Change In Permanent Dipole Moment Research Articles

The CP43 Core Antenna Complex of Photosystem II Possesses Two Quasi-Degenerate and Weakly Coupled Qy-Trap States

The CP43 chlorophyll a-core protein complex plays an important role in funneling excitation energy absorbed by more peripheral antenna complexes of photosystem II (PSII) to the reaction center (RC). Identification and characterization of the lowest energy Qy-states of CP43 is important for understanding the kinetics of excitation energy transfer (EET) from CP43 to the RC. We report the results of several types of spectroscopic experiments performed at liquid He temperatures on the isolated CP43 complex from spinach. Nonphotochemical hole burning (NPHB) and triplet bottleneck hole burning spectroscopies as well as zero-phonon hole (ZPH) action and Stark hole burning spectroscopies were employed. Two quasi-degenerate trap states at 682.9 nm (B state) and 683.3 nm (A state) are identified. The widths of their mainly inhomogeneously broadened Qy-absorption bands are 45 and 120 cm-1, respectively. The uncorrelated site excitation distribution functions (SDF) of the two states are nearly the same as their absorption bands since the electron−phonon coupling is weak (optical reorganization energies of ∼6 cm-1). The NPHB spectra establish that the B state is the primary trap for EET from higher energy Qy-states. The permanent dipole moment change (Δμ) of the S0 → Qy transition for both the B and A states is small, f·Δμ = 0.25 ± 0.05 and 0.47 ± 0.05, respectively, where f is the local field correction factor. These values, together with the weak electron−phonon coupling and other results, indicate that both states are highly localized on a single Chl a molecule. Holewidth measurements led to the remarkable finding that the rates of A → B and B → A EET processes are extremely slow, ∼(6 ns)-1. This suggests that the Chl a molecules of the two states belong to different layers of Chl a molecules located at opposite sides of the membrane. The intriguing question of why CP43 possesses two quasi-degenerate trap states that are so weakly coupled is addressed. The possibility that they play a role in the photoinhibitory and photoregulatory processes is raised.

Stark effect spectroscopy of tryptophan

The change in permanent dipole moment (magnitude of delta mu) for the transition from the 1La state to the ground state of tryptophan is the key photophysical parameter for the interpretation of tryptophan fluorescence spectra in terms of static and dynamic dielectric properties of the surrounding medium. We report measurement of this parameter by means of electric field effect (Stark) spectroscopy for N-acetyl-L-tryptophanamide (NATA) in two solvents, the single tryptophan containing peptide melittin, and 5-methoxytryptophan. The values ranged from 5.9 to 6.2 +/- 0.4 Debye/f for NATA and melittin, where f represents the local field correction. The 1Lb magnitude of delta mu was much smaller. Application of Stark spectroscopy to these chromophores required decomposition of the near-UV absorption into the 1La and 1Lb bands by measurement of the fluorescence excitation anisotropy spectrum and represents an extension of the method to systems where band overlap would normally preclude quantitative analysis of the Stark spectrum. The results obtained for 5-methoxytryptophan point out limitations of this method of spectral decomposition. The relevance of these results to the interpretation of steady-state and time-resolved spectroscopy of tryptophan is discussed.

Matrix-induced and permanent dipole moment of Zn-octaethylporphin in polyvinylbutyral

In this paper we propose to use the Stark-effect technique in combination with persistent spectral hole burning in order to study some structural aspects of metalloporphyrin molecules. Here we focus on the comparison between free base octaethylporphin and the metal analogue Zn-octaethylporphin. This comparison shows that the experimental data can be explained with a model where the metal atom is in an out-of-plane position and causes a permanent dipole moment change of Δμ = 0.15 ±- 0.03 D. These findings are in agreement with some earlier interpretations of X-ray data on porphyrin and heme-systems.

Stark effect spectroscopy of carotenoids in photosynthetic antenna and reaction center complexes

The effects of electric fields on the absorption spectra of the carotenoids spheroidene and spheroidenone in photosynthetic antenna and reaction center complexes (wild-type and several mutants) from purple non-sulfur bacteria are compared with those for the isolated pigments in organic glasses. In general, the field effects are substantially larger for the carotenoid in the protein complexes than for the extracted pigments and larger for spheroidenone than spheroidene. Furthermore, the electrochromic effects for carotenoids in all complexes are much larger than those for the Qx transitions of the bacteriochlorophyll and bacteriopheophytin pigments which absorb in the 450-700 nm spectral region. The underlying mechanism responsible for the Stark effect spectra in the complexes is found to be dominated by a change in permanent dipole moment of the carotenoid upon excitation. The magnitude of this dipole moment change is found to be considerably larger in the B800-850 complex compared to the reaction center for spheroidene; it is approximately equivalent in the two complexes for spheroidenone. These results are discussed in terms of the effects of differences in the carotenoid functional groups, isomers and perturbations on the electronic structure from interactions with the organized environment in the proteins. these data provide a quantitative basis for the analysis of carotenoid bandshifts which are used to measure transmembrane potential, and they highlight some of the pitfalls in making such measurements on complex membranes containing multiple populations of carotenoids. The results for spheroidenone should be useful for studies of mutant proteins, since mutant strains are often grown semi-aerobically to minimize reversion.

Stark effect in P700, the primary electron donor of Photosystem I

Quadratic Stark effect in CP1 pigment-protein complex was examined at low temperatures in the red spectral region. The Stark spectra of samples containing P700 in reduced form exhibit a strong negative band at 704 nm, which disappears on chemical oxidation of P700. The change in permanent dipole moment, Δμ, of P700 on electronic excitation estimated from these spectra was found to be between 4.7 and 7.7 Debye units. It is suggested to reflect the charge-transfer contribution to the excited state of P700. For antenna chlorophyll, Δμ≌ 1 D was obtained in accordance with the data for monomeric chlorophyll.

ChemInform Abstract: VIBRONIC EFFECTS IN THE 7000 A STATE OF AZULENE

Vibronic perturbations in the 7000 A S1(B1) ← S0(A1) absorption system of azulene due to a large number of b1 vibrations have been identified. The total induced oscillator strength for this transition is 1.2×10−3 while the allowed oscillator strength is 7.8×10−3. Accurate intensities for the b1 vibronic origins relative to the allowed origin are reported. Calculations are presented which indicate that the two most probable coupling routes for the vibronic interactions (intensity) involve the second excited 1A1 state (S4) and the ground state, S0. The latter route exists due to the large change in permanent dipole moment between S0 and S1, ∼ 2 D. Assignments for all 12 of the excited state a1 fundamentals (excluding CH) are given. The differences between the 7000 A 1B1 ← 1A1 and 3500 A 1A1 ← 1A1 absorption systems are discussed in relation to the anomalous fluorescence behavior of azulene.

Vibronic effects in the 7000 Å state of azulene

Vibronic perturbations in the 7000 Å S1(B1) ← S0(A1) absorption system of azulene due to a large number of b1 vibrations have been identified. The total induced oscillator strength for this transition is 1.2×10−3 while the allowed oscillator strength is 7.8×10−3. Accurate intensities for the b1 vibronic origins relative to the allowed origin are reported. Calculations are presented which indicate that the two most probable coupling routes for the vibronic interactions (intensity) involve the second excited 1A1 state (S4) and the ground state, S0. The latter route exists due to the large change in permanent dipole moment between S0 and S1, ∼ 2 D. Assignments for all 12 of the excited state a1 fundamentals (excluding CH) are given. The differences between the 7000 Å 1B1 ← 1A1 and 3500 Å 1A1 ← 1A1 absorption systems are discussed in relation to the anomalous fluorescence behavior of azulene.