Effect of sulfur oxidation state on aggregation-related and interfacial behavior of S-linked octyl α-d-mannosides

A structurally matched series of S-linked octyl α-d-mannoside derivatives containing sulfide (1), sulfoxide (2), and sulfone (3) linkages was prepared to investigate the effect of the sulfur oxidation state on aggregation-related behavior in water. The structural assignments were supported by NMR spectroscopy, and the sulfoxide framework was further validated by single-crystal X-ray crystallography of the corresponding acetylated derivative. Nile Red fluorescence measurements revealed concentration-dependent responses for all three compounds, yielding apparent critical aggregation concentration values of 1.86 mM for 1, 2.72 mM for 2, and 1.52 mM for 3. These results indicate that the sulfur oxidation state influences the fluorescence-detected aggregation-related behavior in bulk aqueous solutions. In contrast, surface tension measurements revealed a clear breakpoint only for the sulfide derivative (1.69 mM), whereas no distinct breakpoint was observed for the sulfoxide or sulfone derivatives within the measured concentration range. Dynamic light scattering and transmission electron microscopy qualitatively supported the aggregation-related features in the series. Together, these findings indicate that, in this S-linked octyl α-d-mannoside series and under the conditions examined, the sulfur oxidation state affects both fluorescence-detected aggregation-related behavior in water and its correspondence with interfacial behavior detected by surface tension measurements.