J . A m . Chem. SOC.1989, 11 1 , 6190-6196
6190
Photoinduced Electron Transfer on a Single a-Helical Polypeptide Chain Masahiko Sisido,*,t Ryo Tanaka,* Yoshihito Inai,f and Yukio Imanishif Contribution from the Research Center for Medical Polymers and Biomaterials and Department of Polymer Chemistry, Kyoto University, Sakyo- ku, Kyoto 606, Japan. Received January 27, 1989
Abstract: Electron transfer on an a-helical polypeptide carrying the sequence L-p(dimethy1amino)phenylalanine (dmaPhe)-L-alanine-L- 1-pyrenylalanine (pyrAla) at the midpoint of an a-helical poly(y-benzyl L-glutamate) chain was studied. Conformational energy calculation for the side-chain orientations predicted that only one type of orientation is allowed for both the dmaPhe and the pyrAla units. The center-to-center (edge-to-edge) distance between the two chromophores was estimated to be 13.2 (9.4) A. The fluorescence spectrum showed no exciplex emission in the polypeptide, in contrast to the strong exciplex observed for a model tripeptide having the same dmaPhe-Ala-pyrAla sequence. The rate of electron transfer was calculated from the decay times of pyrenyl fluorescence of the polypeptide in trimethyl phosphate and in tetrahydrofuran solutions. The k,, was on the order of los (8).The activation enthalpy was 1.4 kcal mol-l in trimethyl phosphate and smaller than 1 kcal mol-‘ in less polar solvents near room temperature. It was even smaller at lower temperatures. The activation entropy was less than -25 eu, suggesting a nonadiabatic electron transfer. In contrast to the slow electron transfer in the polypeptide, the rate constant for the model tripeptide was on the order of 107-108 (s?) around room temperature, and the activation enthalpy was higher than that in the polypeptide case.
Electron transfer (ET) in biological systems has been studied
(CH313CO
-CO -NHCHCO-NHCHCO-NHCHCO-
I
on modified proteins carrying metal c~mplexesl-~ and on synthetic polypeptides carrying an electron donor-acceptor pair a t both ends of a The protein system is complicated because of the presence of side-chain aromatic groups that work as electron mediators and the ill-defined structures of the polypeptide main chain. The structure of the synthetic polypeptide system is much simpler and well-defined. Therefore, synthetic polypeptides are suited for obtaining basic information on the ET. However, as recently pointed out by Schanze and Sauer,’ the terminal portions of an a-helical polypeptide chain are often unfolded and the end-to-end distance or the terminal donor-acceptor distance becomes statistical. In this article, we will report the first attempt to attach an organic electron donor, p-dimethylanilino (D) group, and an organic photosensitizer, pyrenyl (P) group, a t the midpoint of an a-helical poly(y-benzyl L-glutamate) chain (I). The chromophores are linked to the main chain by the shortest, spacer (methylene group) to minimize their orientational freedom. H-(NHCHCOh-NHCHCO-NHCHCO-NHCHCO-
I
(CHz)2 I
I
FH2
I CH3
I
CHz
I
I NHCHCO), -0CH2
I
H3C’
OCH3
I
CH3
CHz
CH2
I
‘CH3
11.Boc - dma P he - A I a - p y r A I a - OM e (B DP M )
lypeptides carrying only one pyrenyl group (111) and one (dimethy1amino)phenyl group (IV), respectively, were also prepared. H(NHCHCO)n-NHCHCO
-(NHCHCO)4-OCHZ
I
I CH2
I (CH2)2
(CH2)2
I
I
I I
co
I I
I
0
I
CH2
(CH2)2
I
CH2 I
n=45
111,G l u ( O B z l ) 4 5 - p y r A l a - G l u ( O B z
l)4-
OBzl (GnPG4)-
n = 45
I ,G I u l O E ~ l ) ~ ~ - d m a P hAel a-- ~yrAla-Glu(OBzl)~ - 0 B z I (GnOAPG4)
Poly(y-benzyl L-glutamate) is a typical polypeptide that takes an a-helical conformation in various solvents. The tetrapeptide unit of y-benzyl L-glutamate on the right-hand side of I corresponds to one turn of the a-helix and is expected to stabilize the a-helical conformation a t the pyrAla unit. As a flexible model compound, an oligopeptide carrying the same D-P pair (11) was also synthesized. To examine the intrinsic property of the pyrenyl and (dimethy1amino)phenyl groups fixed on the polypeptide chain, po*To whom correspondence should be addressed at the Research Laboratory of Re ources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Mido .ku, Yokohama 227, Japan. ‘R ,earth Center for Medical Polymers and Biomaterials. *Department of Polymer Chemistry.
0002-7863/89/1511-6790$01.50/0
co I I
Q
i
0
I
co 0
I
n = 23 I V , G l u ( 0 B z l )23-dmaPhe- Glu(0Bz I ) 4
- OBzl
(Gn DG4)
(1) McLendon, G. Acc. Chem. Res. 1988, 21, 160, and references cited therein.
