Synthesis and Self-Assembly of Globotriose Derivatives:  A Model System for Studies of Carbohydrate−Protein Interactions

Self-assembled monolayers (SAMs) on gold exposing α-<SCP>d</SCP>-Galp-(1→4)-β-<SCP>d</SCP>-Galp-(1→4)-β-<SCP>d</SCP>-Glcp (globotriose) are described. A synthetic pathway for the preparation of the bioactive carbohydrate globotriose coupled directly to bis(16-hydroxyhexadecanyl) disulfide ((HOC<INF>16</INF>H<INF>32</INF>S)<INF>2</INF>), as well as via tetra- or di(ethylene glycol) spacers, was developed. The SAMs were characterized by ellipsometry, contact angle goniometry, infrared reflection−absorption spectroscopy, and by their interactions with monoclonal antibodies. The ellipsometry measurements of mixed SAMs revealed thicknesses between 22 and 40 Å, depending on the ratio between carbohydrate and non-carbohydrate disulfides in the preparation solution. When the solution contained 10% or more of the carbohydrate adsorbate, the modified gold substrates displayed total wetting. Infrared reflection−absorption spectroscopy conferred well-ordered SAMs with a high degree of crystallinity. Furthermore, two monoclonal antibodies (IgM, MAHI 419 and IgG, MAHI 5) showed different affinity for mixed SAMs depending on the fraction and the structure of the carbohydrate component used (globotriose or globotriose tetra(ethylene glycol)), with the largest amount of protein bound for MAHI 419 at 1−10% of surface carbohydrate. These results demonstrate the usefulness of the SAM approach to explore molecular details such as the effect of a spacer or antigen distribution on antibody interactions at interfaces.