Magnetic circular dichroism and fluorescence spectra of 5-deazaflavins

058‘%3539/81/010051-5$02.00/O

Spebodmica Acta Vol. 37A, pp. 51 to 55 Pergamon Press Ltd., 1981. Printed in Great Britain

Magnetic circulardichroismand fluorescencespectraof J-deazaftavins H.

YAhfAGUCHI

Department of Chemistry, Faculty of Science, Kumamoto University, 2-39-l

Kurokami, Kumamoto 860, Japan

A. KOSHIRO and Y. HARWA Department of Pharmacy, Yamaguchi University Hospital, Kogushi, Ube 755, Japan and K. MORI and F. YONEDA Department of Pharmaceutical Sciences, Kumamoto University, Oe-honmachi, Kumamoto 862, Japan (Received 28 June 1980) --Magnetic circul& dichroism and fluorescence spectra of Sdeazatlavins have been measured. It has been shown that there is a good correlation between the magnitude of the Faraday El values for the first three bands and the Hammett aWa values. It has been found that the fluorescence quantum yields are sensitive to the electronic properties of the substituents.

INTRODUCTION

The pyrimido [4, 5 - b] quinolink - 2(3H), 4(OH) diones (Sdeazaflavins) have aroused considerable recent interest in the chemistry of this ring system because of the recent discovery that FdzOpossessing Sdeaza,ilavin moiety is the co-enzyme of methaneproducing bacteria [l, 21. In order to elucidate the role of flavins in photobiological processes such as photodynamic action, photoaxis and photosynthesis [S], a considerable amount of work on the photophysics and photochemistry of flavins in vitro has been published [4,5]. The excited state energies and optical spectra of flavins and 5-deazallavins have been investigated by many authors [6-S]. Recently, the He(I) and He(II) photoelectron spectra of a 5-deazaflavin derivative and several flavins have been measured by EWEG et al. [lo]. They have pointed out that the first and second ionization potentials of these compounds come from the a-MO%. The magnetic circular dichroism (MCD) spectra have some advantage over ordinary spectrophotometric techniques in that the appearance of a negative band and the sharp sensitivity to changes of electronic structures are observed. Because of this sensitivity, the MCD spectra provide valuable informations with respect to structural differences between similar compounds [11-191. Fluorescence spectra, polarization, quantum yields and the phosphorescence lifetime of flavins, 3methyl-5-deazaflavin and 5-deazariboflavin have been measured by SONG et al. [6,8,20]. The aim of the present paper is to report the substituent effects on the Faraday B values and fluorescence quantum yields of 8-chloro-3,10dimethyl-5-deazaflavin (I), 3-methyl-5-deazaflavin (II), 3,8,10-trimethyl-5-deazatlavin (III), 3,10-dimethyl-8-methoxy-5-deazaflavin (IV) and 3,10-di-

it? (rr R=Cl (lI)R=H (IU)R=Me (IVhR

=OMe

h’)R=OH

Fig. 1. Numbering of 5-deazaflavins. methyl-8-hydroxy-5-deazaflavin

(V)

the

spectra

MCD

and

fluorescence

by measuring

of

these

compounds. E-AL

Materials 8-chloro-3,10-dimethyl-Sdeazaflavin (I), 3-methyl-5deazallavin (II), 3,8,10-trimethyl-5-deazallavin (III), 3,10-dimethyl-8-methoxy-5-deazatlavin (IV) and 3,10-dimethyl-8-hydroxy-5-deazaflavin (V) were prepared according to the reported procedures [Zl]. These compounds were recrystallized five times from ethanol. They were free of fluorescent impurities as judged from the independence of the excitation spectra upon wavelength of fluorescence over the entire fluorescence bands. Ethanol used as solvent was carefully purified by distillation. Its purity was checked on a fluorescence spectrometer at maximum sensitivity and no detectable luminescerice which would interfere with the 5-deazaflavin fluorescence was observed. It is very valuable for the investigation of photochemical processes to obtain the phosphorescence data. The 5-deazallavin derivatives were moreover purified by thin-layer chromatography with chloroform-acetone (1 : 2 v/v) as the developing solvent but the purities were not enough for getting the phosphorescence data. Measurements Absorption spectra were taken on a Hitachi 200-20 recording spectrophotometer. The MCD spectra were 51

52

H.

