A cyanine-type homolog of the red hair bibenzothiazine chromophore combining reversible proton-sensing with a hydrophobic-to-hydrophilic switching response

The Multifaceted Opportunities Provided by the Pheomelanin-Inspired 1,4-Benzothiazine Chromophore: A Still-Undervalued Issue

“…480 nm (red color) at pH ≥ 4.0 and at ca. 610 nm (light blue color) and −640 nm (deep blue color) at lower pH values (pH 2 and pH −0.4, respectively) [52]. A pH sensor paper was, therefore, prepared by dipping chromatographic paper into a solution of the compound, allowing for visual detection of very acidic pHs compared to commercial pH indicators (Figure 4) [52].…”

“…This, of course, may also result in a switch of the compound from hydrophobic to hydrophilic, which can be exploited for tuning and monitoring the permeability of filtering material functionalized with a proper 1,4-benzothiazine scaffold. As a proof of concept, a paper filter dipped into a methanolic solution of the (1,2-ethanediylidene)bis(3-phenyl-2H-1,4-benzothiazine) described above and left to dry (red in color) was found to be impermeable in water but not acid solutions, which rapidly passes through the filter with the associated color change to blue, allowing one, therefore, to monitor the hydrophobic/hydrophilic state of the device (Figure 6) over several cycles given the high reversibility and robustness of the system compared to other cyanines [52]. Similarly, the condensation products of the same 3-phenyl-2H-1,4-benzothiazine with indole-3-carboxaldehyde showed a marked and reversible (up to 15 cycles) acidichromic behavior, with a shift from a yellow (pH > 4) to a purple (pH < 3) color, even when applied as a coating or adsorbed on different materials (glass, nylon, cotton, paper) (Figure 5) [53].…”

“…As an example, the condensation product of 3-phenyl-2H-1,4-benzothiazine with glyoxal described above was also exploited to functionalize poly(lactic acid) films, alginate hydrogels, or chromatographic papers to visually detect volatile amines produced during fish spoilage or organic acids deriving from bacterial spoilage of milk (Figure 7). Although the safety for use in foods still has to be assessed, the high stability and easy availability through a scalable and easy-to-run synthetic procedure makes this compound a promising alternative to natural cyanins for food freshness monitoring [52].…”

The Benzothiazine Core as a Novel Motif for DNA-Binding Small Molecules

“…Frequently, the benzothiazine unit is part of a larger ring-fused system, such as ∆ 2,2 -bi-(2H-1,4-benzothiazine), which occurs in trichochrome pigments found in red hair and feathers [15]. This nature-inspired chromophore has been fashioned into a cyanine-type dye and employed as a colorimetric pH sensor [16]. Various transition metal complexes of oxicam drugs have been synthesized, characterized and assessed as DNA-binding small molecules [17,18], but there are no systematic studies on benzothiazine substituents controlling the DNA-binding affinity and spectroscopic response, which prompted us to prepare and investigate the novel benzothiazine compounds.…”