Exciton coupling technique

This class includes only agelastatins A-D (60-65), a family of tetracyclic oroidin derivatives isolated from Agelas dendromorpha [92,93] and Cymbastela sp. [94]. The absolute configuration of these molecules was deduced through the application of exciton-coupling techniques [93]. 

We shall conclude this chapter with a few speculative remarks on possible future developments of nonlinear IR spectroscopy on peptides and proteins. Up to now, we have demonstrated a detailed relationship between the known structure of a few model peptides and the excitonic system of coupled amide I vibrations and have proven the correctness of the excitonic coupling model (at least in principle). We have demonstrated two realizations of 2D-IR spectroscopy a frequency domain (incoherent) technique (Section IV.C) and a form of semi-impulsive method (Section IV.E), which from the experimental viewpoint is extremely simple. Other 2D methods, proposed recently by Mukamel and coworkers (47), would not pose any additional experimental difficulty. In the case of NMR, time domain Fourier transform (FT) methods have proven to be more sensitive by far as a result of the multiplex advantage, which compensates for the small population differences of spin transitions at room temperature. It was recently demonstrated that FT methods are just as advantageous in the infrared regime, although one has to measure electric fields rather than intensities, which cannot be done directly by an electric field detector but requires heterodyned echoes or spectral interferometry (146). Future work will have to explore which experimental technique is most powerful and reliable. 

Since the CD of oriented molecules are extremely sensitive to the interaction of chromophores, such data could be very useful in the investigation of exciton phenomena. The technique also allows one to determine the degree of order and orientation of chromophores in a macromolecule. Thus, CD studies on oriented visual pigments, and possibly on some selected visual pigment analogs, could be very informative in clarifying the role played by induced optical activity of the chromo-phore and/or exciton coupling. 

The B850 ring forms a densely packed excitonically coupled aggregate. Shortly after the structure of the LH2 became available, the question of the extent of exciton delocalisation in B850 became a hot research subject. Different experimental techniques and theoretical methods lead... 

Both exciton-coupled CD and X-ray techniques are nonempirical methods for determining the absolute configuration of chiral compounds. Although these two methods differ in that CD is for solutions whereas X-ray is for crystals, both methods naturally give the same absolute configuration. 

Exciton-phonon coupling, 42 Excursion time, 65 Exploding-foil technique, 152 External injection, 172... 

Paramagnetic species formed by reactions of the radiation-produced transient species in ice and frozen aqueous systems have been studied by ESR technique. The radiation-produced electrons have been found to react e.g. with acidic solutes to form H-atoms and with group 11(b) metal ions to give the corresponding univalent radical ions, while the holes can react with anions such as S04 2 and H2P04 giving the radical ions S04 and HP04. Evidence that the electron and hole are coupled to each other, and may in fact exist in irradiated pure ice primarily in an (exciton-like) bound state has been discussed. The present work provides evidence for the reactions of the radiation-produced positive holes apart from the reactions of the electrons. 

In resonant infrared multidimensional spectroscopies the excitation pulses couple directly to the transition dipoles. The lowest order possible technique in noncentrosymmetrical media involves three-pulses, and is, in general, three dimensional (Fig. 1A). Simulating the signal requires calculation of the third-order response function. In a small molecule this can be done by applying the sum-over-states expressions (see Appendix A), taking into account all possible Liouville space pathways described by the Feynman diagrams shown in Fig. IB. The third-order response of coupled anharmonic vibrations depends on the complete set of one- and two-exciton states coupled to thermal bath (18), and the sum-over-states approach rapidly becomes computationally more expensive as the molecule size is increased. 

A full report of the c.d. of steroidal diol bis-(p-dimethylaminobenzoates) confirms the reliability of theoretical calculations of the coupled Cotton effects of the remote ester groups (exciton chirality method). The c.d. curves show two maxima of opposite signs, separated by some 27 nm, and with intensity inversely proportional to the square of the interchromophore distance. The profile of the observed c.d. curve results from the superimposition of two component curves, each of asymmetric shape.A review of the uses of chiroptical techniques for structural and conformational studies includes examples of the assignment of stereochemistry to steroids and terpenoids, among a wide variety of natural products. The Octant Rule for carbonyl compounds and the rules applicable to other chromophoric systems are discussed critically. 

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