YAMAGUCHI

measured on a JASCO J-40A recording spectropolarimeter equipped with a 15.2 kG electromagnet. In order to obtain adequate signal to noise, multiple scanning and averaging was accomplished with a computer. Each memory unit in the computer stored the MCD signal for a spectral band of 0.1 nm. Fluorescence spectra were obtained with a Shimadzu RF-502 recording spectrophotometer. Relative quantum yields were obtained from the corrected fluorescence spectra. The correction factors were obtained using the method given by WHITE et al. [22]. The fluorescence quantum yield of riboflavin was calculated based on the fluorescence quantum yield (+) of fluorescein in 0.1 N NaOH when excited at either 366 nm, rj+ = 0.84, or 436 nm, 4 = 0.92[23]. Riboflavin was used as the reference compound for 5-deazatlavins investigated. All samples for relative intensity measurements were kept under the same conditions. The relative quantum yields of I, II, III, IV and V were then estimated using the formula proposed by PARKER and Rnns [24]. This formula can be applied only to sufficiently dilute solution (absorbance less than 0.1). The absorbances of the sample and reference solutions were very nearly equal. Fluorescence quantum yield of riboflavin evaluated using fluorescein as the reference were found to be 0.32* 0.02 in ethanol at 25°C when excited at 436nm. The value is in complete agreement with previous data [6]. The wavelength of all instruments was calibrated at 253.7 and 296.8nm by using a HZ lamp and was accurate to within *O.l nm. All measurements were made on deaerated samples at 25°C. RESULTS

AND

DISCUSION

As the absorption and MCD spectra of I, II, III, IV and V resemble each other, the absorption and

et

al.

MCD spectra of II are presented in Fig. 2 as representative examples. The negative, negative and positive MCD bands correspond to the first (around 25.0 x lo3 cm-‘), second (around 32.0 x lo3 cm-‘) and third (around 40.0~ lO’cm-r) absorption bands, respectively. The molecular symmetry of I, II, III, IV and V is so low that only the Faraday B value is extracted from the MCD spectra by use of the formula: B = -(33.53)-l

(CE’l&) dv d and

where u is frequency in cm-l and [@jr,, is molar ellipticity per unit field in units of deg. 1. m-r mol-’ G-l[ll, 121. The extracted Faraday B value is surnrnarized in Table 1. The plot of the absolute values of the Faraday B values of the first, second and third bands of I, II, JII,.XV and V against the Hammett a,,, values [25] are shown in Fig. 3. As shown in both mono- and di-substituted benzenes [26], there is a good correlation between the magnitude of the Faraday B values of S-deazaflavins and the Hammett a,, values of substituents. Figure 3 indicates the facts that as the Hammett a,,, values increase the Faraday B values of the first and second bands increase and those of the third bands decrease. The quantum mechanical equation for the Faraday B term can be expressed by the magnetic

Wavenumber,

kcm-I

Fig. 2. The MCD (top) and U.V. (bottom) spectra of 3-methyl-5-deazatlavin in ethanol at room temperature.

Magnetic circular dichroism and fluorescence spectra of 5-deaztiavins

53

Table 1. Absorption maximum (v,,,,), intensity (log E) and Faraday B value (B) Compound

I

11

III

IV

V

v ma**

log E

Bt

25090

4.13

0.471

31120

3.99

1.65

37970

4.59

-0.847

44500

4.60

25060

4.07

0.401

31290

3.99

1.61

38090

4.64

-0.910

45568

4.53

25190

4.11

0.349

30690

4.00

1.59

38290

4.56

-0.105

44820

4.54

25530

4.31

29850

3.82

1.52

39080

4.50

-0.110

43197

4.58

24400

4.11

0.318

0.294

33330

3.41

1.46

39800

4.30

-0.108

42900

4.25

* In cm-‘.

t In 10e3 Bohr magneton D* cm-i.

-f 1.8 t

f\

-0.4

-0.2

0

0.2

Fig. 3. The plot of the Faraday B values of the lirst (top), second (centre) and third (bottom) bands against the Hammett up_ values. For numbering system, see Fig. 1.

transition moments, electric transition moments and transition energies among various electronic states [ll, 121. The changes in the Faraday B values induced by substituents may arise from changes in the electric transition moments, changes in the magnetic transition moments and changes in the energy levels. Based on the simple perturbation theory, Mrcru has proposed the equations for the changes of the Faraday B values [14, 15, 191. However, as it is very diicult to obtain the excellent MO’s for II, the changes of the Faraday B values can not be accurately evaluated by using the equations. If the changes in the electric transition, moments might be parallel or nearly so and the changes in the energy levels might be parallel to the changes in the magnetic transition moments in the series of S-deazaflavins, the changes of the Faraday B values might be proportional to the magnitude of changes in the magnetic moments. According to the electromagnetic theory [27], the magnitude of the magnetic moment is proportional to an amount of electron flowing in the framework of molecule. As shown in fervenulins and toxoflavins [16], the frrst band may be originated from a transition related with the electron migration from nitrogen to carbony1 oxygen as shown in (A) of Fig. 4. It may be

