The electronic absorption line shape and Stark spectrum of the lowest energy Q y transition of the special pair in bacterial reaction centers contain a wealth of information on mixing with charge transfer states and electronic asymmetry. Both vary greatly in mutants that perturb the chemical composition of the special pair, such as the heterodimer mutants, and in mutants that alter interactions between the special pair and the surrounding reaction center protein, such as those that add or remove hydrogen bonds. The conventional and higher-order Stark spectra of a series of mutants are presented with the aim of developing a systematic description of the electronic structure of the excited state of the special pair that initiates photosynthetic charge separation. The mutants L168HF, M197FH, L131LH and L131LH/M160LH/ M197FH are known to have different hydrogen-bonding patterns to the special pair; however, they exhibit Stark effects that are very similar to wild type. By contrast, the addition of a hydrogen bond to the M-side keto carbonyl group of the special pair in M160LH greatly affects both the absorption and Stark spectra. The heterodimer special pairs, L173HL and M202HL, exhibit much larger Stark effects than wild type, with the greatest effect in the M-side mutant. Double mutants that combine the M-side heterodimer and a hydrogen-bond addition to the L-side of the special pair decrease the magnitude of the Stark effect. These results suggest that the electronic asymmetry of the dimer can be perturbed either by the formation of a heterodimer or by adding or deleting a hydrogen bond to a keto carbonyl group. From the pattern observed, it is concluded that the charge transfer state P L + P M -has a larger influence on the excited state of the dimer in wild type than the P L -P M + charge transfer state. Furthermore, asymmetry can be varied continuously, from extreme cases in which the heterodimer and hydrogen-bond effects work together, to cases in which hydrogen bonding offsets the effects of the heterodimer, to cases in which the homodimer is perturbed by hydrogen bonds. This leads to a unified model for understanding the effects of perturbations on the electronic symmetry of the special pair, and this can be connected with perturbations on the properties of many other systems such as donor-acceptor-substituted polyenes.Photoexcitation of a strongly interacting pair of bacteriochlorophyll (BChl) 1 molecules called the special pair or P in bacterial photosynthetic reaction centers (RCs) initiates a series of light-driven charge separation reactions. The singlet excited state of P, 1 P, transfers an electron within a few picoseconds to H L , a bacteriopheophytin (BPhe) molecule that is the initial electron acceptor (1). As shown in the upper part of Figure 1, there are two similar sets of chromophores related by a local C 2 axis of symmetry that can serve as electron acceptors, yet only the chromophores on the L side (the right side as illustrated, sometimes denoted the A branch) participate in the i...

Stark spectra for nonoriented molecules with typical broad inhomogeneous line widths are obtained by applying an ac electric field and detecting the change in absorbance at the second harmonic of the field modulation frequency. The analysis of such data in terms of molecular parameters such as changes in dipole moment and polarizability is often difficult. We outline a new method, called higher-order Stark spectroscopy, in which the higher even-powered harmonics of the response to an ac field are measured. The method is applied to electronic states of photosynthetic pigments, including pure monomeric bacteriochlorophyll, the homoand heterodimer special pair primary electron donors in photosynthetic reaction centers, and the carotenoid spheroidene, both pure in an organic matrix and in the B800-850 antenna complex. It is shown that even at a qualitative level these systems divide into two groups: those where the change in dipole moment dominates the change in polarizability (pure bacteriochlorophyll, the heterodimer, and spheroidene in the antenna complex) and those where the polarizability dominates the change in dipole moment (pure spheroidene and the homodimer special pair).A quantitative analysis of the Stark effect spectrum provides information on molecular properties associated with the movement of charge such as the change in dipole moment (&), polarizability (Aa), and hyperpolarizability for an electronic or vibrational2 transition. For a uniaxially oriented system, the contributions from Ap and A a can be readily distinguished as the former depends linearly on the field, while the latter depends quadratically on the field. However, for isotropic, immobilized samples (frozen glasses or polymer films), which are far easier to study and are often the only conditions under which samples can be studied, the contributions from Ap and Aa, as well as all other electrooptic parameters, depend on the same power of the field; consequently, the contributions can only be obtained by analyzing the Stark line shape. In the following we outline a new experimental method, called higher-order Stark spectroscopy, for obtaining more information than was previously possible about the electrooptic properties of molecules. We specifically apply this method to several photosynthetic pigments. By comparing the higher-order Stark spectra for several different types of chromophores in different environments, it is possible, even at a qualitative level. to obtain information on the relative contributions of Ap and Aa. We find directly that the first electronic excited state of the special pair primary electron donor P in photosynthetic reaction centers (RCs) has a very large polarizability, a conclusion which was previously suggested from conventional low-temperature Stark spectroscopy via a complex line shape analysi~.~ In a subsequent paper, we will show that a quantitative analysis of the higher-order Stark spectra can be used to obtain the diagonal elements of the polarizability tensor of P, and this information can be used to ...

