Thermodynamic parameters for the intramolecular disordered-to-beta transition of poly(L-tyrosine) in aqueous solution

We have examined the disordered-to-beat transition of two samples of high molecular wieght poly(L-tyrosine), a polypeptide with an ionizing side chain, by potentiometric titration of solutions of this polymer in aqueous buffer. The degree of ionization was determined spectrophotometrically from the absorption of the tyrosinate anion at 293.5 nm. By using polymer solutions of high dilution the region of the conformational transition was separated from that of aggregation. As a consequence it was possible to obtain titration data for the intramolecular transition which we believe are free of interference from aggregation. For this transition we found that deltaHO=-1330 cal/mol of residue. The two samples gave values of -3.42+/-0.14 and -3.85+/-0.10 u for the change in entropy. Possible artifactual origins for this distinction have been examined and may be dicounted. Rather, it probably reflects a difference in the beta structures formed which depends on molecular weight.