H. YAMAGUCHI et al.

54

MeJi+ Me A

I

Me B

R Me C

Fig. 4. The electron migration for the first (top), second (centre) and third (bottom) transitions of 5deazatlavins. considered that the electron migration is enhanced by the electron-withdrawing power of a chlorine atom of I and then the Faraday B value of I becomes larger than the Faraday B value of II. The

migration is interfered by the electron-donating power of methyl, methoxy and hydroxy groups and consequently the Faraday B values of III, IV and V turn out smaller than the Faraday B value of II. As the second band may correspond to the electron migration indicated in (B) of Fig. 4, it may be expected that the substituent effect on the second band is the same as the substituent effect on the first band. The third band may arise from a transition involving the electron migration illustrated in (C) of Fig. 4. In this case, as the presence of the electron-withdrawing chlorine atom of I weakens the migration, it may be considered that the substituent effect on this band is reverse to the substituent effect on the first and second bands. The relative fluorescence quantum yields (4) of I, II, III, IV and V are summarized in Table 2 along with the wavenumber (z+) of the band maxima of the fluorescence spectra. The 6 value of II is in close agreement with the value given by SUN and SONG [S]. As the +f values are comparable to those of flavins (their fluorescence states are the lowest T--~F* states [6]) and the first and second ionization potentials of 3,7,8-trimethyl-5-deazaflavin arise from the T-MO’S [lo], it can be considered that the fluorescence of 5-deazaflavins emits from the lowest 7~---7~*state. Table 2 indicates the fact that the fluorescence quantum yields of 5-deazatlavins are very sensitive to the electron-donating and electron-accepting properties of substituents.

Table 2. Fluorescence maximum (I+) and quantum yield (+) * Compound

*

Vf

"f

I

22370

0.32+0.02

II

22320

0.36tO.02

III

22550

0.43tO.02

IV

23090

0.5ltO.02

V

21740

0.53kO.02

In cm-‘.

[8] M. SUN and P. S. SONG, Biochemistry 12, 4663 (1973). [l] L. D. EIRICH, G. D. VOGELS and R. S. WOLFE, [9] K. NAKANO,T. SUGIMOTOand H. SUZUKI,J. Phys. Biochemistry 17, 4583 (1978). Sot. Jpn 45, 236 (1978). [2] W. T. ASHTON,R. D. BROWN,F. JACOBSONand C. [ 101 J. K. EWEG,F. M-R, H. VAN DAM, A. TERPSTRA and A. OSKAM, J. Am. Chem. Sot. 102, 51 WALSH,J. Am. Chem. Sot. 101,4419 (1979). (1980). [31J. B. THOMAS,Primary Photoprocesses in Biology. Wiley, New York (1965). [ll] A. D. BUC~G~ and P. J. S-HENS, Ann. Reu. [4] G. R. PENZER and G. K. RADDA,Quart. Rev. Chem. Phys. Chem. 17,399 (1969). Sot. 21, 43 (1967). [12]P. N. SCHATLand A. J. MCCAFFREY,Quart. Rev. [5]P. S. SONG,Flauins and FZauoproTeins, (edited by H. Chem. Sot. 23, 552 (1969). Kamin), LI 37. Universitv Park Press. Baltimore. [13] D. CALDWELL, J. M. ‘I~ORNEand H. EYRING,Ann.

rim

(i97i).

[6] M. SUN, T. A. MOORE and P. S. SONG,J. Am. Chem. Sot. 94, 1730 (1972). [7] S. P. SONG,T. A. MOORE and W. E. KURTIN, Z. Naturforsch.

27B,

1011 (1972).

Reu. Phys. Chem. 22, 259 (1971). [14] J. MICHL,J. Am. Chem. Sot. 100,6801,6812,6819 (1978). [15] M. R. WHIWLE, M. VAS.&Kand J. MICHL, J. Am. Chem. Sot. 100,6844 (1978).

Magnetic circulardichroism and fluorescencespectra of 5-deazaavins [16] H. YAMAGUCHI,R. KUWATAand F. YONFDA, J. HeterocyclicChem. 15,615 (1978). [17] M. HIGASHJ and H. YAMAGUCHI, J. Chem. Phys. 70, 2198 (1979). [18] H. YAMAGUCHI,Y. -0~0, T. II(EDA and F. YONEDA.J. Chem. Sot. Faradav _ II, 75, 1506 (1979).

[19] J. MICXIL,Intern.J. Quantum Chem. Symp. 10,107 (1976). [20] P. S. SONG,J. D. CHOI,R. D. FUGATEand K. YAGI, Flauins and Flauoproteins,(edited by T. P. Singer), ch. 40. Elsevier Scientific, Amsterdam (1976). [21] F. YONEDA, K. MORI, Y. SAKUMA and H. YAMAGUCHI, J. Chem. Sot. Parkin I, 978 (1980).

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[22] C. E. WHITE,M. Ho and E. Q. WEIMER,Anal. Chem. 32, 438 (1960). [23] W. R. DAWSON and M. W. WINDSOR, J. Phys. Chem. 72, 3251 (1968). [24] C. A. PARKERand W. T. REES,Analyst 85, 587 (1960). Z$e Modem StructuralTheory of [‘=I L. N. FERGUSON, Organic Chemistry. Prentice-Hall, Englewood Cliffs, NJ (1963). [26] J. G. Foss and M. E. MCCARIXLLE, J. Am. Chem. sot. 89, 30 (1967). 1271 H. EYRING,J. WALTERand G. E. KIMBALL,Quantum Chemistry, p. 127. John Wiley, New York, N.Y. (1960).