Abstract
Binol quinone methides (BQMs) can be generated from 1,1′-(2,2′-dihydroxy-1,1′-binaphthyl-6,6′-diyl)bis(N,N,N-trimethylmethanamiuium) bromide (BQMP-b) in a 1:1 MeCN:H2O mixed solution via a ground state intramolecular proton transfer (GSIPT), as mentioned in our previously reported studies. Here, the photoreaction of BQMP-b in neutral and basic aqueous solution (pH = 7, 10, 12) was investigated to explore the possible mechanisms and the key intermediates produced in the process of the photoreaction and to examine whether they are different from those in a neutral mild-mixed MeCN:H2O solution. The studies were conducted using femtosecond transient absorption (fs-TA), nanosecond transient absorption (ns-TA), and nanosecond time-resolved resonance Raman spectroscopy (ns-TR3) in conjunction with results from density functional theory (DFT) computations. The results showed that BQMP-b was deprotonated initially and produced BQMs species more effectively through an E1bc elimination reaction in a strong basic aqueous condition (pH = 12), which differed from the reaction pathway that took place in the solution with pH = 7 or 10. A related single naphthol ring molecule 1-(6-hydroxynaphthalen-2-yl)-N,N,N-trimethylmethanaminium bromide (QMP-b) that did not contain a second naphthol ring was also investigated. The related reaction mechanisms are elucidated in this work, and it is briefly discussed how the mechanisms vary as a function of aqueous solution pH conditions.
Highlights
In recent years, bifunctional quinone methides (QMs) have attracted more attention from scientists because they have been discovered to be important intermediates to alkylate and crosslink DNA, giving them the potential to be applied in cancer therapy [1,2,3,4,5,6]
In the S0 and S1 states were investigated. These results showed that the S1 of BQMP-b− had a shorter bond between C2-O1 (1.26 Å, right) in the binaphthol ring group compared to that of BQMP-b− in the ground state (1.29 Å, left), which further suggested that this C–O bond had high tendencies to form the C=O bond
The study detailed here has given a helpful elucidation of the electrophilic properties of BQMP-b in neutral and basic aqueous solutions
Summary
Bifunctional quinone methides (QMs) have attracted more attention from scientists because they have been discovered to be important intermediates to alkylate and crosslink DNA, giving them the potential to be applied in cancer therapy [1,2,3,4,5,6]. Freccero et al reported on the chemical and spectroscopic behavior of binol quaternary ammonium derivatives and studied their bis-alkylation processes in water and DNA interstrand cross-linking reactions. They pointed out that the reactivity of binol quinone methides (BQMs) generated from the derivatives was very stable and that they were useful DNA cross-linkers with submicromolar potency using UV-visible light activation [15,16]. In light of the importance of BQMs, the elucidation of the mechanism of their formation is important to Molecules 2018, 23, 3102; doi:10.3390/molecules23123102 www.mdpi.com/journal/molecules
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