A novel disulfide, which carried two pepstatin fragments at both ends, was prepared by the coupling of 11,11'-dithiobisundecanoic acid (DTUA) with a fragment (Val-Val-Sta) carrying a n-hexyl end (Pepsta(h)). The compound obtained (DTUA-Pepsta(h)) formed a self-assembled monolayer (SAM) on a gold electrode and vacuum-evaporated gold thin film as proven by cyclic voltammetry and reflection absorption infrared spectroscopy, respectively. When the SAM-modified gold electrode was incubated with a solution of aspartyl protease, pepsin, a decrease in both anodic and cathodic peak currents and an increase in potential difference were observed in the cyclic voltamogram of hydroquinone as a probe, whereas a coexistence of free pepstatin fragment inhibited these phenomena, indicating the specific binding of pepsin to the fragment at the exterior of the SAM. The binding rate of the enzyme to the SAM was largely dependent on the surface density of the fragment moiety in the SAM. Furthermore, when the SAM of DTUA-Pepsta(h) on a gold colloid array deposited on an amino group-modified glass plate was immersed in a pepsin solution, absorption of the glass plate at 550 nm corresponding to a localized surface plasmon resonance of the gold colloid abruptly increased and slightly red-shifted, and a further addition of pepstatin A gradually decreased the absorbance. From the increasing and decreasing profiles of absorbance, the association constant (K(assoc)) for pepsin with the fragment on the SAM was determined. Similar phenomena were observed upon immersion of the fragment-modified SAM in a solution of HIV-1 protease, suggesting a usability of the pepstatin fragment SAM for the detection and removal of the enzyme from biological fluids.
The molecular recognition of various kinds of bisphenols (BPs) by a self-assembled monolayer (SAM) of thiolated calix[6]arene on a gold electrode was examined using cyclic voltammetry (CV). Based on the inhibitory effect of BPs on the inclusion of hydroquinone (HQ) as a probe bythe surface-confined calix[6]arene, the association constants (Kassoc) of BPs with the immobilized calix[6]arene were estimated. The Kassoc values for BPs with the SAM of thiolated hexasodium calix[6]arenehexasulfonic acid (thioSCX6) were much smaller than those in the free calix[6]arene derivative systems reported previously. The order of the Kassoc values for BPs with thioSCX6 was bisphenol A (BPA) > bisphenol S (BPS) > bisphenol B (BPB) > bisphenol F (BPF). The Kassoc values for BPA and BPS with thioSCX6 were larger than that for BPB, despite the larger hydrophobicity of BPB than that of BPA and BPS. This is probably because the inclusion phenomena in this system are not simply driven by the hydrophobic interaction, but are significantly affected by several steric and structural factors with immobilization of the host. Those are (1) a decrease in flexibility of the SCX6 cavity by the formation of SAM, (2) a decrease in inclusion ability of the SCX6 by the presence of Au surface just beneath it, and (3) a repulsion of hydrophobic guests bythe presence of sulfonate groups at the top of the SAM. Moreover, the association phenomena (adsorption and desorption processes) of a bisphenol with the SCX6 SAM were examined directly by using localized surface plasmon resonance spectroscopy.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.