3PN + O2•‒ → PN•‒ + 1O2 18) PN•+ + O2•‒ → 3PN + 1O2 In this case, a reaction rate for the type II mechanism (ktype II) was obtained, according to the following equation: ./#0 11 = 3 + 43

(10)

The reaction rate calculated for the type II mechanism was of 1.64 x 1010 M-1 s-1. Even though it was known that the products of electron transfer reactions cannot be stabilized by the lipid media; however, in this study excited species such as 1PN* and 3PN were considered


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and these molecules shown that they are able to react through an electron transfer reaction because of their own reactivity, as it has been reported.63,64 According to the photosensitizer capacity of phenalenone in aqueous and lipid media studied, in aqueous media the photosensitizer capacity of phenalenone occurred through type I and II mechanisms. The reaction rates calculated for the mechanisms type I and II in aqueous media were of 1.58 x 1010 and 1.73 x 1010 M-1 s-1, respectively. In lipid media, the photosensitizer capacity of phenalenone occurred through the mechanism type II. The reaction rate observed for the mechanism type II in lipid media was of 1.64 x 1010 M-1 s-1. These results are according to the experimental results observed, in where it was proposed that the phenalenone is a better photosensitizer through type II mechanism in lipid media than in aqueous media.27

CONCLUSIONS The photosensitizier capacity of phenalenone was studied in aqueous and lipid media through the single electron transfer reactions, employing the density functional theory. Physiological conditions were considered, therefore the 3O2 and O2•– species were taking into account. In aqueous media the phenalenone photosensitizer capacity was carried out through both types I and II mechanisms; in the type I mechanism the reactions 2 and 20 were involved and the reaction rate calculated for this mechanism was of 1.58 x 1010 M-1 s-1; while for the type II mechanism were reactions 6, 8 and 16 were involved and the reaction rate for this mechanism was of 1.73 x 1010 M-1 s-1. The overall reaction rate of the photosensitizer ability of phenalenone in aqueous media was of 3.31 x 1010 M-1 s-1. In lipid media, the photosensitizer ability was attributed to the type II mechanism and involved reactions 8 and 18; the reaction rate calculated for this mechanism was of 1.64 x 1010 M-1 s-1. The results indicated that the photosensitizer


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ability of phenalenone shown a heavy reliance on the media in which the reactions occurs depending on whether it is an aqueous or lipid media. According to the results obtained in this study, phenalenone is a better photosensitizer through the type II mechanism in lipid media than in aqueous media. Finally, the electron transfer reactions can be employed to study the photosensitizer ability of molecules.

ASSOCIATED CONTENT Supporting Information. Optimized geometries of neutral, excited, oxidized and reduced phenalenone. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENT We gratefully acknowledge the Laboratorio de Visualización y Cómputo Paralelo at Universidad Autónoma Metropolitana-Iztapalapa and thematic network of collaboration between academic groups UV-CA-354, PRODEP 2015. M. E. Medina thanks CONACYT for the fellowship (Retention program 243932).

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Table of Contents Graphic:

The photosensitizer ability of phenalenone shown a heavy reliance on the media in which the reactions occur and is a better photosensitizer through the type II mechanism in lipid than in aqueous media.


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Figure 1. Phenalenone, PN. Figure 1 190x107mm (300 x 300 DPI)


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Scheme 1. Main reactions involved in the photosensitizer capacity of phenalenone in aqueous media. Scheme 1 1350x1190mm (96 x 96 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Scheme 2. Main reactions involved in the photosensitizer capacity of phenalenone in lipid media. Scheme 2 1496x1190mm (96 x 96 DPI)


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Theoretical Study on the Photosensitizer ... - ACS Publications

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