Higher order Stark spectroscopy has recently been introduced and applied to characterize the electrooptic properties of chromophores in bacterial photosynthetic reaction centers. 1 In the course of these studies, an unusually large and broad higher order Stark effect with a novel line shape was discovered in the region of the monomeric bacteriochlorophyll absorption band. The origin of this new feature has been explored by comparing results from reaction centers in which the chromophores are modified or the environment around the chromophores has an altered amino acid residue composition. Taken together, these results demonstrate that this unusual higher order Stark effect is related to both the monomeric bacteriochlorophyll and bacteriopheophytin on the electron-transfer pathway of the reaction center. The effects of mutations and the oxidation of the special pair on this signal specifically suggest the involvement of charge-separated species between these monomeric chromophores. In part 2 (following paper in this issue) we develop a general treatment of this phenomenon based on a charge resonance interaction between a strongly allowed transition and a charge-separated state. This leads to a variety of predicted higher order Stark line shapes which span the range observed in part 1 and from which we can obtain information on these potentially important, but heretofore experimentally inaccessible, charge-separated states. There has been extensive experimental and theoretical work directed toward a deeper understanding of the mechanism of the initial electron-transfer steps in photosynthesis. A schematic diagram of the chromophore arrangement derived from the X-ray structure 2,3 is shown in Figure 1. Two closely interacting bacteriochlorophylls (BChls) form the dimeric special pair P, which is the primary electron donor. There are two accessory BChls designated B, two bacteriopheophytins (BPhe) designated H, and two quinones (not shown). Despite the structural pseudo C 2 symmetry obvious from the structure, electron transfer occurs predominantly along the L-branch of monomeric chromophores, as illustrated with a quantum yield approaching unity. 4-6 This extremely efficient and unidirectional electron transfer has stimulated much experimental and theoretical interest. 7-10 The role of the accessory BChl on the functional side (B L) in facilitating the electron-transfer reaction from 1 P to P + H L-is an unresolved problem. Two limiting mechanisms have emerged: a two-step mechanism in which electron transfer occurs sequentially from 1 P to form the intermediate state P + B Land subsequently P + H L-11,12 and a direct one-step electron transfer from 1 P to form P + H L-, where the P + B L-state serves as a virtual intermediate to enhance the electronic coupling between 1 P and P + H L-by superexchange. 10,13,14 In either case, the B L molecule plays a crucial role, whether as P + B L-or B L + H L-. 15,16 The role of B L no doubt is also critical to understanding the origin of unidirectional electron transfer. All in...

The electronic absorption spectrum of the homodimeric special pair in bacterial photosynthetic reaction centers is greatly modified when one of the bacteriochlorophylls is replaced by a bacteriopheophytin to form a heterodimer. The absorption exhibits further unusual changes when hydrogen bonds from the protein to conjugated carbonyl groups are added or removed. In order to explain these absorption features, we postulate a charge resonance interaction between the lowest energy exciton state of the special pair and an intradimer charge transfer state. A general theory is developed that is closely related to the formalism of Fano's treatment for atomic absorption line shapes associated with autoionization (Fano, U. Phys. ReV 1961 124, 1866). Three different charge resonance limits are discussed, which depend on the relative magnitudes of the electronic coupling between the exciton state and charge transfer state and the vibronic bandwidth of the charge transfer state. In the intermediate charge resonance limit, two broad bands are predicted, and this corresponds closely to the unusual absorption line shape observed for the heterodimer. Furthermore, the systematic variations in the absorption line shapes for four different heterodimer/hydrogen bond mutants can be satisfactorily explained by shifting the relative energies of the exciton and charge transfer states. This leads to the conclusion that the BChl + BPheintraheterodimer charge transfer state is primarily responsible for the charge resonance interaction, providing information on the absolute energy of this functionally-relevant state and the electronic coupling. This treatment is generally applicable to absorption line shapes in related systems and can be used to provide a unified treatment of the absorption spectra of a large variety of available perturbed homo-and heterodimers.

