Donor acceptor higher order complexes

However, these observations are not proof of the role of a donor-acceptor complex in the copolymcrization mechanism. Even with the availability of sequence information it is often not possible to discriminate between the complex model, the penultimate model (Section 7.3.1.2) and other, higher order, models.28 A further problem in analyzing the kinetics of these copolyincrizations is that many donor-acceptor systems also give spontaneous initiation (Section 3.3.6.3). 

The potential for RNA to act as a catalyst is dictated by its structure as a linear polymer of the four common ribonucleotides. Like DNA, RNA can form double stranded, antiparallel helices via traditional Watson-Crick base pairing. However, the backbone of nucleic acid is highly flexible and RNA can form complex tertiary structures that often involve non-Watson-Crick base pairing to create active site crevices for catalysis. The phosphodiester backbone is charged negatively and interacts electrostatically as well as by direct coordination with solution divalent cations. Ribose, purines, and pyrimidine bases contain both H-bond donors and acceptors that help stabilize higher-order stmcture and provide for substrate positioning, as well as participate in active site interactions. 

In a pentacarbonylchromium complex of cr-s-bonded 2-diethylboryl-5-methylthiophene, a donor-acceptor interaction between boron and a carbonyl group is suggested in order to rationalize the higher field shift of its "B signal (77JOM(125)155) (Fig. 6). 

Jayaprakash et al. [198] reported the series of push-pull bimetallic polyene complexes [(CO)5M=C(OCH3)(-CH=CH-) (t C5H4)Fe(r C5H5)] (M = W, Cr n = 1-4) with ferrocene as donor and a carbene complex of Cr or W as acceptor. The jSi o6(HRS) values increase with increasing the conjugation of the n linker for the same n linker, the chromophore based on W exhibits higher second-order NLO responses than that based on Cr, as expected for a more efficient n backdonation from the W atom. The quadratic hyperpolarizability of these bimetallic push-pull... 

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