Host–Guest Complexation of Amphiphilic Molecules at the Air–Water Interface Prevents Oxidation by Hydroxyl Radicals and Singlet Oxygen

Abstract

The oxidation of antioxidants by oxidizers imposes great challenges to both living organisms and the food industry. Here we show that the host–guest complexation of the carefully designed, positively charged, amphiphilic guanidinocalix[5]arene pentadodecyl ether (GC5A‐12C) and negatively charged oleic acid (OA), a well‐known cell membrane antioxidant, prevents the oxidation of the complex monolayers at the air–water interface from two potent oxidizers hydroxyl radicals (OH) and singlet delta oxygen (SDO). OH is generated from the gas phase and attacks from the top of the monolayer, while SDO is generated inside the monolayer and attacks amphiphiles from a lateral direction. Field‐induced droplet ionization mass spectrometry results have demonstrated that the host–guest complexation achieves steric shielding and prevents both types of oxidation as a result of the tight and “sleeved in” physical arrangement, rather than the chemical reactivity, of the complexes.

Probe‐Free Direct Identification of Type I and Type II Photosensitized Oxidation Using Field‐Induced Droplet Ionization Mass Spectrometry

“…In the current study, we adopt our unique home-built field-induced droplet ionization mass spectrometry (FIDI-MS) methodology (Figure 1), which is capable of selective online sampling of molecules that reside at the air-water interface and suffers minimal influence from the bulk of the solution. [27][28][29][30] Amphiphilic lipids, the major oxidation target of PDT, [1] and their oxidation products especially those unstable products that could easily degrade, can be detected by FIDI-MS in an in situ manner. The posterchild PS, 5,10,15,20-tetra(m-hydroxyphenyl)chlorin (temoporfin, T for short), is selected in this study for its high quantum yield and benign, tissue-penetrating activation wavelength (652 nm).…”

“…Since the progeny microdroplets originate from the surface of the original hanging droplet, this techniques features high interface selectivity. [27][28][29][30] Each measurement by FIDI-MS takes only several minutes. Figure 2 a exhibits the FIDI-MS sampling of the POPC oxidation products with different concentrations of T after illuminated by a 660 nm laser pointer for 1 minute.…”

“…We next discuss the mechanisms of forming these products ( Figure 3). Type II is the well-accepted mechanism of 1 O 2 reacting with an olefin, [28,30,35] which is applicable to the C=C double bond on the oleyl chain of POPC. Basically the double bond uptakes a 1 O 2 molecule, after which the terminal O atom abstracts one H atom from either of the neighboring carbon atoms, resulting in an allyl hydroperoxide molecule where the C = C bond is shifted by one carbon from the original position, resulting in the + 32 Da products at m/z 793 and 815.…”

Probe‐Free Direct Identification of Type I and Type II Photosensitized Oxidation Using Field‐Induced Droplet Ionization Mass Spectrometry

“…In view of the above discussions, a facile, direct, in situ technique that can capture unstable reaction intermediates and products but does not require the addition of any chemicals other than the PS and the substrates seems to be in great need for the design, synthesis, and mechanistic screening of novel photosensitizers. In the current study, we adopt our unique home‐built field‐induced droplet ionization mass spectrometry (FIDI‐MS) methodology (Figure 1), which is capable of selective online sampling of molecules that reside at the air–water interface and suffers minimal influence from the bulk of the solution [27–30] . Amphiphilic lipids, the major oxidation target of PDT, [1] and their oxidation products especially those unstable products that could easily degrade, can be detected by FIDI‐MS in an in situ manner.…”

“…The progeny droplets ejected into the mass spectrometer yield ions that are subjected to mass analysis. Since the progeny microdroplets originate from the surface of the original hanging droplet, this techniques features high interface selectivity [27–30] . Each measurement by FIDI‐MS takes only several minutes.…”