Higher order Stark spectroscopy has recently been introduced and applied to characterize the electrooptic properties of chromophores in bacterial photosynthetic reaction centers. 1 In the course of these studies, an unusually large and broad higher order Stark effect with a novel line shape was discovered in the region of the monomeric bacteriochlorophyll absorption band. The origin of this new feature has been explored by comparing results from reaction centers in which the chromophores are modified or the environment around the chromophores has an altered amino acid residue composition. Taken together, these results demonstrate that this unusual higher order Stark effect is related to both the monomeric bacteriochlorophyll and bacteriopheophytin on the electron-transfer pathway of the reaction center. The effects of mutations and the oxidation of the special pair on this signal specifically suggest the involvement of charge-separated species between these monomeric chromophores. In part 2 (following paper in this issue) we develop a general treatment of this phenomenon based on a charge resonance interaction between a strongly allowed transition and a charge-separated state. This leads to a variety of predicted higher order Stark line shapes which span the range observed in part 1 and from which we can obtain information on these potentially important, but heretofore experimentally inaccessible, charge-separated states.There has been extensive experimental and theoretical work directed toward a deeper understanding of the mechanism of the initial electron-transfer steps in photosynthesis. A schematic diagram of the chromophore arrangement derived from the X-ray structure 2,3 is shown in Figure 1. Two closely interacting bacteriochlorophylls (BChls) form the dimeric special pair P, which is the primary electron donor. There are two accessory BChls designated B, two bacteriopheophytins (BPhe) designated H, and two quinones (not shown). Despite the structural pseudo C 2 symmetry obvious from the structure, electron transfer occurs predominantly along the L-branch of monomeric chromophores, as illustrated with a quantum yield approaching unity. [4][5][6] This extremely efficient and unidirectional electron transfer has stimulated much experimental and theoretical interest. 7-10 The role of the accessory BChl on the functional side (B L ) in facilitating the electron-transfer reaction from 1 P to P + H L -is an unresolved problem. Two limiting mechanisms have emerged: a two-step mechanism in which electron transfer occurs sequentially from 1 P to form the intermediate state P + B Land subsequently P + H L -11,12 and a direct one-step electron transfer from 1 P to form P + H L -, where the P + B L -state serves as a virtual intermediate to enhance the electronic coupling between 1 P and P + H L -by superexchange. 10,13,14 In either case, the B L molecule plays a crucial role, whether as P + B L -or B L + H L -. 15,16 The role of B L no doubt is also critical to understanding the origin of unidirectional electron tra...

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Rotary ultrasonic machining has been widely used for machining of hard and brittle materials due to the advantages of low cutting force, high machining accuracy, and high surface integrity. Focusing on the development of specialized rotary ultrasonic machining systems, this article summarizes the advances in the functional components and key technologies of rotary ultrasonic machining systems for hard and brittle materials, including the ultrasonic generator, power transfer structure, transducer, ultrasonic horn, and cutting tool. Developments on the automatic frequency tracking method, the establishment of an electrical compensation model for power transfer, the energy conversion characteristics of piezoelectric materials and giant magnetostrictive materials, and the design methods for the ultrasonic horn and cutting tool were elaborated. The principle of magnetostrictive energy conversion, output amplitude characteristics of a giant magnetostrictive transducer, and high-power giant magnetostrictive rotary ultrasonic machining systems were also presented. Future research and developments of rotary ultrasonic machining systems regarding the ultrasonic generator, amplitude stability, energy conversion efficiency, vibration mode, and system integration were finally discussed.

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