Xiguang Zheng

Studies on the energy transfer among the rod-core complex from phycobilisome of Anabaena variabilis by time resolved fluorescence emission and anisotropy spectra

Abstract Picosecond time-resolved fluorescence emission and polarization spectra were recorded for the rod-core complex ( α β ) 6 PC L RC 27 ( α β ) 3 AP L C 8.9 from cyanobacterium Anabaena variablis . Fluorescence decay was measured by single-photon timing and resolved to kinetic components by global data analysis with two different excitation wavelengths at 580 nm and 640 nm. Fluorescence anisotropies were recorded and analyzed at different excitation/emission pairs, such as, 590 nm/625 nm, 625 nm/640 nm, 635 nm/652 nm and 650 nm/665 nm. It was expected through this work to investigate the energy transfer processes between C-PC and APC, as well as those in a C-PC hexamer. Our data indicated that the rod-core complex acted as a functional entity with respect to excitation energy transfer. The kinetic component of 18 ps, observed with excitation at 580 nm and 640 nm, was interpreted as the excitation energy transfer from the three β 84 chromophores of terminal C-PC trimer to an α 84 chromophore in the APC core. The 55 ps component, observed with excitation at 580 nm, most likely represent the transfer from β 155 -PCB to β 84 -PCB in a C-PC monomer. The component of 110 ps, observed with excitation at 640 nm, may originate from the transfer between α 84 - and β 84 -PCB chromophores in a APC monomer. The 10 ps component was assigned to the energy transfer processes between the identical β 155 -PCB or α 84 -PCB chromophores in two trimers while the 40±2 ps and 45 ps components to those among the three β 84 -PCB chromophores in a C-PC trimer and an APC trimer, respectively.

DIRECT MEASUREMENT OF EXCITATION TRANSFER DYNAMICS BETWEEN TWO TRIMERS IN C-PHYCOCYANIN HEXAMER FROM CYANOBACTERIUM ANABAENA VARIABILIS

Abstract We provide the first experimental evidence for the excitation transfers between two trimers of an isolated C-phycocyanin hexamer (αβ) 6 PC L RC 27 , at the end of the rod proximal to the core of PBS in cyanobacterium of Anabaena variabilis , with picosecond time-resolved fluorescence spectroscopy. Our results strongly suggest that the observed fluorescence decay constants around 20 and 10 ps time scales, shown in anisotropy decay, not in isotropic decay experiments arose from the excitation transfers between two trimers via two types of transfer pathways such as 1β 155 ↔6β 155 (2β 155 ↔5β 155 and 3β 155 ↔4β 155 ) and 2α 84 ↔5α 84 (3α 84 ↔6α 84 and 1α 84 ↔4α 84 ) channels and these could be described by Foster dipole-dipole resonance mechanism.

Studies of the Energy Transfer among Allophycocyanin from Phycobilisomes of Polysiphonia urceolata by Time‐Resolved Fluorescence Isotropic and Anisotropic Spectroscopy

Abstract— The excitation energy transfer processes in the allophycocyanin (APC) monomer and trimer from phycobilisomes of Polysiphonia urceolata were studied using picosecond time‐resolved fluorescence isotropic and anisotropic spectroscopy. Based on our experimental results, conclusions could be drawn as follows: (1) After the processes of exciton localization are finished, the localized excitation energy on any chromophore can be transferred to the other chromophores due to the weak couplings between them, and the processes among three p84‐phycocyaninbilin (PCB) chromophores in the center of the ring shape of the APC trimer are more important than those of between a84‐ and p84‐PCB chromophores in the same monomer. (2) The decay time constants of 95 ± 5 ps and 40 ± 5 ps components, observed by us in this work, were assigned to the excitation energy transfer or redistribution between α84‐ and β84‐PCB chromophores in the same monomer of the APC trimer and among three β84‐PCB chromophores in the center of the ring shape of the APC trimer, respectively. Specifically, the assignment of the decay constants for the 40 ± 5 ps component was different from those of previous results. (3) Based on the model of Debreczeny, and using the fluorescence residual anisotropy r(∞) with a probing wavelength of 650 nm, the angles between the C3 symmetry axis and transition dipoles of α84‐ and ‐PCB chromophores were found to be φa84= 67° and φβ84= 148°, respectively, which are in agreement with the prediction of the X‐ray crystal structure of APC. (4) The results show that anisotropy decays, observed with the APC trimer, did exhibit a strongly probing wavelength dependence that did not show up in the monomer.

