Simultaneous Determination of Interfacial Molarities of Amide Bonds, Carboxylate Groups, and Water by Chemical Trapping in Micelles of Amphiphiles Containing Peptide Bond Models

Abstract

Chemical trapping is a powerful approach for obtaining experimental estimates of interfacial molarities of weakly basic nucleophiles in the interfacial regions of amphiphile aggregates. Here, we demonstrate that the chemical probe 4-hexadecyl-2,6-dimethylbenzenediazonium ion (16-ArN(2)(+)) reacts competitively with interfacial water, with the amide carbonyl followed by cleavage of the headgroups from the tail at the amide oxygen, and with the terminal carboxylate groups in micelles of two N-acyl amino-acid amphiphiles, sodium N-lauroylsarcosinate (SLS) and sodium N-lauroylglycinate (SLG), simple peptide bond model amphiphiles. Interfacial molarities (in moles per liter of interfacial volume) of these three groups were obtained from product yields, assuming that selectivity toward a particular nucleophile compared to water is the same in an aqueous reference solution and in the interfacial region. Interfacial carboxylate group molarities are ~1.5 M in both SLS and SLG micelles, but the concentration of the amide carbonyl for SLS micelles is ~4.6-5 times less (ca. 0.7 M) than that of SLG micelles (~3 M). The proton on the secondary N of SLG helps solubilize the amide bond in the aqueous region, but the methyl on the tertiary N of SLS helps solubilize the amide bond in the micellar core, reducing its reaction with 16-ArN(2)(+). Application of chemical trapping to proteins in membrane mimetic interfaces should provide insight into the topology of the protein within the interface because trapping of the amide carbonyl and cleavage at the C-N bond occurs only within the interface, and fragment characterization marks those peptide bonds located within the interface.

Self-assembled cationic amphiphiles as antimicrobial peptides mimics: Role of hydrophobicity, linkage type, and assembly state

“…The surface tensions ( γ ) of CAms were measured using a Fisher Surface Tensiometer model 21 (Waltham, MA) at room temperature. 24 The platinum ring was rinsed with hexane, methanol, and DI water followed by heating with a Bunsen burner before use. A stock solution of CAms (5 mL) was transferred into a carefully cleaned vessel and γ was measured repeatedly at least three times until the variation was smaller than 0.2 mN/m.…”

To Model Chemical Reactivity in Heterogeneous Emulsions, Think Homogeneous Microemulsions

“…16-ArN 2 + is a unique chemical probe that reacts with AOs via an oxidation/reduction mechanism . We selected this cation for several reasons: (a) because of the significant level of current understanding of the rich and complex chemistry of arenediazonium ions; (b) because we successfully used a structurally similar 2,4-dimethyl-4-hexadecylarenediazonium ion under conditions in which it reacts heterolytically, to monitor the interfacial molarities of a wide variety of weakly basic nucleophiles, including water, in association colloids , and to cleave peptide bonds; (c) because the preparation of 16-ArN 2 + and other substituted arenediazonium ions as their fluoroborate (BF 4 – ) salts from their aniline precursors is straightforward , and occurs in high yields (>70%); (d) because the reactive group of the cation is located within the interfacial region of the aggregates; and (e) because the oxidation/reduction reaction of 16-ArN 2 + with AOs is significantly faster than its spontaneous heterolytic dediazoniation in different solvents, association colloids, and emulsions. ,− …”

Ion-Specific Effects on Vesicle-to-Micelle Transitions of an Amino Acid Surfactant Probed by Chemical Trapping

“…The interfacial region between aggregates and bulk water may be compared to a “black box”, in that common experimental methodology is unable to focus on this specific area. The CT method was established by the Romsted group and has been employed to determine the interfacial molarities of weakly basic nucleophiles, such as halide ions, water, urea, and alcohols in association colloid solutions. − As has been stated many times in previous publications, − there are two important assumptions for the CT method: (1) the pseudophase model work and (2) dynamic equilibrium.…”

“…The CT reaction pathways with 16-ArN 2 + in SLG aqueous solutions have been studied previously by Zhang, as summarized in Scheme . The same reactions occur with 1-ArN 2 + in the reference solutions .…”

“…The reaction between the probe and amide oxygen is slightly more complicated, and both z-ArEC12 and z-ArOH h are the products. The mechanisms , of the reactions are summarized in Scheme S1 in the Supporting Information. Therefore, the interfacial molarities of water, H 2 O m , carboxylate groups, COO m , and amide bonds, CON m , are estimated by the corresponding product yields of %16-ArOH w , %16-ArGE, and %16-ArOI (equals to %16-ArEC12 + %16-ArOH h ), respectively, and the selectivity is determined by the results for 1-ArN 2 + .…”