Formation of the Pb—Boron Bond

The interactions of PABA with RNA were investigated with UV-Vis and PM-IRRAS spectroscopy. Figure 3.33 shows the absorption spectra of a PABA layer (both salt and base form) and a PABA/RNA bilayer. The layer of PABA in the salt form (Figure 3.33, a) exhibits the characteristic absorption bands around 400 and 800 nm attributed to tt-tt and bipolaron band transitions, respectively [53, 54]. The blue shift in the bipolaron band of the base form of the PABA layer (Figure 3.33, b) from 800 to 740 nm was observed upon exposure to PBS at pH 7.4 because of the removal of D-fructose and fluoride [37]. Subsequent complexation of the PABA layer in its base form with RNA resulted in a red shift in the bipolaron band from 740 to 800 nm, together with a small increase in the absorbance. These results reportedly confirmed the complexation of RNA with PABA under neutral conditions by the formation of the bilayer through anionic boronate esters, and subsequent conversion of the base form of PABA back to a self-doped salt form. The creation of the anionic tetrahedral boron forms the basis of multilayer formation. Further, the formation of boronate esters and a boron-nitrogen dative bond, as well as electrostatic interactions of anionic phosphates with cationic amines is supported by PM-IRRAS spectroscopy. After complexation... 

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