Bond, chemical types disulfide

The first line of mucoadhesive polymers were based on polymer-mucin interaction through physical and/or chemical non-covalent bonds. However, there have been attempts to improve mucoadhesive properties via covalent bonds such as disulfide links between the polymer and the mucin-type glycoproteins. This approach has lead to the development of thiolated polymers where a small... 

In an attempt to sensitize the thiosulfate bond cleavage, benzophenone (10% by weight) was incorporated into the polymer film. Upon photolysis at 366 nm, the 639 cm 1 thiosulfate band was reduced (Figure 10) as in the case of direct photolysis at 254 nm and 280 nm. Since benzophenone is a known triplet sensitizer it is likely that the S-S bond cleavage in the thiosulfate group occurs from a triplet excited state in the sensitized reaction. Incidentally photolysis of a PATE film at 366 nm in the absence of benzophenone resulted in no loss of the 639 cm 1 IR peak. Unfortunately due to the film thickness, we were unable to obtain accurate quantum yields for either the direct or sensitized photolysis. Finally it should be noted that no chemical evidence has been presented to confirm disulfide formation. Results from the photolysis of a PATE-type model compound will be offered to substantiate the claim of disulfide formation as well as quantitate the primary photolysis step. But first, we consider photolysis of a PASE polymer film. 

The simplest approach to disulfide bond formation of chemically synthesized peptides and proteins involves (i) complete deprotection including that of the cysteine thiol functions, and (ii) mild oxidation of the thiol groups to form the folded product with the native cystine connectivity. With careful attention to experimental conditions, the native-type folding of the peptide can be accomplished however, misfolded disulfide isomers are often produced as the main products in spite of efforts to optimize the reaction conditions. 

Fig. 1. Different strategies of coupling cargoes to cell-penetrating peptides. (I) Non-covalent coupling by electrostatic interactions (A) (44,46), specific pairing of nucleotides (B) (39,48), biotin-avidin interaction (C) (37,66) or mixed-type interactions (D) (26). (II) Covalent coupling by connecting CPP and cargo molecule into a continuous chain (E) (36), forming a disulfide bond (F) (49,56) or chemical crosslinking (G) (37,57).

From a purely chemical point of view, the fundamental fact is that proteins contain covalent bonds which are split by rather drastic means and more labile, noncovalent bonds which may be split or at least altered by denaturation alone. Two types of covalent structures exist, involving peptide bonds and disulfide bridges respectively. Both structures should be clearly differentiated since they are destroyed by quite different chemical treatments (hydrolysis for the first, oxidation or reduction for the second). They will be called for this reason, respectively, the primary and the secondary structures. Conversely, several inter- or intrachain, noncovalent... 

Types of Bonds in Proteins. Two kinds of bonds are usually present. The main covalent bonds are the peptide bonds between the amino acid residues and the disulfide bonds which are the cross-links. Both of these are subject to chemical modification. With very few exceptions, methods for chemical modifications must not affect the peptide bonds. In addition, there are certain other types of cross-linkages found in specialized proteins such as in the structural proteins collagen and elastin. 

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