ENERGY TRANSFER IN ALLOPHYCOCYANIN HEXAMER FROM ANABAENA VARIABILIS BY TIME-RESOLVED SPECTROSCOPY

Abstract The process and mechanism for energy transfer in an allophycocyanin hexamer from Anabaena variabilis have been studied by time-resolved fluorescence spectroscopy. Deconvolution of isotropic time-resolved fluorescence spectra shows that the energy transfer time between two allophycocyanin trimers is about 14 ps, while the energy transfer time from the linker polypeptide (L cm 42 ) to the final emission is about 65 ps. The anisotropic results of time-resolved fluorescence spectra show that the energy transfer process among three ring-like ( α - 84 / α - 84 ) chromophore pairs still plays an important role with a time constant of about 45 ps. The facts support the idea that the mechanism for energy transfer in the allophycyanin hexamer should be described by Foster dipole—dipole interaction rather than by an exciton interaction.

Model of chromophore coupling in allophycocyanin aggregation

A new model is proposed to study the interaactions among chromphores in APC trimer. Using this model, the electronic transition frequencies are calculated. The results are in good agreement with the experimental.

EXCITATION ENERGY TRANSFER MECHANISMS IN ALLOPHYCOCYANIN (I) -ENERGY TRANSFER MECHANISMS IN MONOMER

The mechanisms of excitation energy transfer within allophycocyanin monomer with the theory of generalized master equation (GME) and the technique of time-resobed fluorescence anisotropic spectroscopy are studied. In the case of known information of its structure and spectra, the theory applied is based on the assumption that the coupling interaction between two chromophores is fairly weak. The theory correctly predicts the experimentlly observed rate for excitation energy transfer in allophycocyanin monomer. Based on the results, the energy transfer mechanism can be described as Förster and these processes cannot take place from the high vibrational levels of donor to acceptor.

Synthesis and energy transfer of model conjugate of phycobilisome rods

The conjugate of R-phycoerythrin (R-PE) and C-phycocyanin (C-PC) was synthesized through a heterobifunctional coupling reagent, N-succinimidyl 3-(2-pyridyldithio) propionate. The molar ratio of R-PE to C-PC was determined by absorption spectra, and the result was 2:1. The energy transfer phenomena were observed from steady-state fluorescence spectra. The calculated result showed that the energy transfer efficiency from R-PE to C-PC was 88%. The energy transfer kinetics was determined by picosecond time-resolved fluorescence spectra. The time constant of energy transfer from R-PE to C-PC was 80 ps, which was much longer than that in the rod of native phycobilisomes.

Ultrashort pulse laser microsurgery on cell

A laser microbeam system has been set up for microsurgery on cell. The relations of laser wavelength, pulse duration and pulse energy to punching effects and self-healing are studied. The experimental results demonstrate that picosecond pulse laser microbeam offers many advantages in cell microsurgery. The mechanism of punching by picosecond microbeam is high field puncture instead of heat effect, and is irrelevant to cell kinds and colors. The diameter and depth of microsurgery can therefore be easily controlled by adjusting the laser pulse energy. The diameter of the minimum aperture is about 0.1 micrometers , much smaller than the theoretical limit ((lambda) /2) for optical microscope due to self- focusing effect. With ultrashort pulse laser microbeam, we can easily cut off any part of a cell. An example is that eight nuclei in the center of unicellular parasite Pneumocystis Carinii can be destroyed one by one by ultrashort pulse laser microbeam without cell wall injury. The holes can also be punched by ultrashort pulse laser microbeam from cell wall to cell nucleus. In a fraction of a second to several seconds after punching, the hole on cell wall or cell membrane can self-heal. Exogenous DNA can be introduced into the cell before its self- healing.

Dynamics of excitation energy transfer in phycobiliproteins

Theoretical descriptions of excitation energy transfer between chromophores in allophycocyanin are presented, including bilateral energy transfer paths between chromophores, and are expressed, based on Foster interaction mechanism, as Pauli master equations. Group analysis in C3 symmetry is performed to carry out analytic expressions for fluorescence decays which is generally of triexponential with effects of chromophore coupling and exciton splitting taken account. It is pointed out that the time constant of each decay component contains mixed information of different energy transfer paths, and therefore show its dependence on subtle configuration of chromophores, probably related to site heterogeneity and thus to inhomogeneous broadening previously observed.

Exciton collision and fluorescence quenching in R-phycoerythrin

Single ps pulse spectroscopic techniques have been employed to investigate excited-state dynamics of R-phycoerythrin (R-PE) isolated from two red algae Polysiphonia urceolata and Porphyra yezoensis, respectively, at various excitation photon densities in a range of 8 X 1014