Thiol esters, absorption bands

The thiol esters examined by Jencks et aU including S-acetylglutathione, all have the C=0 band near 1675 cm (Table 19.3). Thiol esters also absorb maximally near 1675cm in nonpolar solvents. Hauptschein et al. (1952) have reported this absorption to be insensitive to substituent effects. 

Table 19.3. Absorption Bands, in cm of Thiol Esters, (Jencks et a ., i960)...

The interaction of the chromophore and protein part of PYP produces an absorption band with 446 nm as its maximum. As the free chromophore, trans-4-hydroxycinnamic acid, has an absorption maximum at 284 nm (in aqueous solvent around neutral pH), this interaction of the chromophore with the protein induces a large red shift. Several specific interactions can be distinguished. For one, the thiol ester link of the chromophore with Cys69 causes a red shift of -5713 cm (from 284 to 339 nm). This follows from a comparison of the absorption maximum of 4-hydroxycinnamic acid and the denatured form of PYP in aqueous solution at pH 7. An additional red shift of 4310 cm (from 339 to 397 nm) occurs when the chromophore becomes deprotonated, which follows from a comparison of the absorption maximum of the denatured form of PYP in aqueous solution at pH 7 and 11. This leaves a red shift of -2767 cm (from 397 to 446 nm) that is due to interactions with the protein. Though this description is illustrative, it does not provide specific insight regarding the nature of the interactions between the chromophore and the protein. 

---