Peptide bonds form through a process known as dehydration synthesis
The Difference between Hydrolysis and Dehydration Synthesis
Pyridyl disulfide linkers include cleavable disulfide bonds, which facilitates a quantitative evaluation of the reaction efficiency. Jon . calculated the concentration of surface-bound peptide molecules on the nanoparticles by quantifying the released pyridine-2-thione . In an effort to develop integrin-targeted iron oxide nanoparticles as theranostic agents, amine-modified iron oxide nanoparticles were synthesized, and SPDP was added to convert the primary amine groups on the nanoparticles to sulfhydryl-reactive pyridyldisulfide groups. Conjugation between the thiol group-containing cyclic RGD peptides and the SPDP-activated nanoparticles produced pyridine-2-thione, which was immediately collected by spin filtering (at 100 K). The immobilized cRGD molecules were quantified based on the ultraviolet (UV) absorbance at 343 nm of the collected pyridine-2-thione filtrate, indicating that the average number of conjugated cRGD peptides on each nanoparticle was 0.39 wt%. This linker is useful for enhancing the intracellular gene silencing properties of siRNA. Bhatia . studied the gene-silencing efficacy of siRNA-conjugated QDs using cleavable (sulfo-LC-SPDP) or noncleavable (sulfo-SMCC) cross-linkers . They immobilized thiol-modified siRNA specific for EGFP to amine-functionalized QDs via sulfo-LC-SPDP or sulfo-SMCC linkers and quantified the EGFP fluorescence intensity. The siRNA attached QDs via the sulfo-LC-SPDP linker provided greater silencing efficiency than those attached via the sulfo-SMCC linker. The cleavable disulfide cross-linker released siRNA from the nanoparticles into the intracellular reducing environment, which affected the interactions between the siRNA and the RNA induced silencing complex (RISC), which is necessary for gene knockdown.
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Recently, Chen and Sun used the Mannich reaction to couple biomolecules to nanoparticles . In this work, iron oxide nanoparticles functionalized with active hydrogen groups were reacted with amine group-containing cyclic RGD peptides to develop ultrasmall c(RGDyK)-coated Fe3O4 nanoparticles (with an 8.4 ± 1.0 nm hydrodynamic diameter) as tumor-targeted imaging agents. Nonspecific uptake of the iron oxide nanoparticles by RES in the blood stream complicates the development of small biocompatible nanoparticles with targeting capabilities. Initially, they synthesized Fe3O4 nanoparticles via the thermal decomposition of Fe(CO)5 in benzyl ether in the presence of 4-methylcatecol, as a surfactant, followed by air oxidation. The 4-methylcatecol formed a tight thin coating layer over the nanoparticle surface via formation of a strong chelating bond between the iron and the catechol unit. The aromatic ring of the 4-methylcatecol on the nanoparticles was directly coupled with the amine group of a lysine residue in the cyclic RGD peptide, c(RGDyK) (Figure ). High-resolution transmission electron microscopy (HRTEM) images of the nanoparticles indicated an iron oxide core size of 4.5 nm and a coating layer containing the c(RGDyK) peptide 2 nm in thickness, close to the size in water.