Tryptophan donor-acceptor interaction

An expedient and stereoselective synthesis of bicyclic ketone 30 exemplifies the utility and elegance of Corey s new catalytic system (see Scheme 8). Reaction of the (R)-tryptophan-derived oxazaboro-lidine 42 (5 mol %), 5-(benzyloxymethyl)-l,3-cyclopentadiene 26, and 2-bromoacrolein (43) at -78 °C in methylene chloride gives, after eight hours, diastereomeric adducts 44 in a yield of 83 % (95 5 exo.endo diastereoselectivity 96 4 enantioselectivity for the exo isomer). After reaction, the /V-tosyltryptophan can be recovered for reuse. The basic premise is that oxazaborolidine 42 induces the Diels-Alder reaction between intermediates 26 and 43 to proceed through a transition state geometry that maximizes attractive donor-acceptor interactions. Coordination of the dienophile at the face of boron that is cis to the 3-indolylmethyl substituent is thus favored.19d f Treatment of the 95 5 mixture of exo/endo diastereo-mers with 5 mol % aqueous AgNC>3 selectively converts the minor, but more reactive, endo aldehyde diastereomer into water-soluble... 

The (5)-tryptophan-derived oxazaborolidenes utilized in this aldol study have been previously examined by Corey as effective catalysts for enantioselective Diels-Alder cycloaddition reactions [6]. Corey has documented unique physical properties of the complex and has proposed that the electron-rich indole participates in stabilizing a donor-acceptor interaction with the metal-bound polarized aldehyde. More recently, Corey has formulated a model exemplified by 7 in which binding by the aldehyde to the metal is rigidified through the formation of a hydrogen-bond between the polarized formyl C-H and an oxyanionic ligand [7], The model illustrates the sophisticated design elements that can be incorporated into the preparation of transition-metal complexes that lead to exquisite control in aldehyde enantiofacial differentiation. 

The crystal structure of the red picric acid salt of D,L-tryptophan-methanol was determined by X-ray diffraction methods 141). The indole and picrate planes are stacked, with interplanar spacing of 3.3-3.5 A. The stacked pairs are relatively isolated and without k-k interactions between adjacent pairs. The stacking interactions appears to be of the donor-acceptor (charge-transfer) type. The vibrational spectrum of tryptophan picrate contains a strong band at 1740 cm which is not observed in the spectra of either of the components, and is attributed to the C = 0 stretching vibration (250). 

Thus, the efficiency of energy transfer between donors and acceptors randomly distributed in a plane depends on R0, a, and a, and the transfer efficiency is independent of a. The important point was made that surface density of the acceptor could be 1 per 500 phospholipids for R0 > 30 A. Using these equations for different donor and acceptor concentrations, the data were matched against the different theoretical curves to obtain the R0. An example of the application of the method of Fung and Stryer(81) is the study of energy transfer between the tryptophan of a membrane protein (or peptide models of proteins) and DPH,(83) in which it was shown that efficient energy transfer can occur without any special interaction being required between DPH and the proteins in specific areas of the membrane. 

Equations (B5.3.1) and (B5.3.4) also assume that the reacting system is in thermal eqmlibrium with its surroundings. This assumption can be problematic for a reactant that is created by photoexcitation. In addition, Eqs. (B5.3.3) and (B5.3.4) require electronic interactions of the donor and acceptor to be weak. The interaction matrix element//21 for electron transfer depends on the electronic orbital overlap of the reactants, which drops off rapidly as the intermolecular distance increases [101] but can be relatively strong for electron transfer from excited tryptophans to nearby residues and backbone amide groups in proteins [50, 51]. 

This clearly emphasizes an important function for threonine M222 and tryptophan M252 in the reaction center structure. Tryptophan M252 mediates binding of to its site presumably via a charge transfer interaction (6) between the electron donor tryptophan and the electron acceptor and thus optimizes reaction center function. This is especially evident in the crystal structure of Rps. viridis were the Ti-electron rich C2-C3 bond of the indole... 

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