0 1989 American Chemical Society
J. Am. Chem. SOC.. Vol. 111, No. 17, 1989 6191
Electron Transfer on a n a - H e l i x
7
1
30 I
I
i
i
Wavelength (nml
u
Figure 1. Absorption spectra of polypeptide I (-),
tripeptide I1 (- -), polypeptide 111 (-), polypeptide IV (-*-), and the sum of the spectra of 111 and IV ( - - - ) in TMP at room temperature. The polypeptide concentrations were determined by the absorbance at 345 nm using the = 4.47 X lo4). molar absorption coefficient of the tripeptide
-2?90
250
210 230 Wavelength (nm)
Figure 2. CD spectra of polypeptide I (-), polypeptide 111 (- - -), and tripeptide 11 in TMP at room temperature. The ordinate is the value with respect to the molar concentration of amide group. (-e-)
Results and Discussion Synthesis and Characterization of the Samples. The polypeptide I was prepared by the polymerization of y-benzyl L-glutamate N-carboxyanhydride [Glu(OBzl)NCA] by using an oligopeptide (V) as the initiator. The oligopeptide with a (tert-butyloxy)-
300
1
H-NHCHCO-NHCHCO-NHCHCO-(NHCHCO)d-OCH2
240
t
I CH2
I
I
c H3
I
CH2
I
I x N
IBCt
I 260 O
v carbonyl (Boc) protecting group a t the terminal nitrogen was synthesized by a conventional liquid-phase method and characterized by IR, 'H N M R , and UV spectra and by elemental analysis. After the Boc group was removed, the oligopeptide was mixed with the N C A in dimethylformamide. The polymerization was completed after 4 days. The number-average degree of polymerization of the poly(y-benzyl L-glutamate) unit, E, in the final polypeptides was determined from the absorption intensity ~ , of ~ the of the benzyl group in the UV spectruym ( E ~ =~ 218) polypeptide (difference between the bold-faced solid line and the bold-faced broken line in Figure 1). The model oligopeptide I1 was synthesized similarly by a liquid-phase method. The polypeptides 111 and IV were prepared similarly by the polymerization of Glu(OBz1)NCA using the corresponding oligopeptide as the initiator. Absorption and Circular Dichroic Spectra. The UV absorption spectra of the polypeptides and the oligopeptide were measured (2) McLendon, G.; Pardue, K.; Bak, P. J . Am. Chem. SOC.1987, 109, 7540. (3) (a) Isied, S. S.;Kuehn, C.; Worosila, G. J. Am. Chem. SOC.1984, 106, 1722. (b) Bechtold, R.; Gardineer, M. B.; Kazmi, A.; van Hemelryck, B.; Isied, S. S. J. Phys. Chem. 1986, 90, 3800. (4) (a) Axup, A. W.; Albin, M.; Mayo, S. L.; Crutchley, R. J.; Gray, H. B. J. Am. Chem. SOC.1988, 110,435. (b) Karas, J. L.; Lieber, C. M.; Gray, H. B. J . Am. Chem. SOC.1988, 110, 599. ( 5 ) Peterson-Kennedy, S. E.; McGourty, J. L.; Hoffman, B. M. J . Am. Chem. SOC.1984, 106, 5020. (6) (a) Isied, S. S.; Vassilian, A. J. Am. Chem. SOC.1984, 106, 1726. (b) Isied, S. S.;Vassilian, A.; Magnuson, R. H.; Schwarz, H. A. J . Am. Chem. SOC.1985, 107, 7432. (c) Isied, S. S.; Vassalian, A.; Wishart, J. F. J . Am. Chem. SOC.1988, 110, 635. (7) Schanze, K. S.; Sauer, K. J . Am. Chem. SOC.1988, 110, 1180.
I
0' 0
I
60
I
I
180
120
1
I
I
240
300
360
XI
Figure 3. Energy contour map for the side-chain orientation of the in an a-helical pyrenyl group in A~-(Ala)~-pyrAla-(Ala)~-Nh(CH,) main-chain conformation. The interval of the contour lines is 0.5 kcal mol-'. in trimetyl phosphate ( T M P ) (Figure 1). The spectrum of I is essentially the same as the sum of the spectra of 111 and IV. This indicates that the D and P chromophores are properly introduced in the polypeptide and that any strong ground-state electronic interaction is absent between the D and P groups in polypeptide I. The ground-state interaction was also not seen in the model oligopeptide 11. Therefore, the absence of the ground-state interaction is not a result of the geometry of the D-P pair fixed on the polypeptide chain but of the electronic property of the D-P pair in T M P . Circular dichroic (CD) spectra of the polypeptides and the model peptide in the far-UV region are shown in Figure 2. The C D patterns of the polypeptides are those of a right-handed ahelix. The A€ values a t 222 nm (-1 1.6 for I and -10.3 for 111) are very close to the value reported for 100% a-helical polypeptides (A€ = -10.8 to -12).* Conformational Analysis. The C D spectra indicated a righthanded a-helical main chain of the polypeptides I and 111. T o estimate the most probable orientsltions and the extent of thermal fluctuations of the side-chain chromophores, conformational energy calculation was carried out assuming the a-helical main chain (4 = - 5 7 O , $ = - 4 7 O , w = 180°).17 For simplicity, the Glu(OBz1) (8) Woody, R. W. J . Polym. Sci., Macromol. Rev. 1977, 12, 181.
6792 J . Am. Chem. SOC.,VolaI 1 I, No. 17, 1989
Sisido et al.
was 9.26 A, indicating that the thermal fluctuation does not alter the interchromophore distance significantly. As for the thermal fluctuation of the helical main chain, energy 30C contour maps were calculated for three pairs of rotational angles #in A~-(Ala),-dmaPhe-Ala*-pyrAla-(Ala)~-NH(CH~): (dmaPhe)-d(Ala*), d(Ala*)-#(Ala*), and $(Ala*)-d(pyrAla). 240 The a-helical conformation was assumed for the rest of the main chain, and the side chains were set to be in the minimum-energy orientations. The three maps showed only one allowed region at 180 the rotational angles corresponding to the right-handed a-helical X N conformation. The range of thermal fluctuation is very small (
