Tryptophan lactoglobulin

Showing the monoisotopic masses of fragments MH+ of /i-lactoglobulin A cysteine residues are shown in bold script, i.e. cysteine-C aromatic residues are underlined, i.e. phenylalanine-F, tyrosine-F, and tryptophan- mIz expected for singly charged species. 

The six major proteins of milk, asl-, o s2-, and /c-casein, jS-lactoglobulin, and a-lactalbumin, contain at least one tryptophan residue [57], the fluorescence of which allows the monitoring of the structural modifications of proteins and their physicochemical environment during the coagulation processes. Emission fluorescence spectra of the protein tryptophanyl residues were recorded for the milk coagulation kinetics induced by... 

Figure B3.6.8 Quenching of the tyrosine fluorescence of p-lactoglobulin. Spectra were recorded in the absence (a, c) and presence (b. d) of 9.48 M urea, and excited at 275 nm (solid lines, tyrosine and tryptophan excited) and 297 nm (broken lines, tyrosine not excited). Parameters protein concentration 16.5 pM in 0.025 M phosphate, 0.068 M NaCI, pH 6.2 Xex = 8 nm and kem = 9 nm . Perkin-Elmer MPF 2A spectrofluorometer. Reprinted from Creamer (1995) with permission from the American Chemical Society.

The comparison of the fluorescence spectra of p-lactoglobulin in an aqueous solution and in 50% ethanol (v/v) (not shown) demonstrates that the maximum of the tryptophan fluorescence emission is shifted from 332 nm to 338 nm, respectively. Additionally, a concomitant increase in the maximum fluorescence intensity may be observed. Red shift of the emission maximum implies that under the influence of alcohol the tryptophan residues, which in aqueous solutions are sheltered in the hydrophobic interior of a protein molecule [77], become more exposed to a polar environment. 

Also analyzed b-lactoglobulin A, lysoz3nne, phenylalanine, ribonuclease A, tryptophan, tyr amine... 

This behavior is compatible with an increase in the mobility of tryptophans upon permeation using both membranes. The decrease in fluorescence anisotropy observed for the permeate solutions collected with the 10 kDa membrane clearly indicates that permeation through the 10 kDa membrane induces molecular unfolding. The decrease in fluorescence anisotropy was considerably lower upon permeation of P-lactoglobulin through a 30 kDa membrane, indicating, in... 

Table 12.3 Fit of the fluorescence decays obtained for solutions of P-lactoglobulin in feed, retentate and permeates collected after ultrafiltration at different TMP values, using regenerated cellulose (RC) membranes with a nominal cut-off of 10 kDa and 30 kDa. t and An correspond to decay times and amplitudes of the different tryptophan residues, while accounts for the quality of the fits [1],...

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