McKnight R P, Auer H E
Macromolecules. 1976 Nov-Dec;9(6):939-44. doi: 10.1021/ma60054a012.
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.
我们通过对这种聚合物在水性缓冲液中的溶液进行电位滴定，研究了两种高分子量聚（L-酪氨酸）样品（一种带有可电离侧链的多肽）从无序到有序的转变。通过分光光度法从293.5 nm处酪氨酸阴离子的吸收来测定电离度。通过使用高稀释度的聚合物溶液，构象转变区域与聚集区域得以分离。因此，有可能获得我们认为不受聚集干扰的分子内转变的滴定数据。对于这种转变，我们发现ΔH° = -1330 cal/mol残基。两个样品的熵变值分别为-3.42±0.14和-3.85±0.10 u。已经研究了这种差异可能的人为起源并可予以排除。相反，这可能反映了所形成的β结构的差异，这种差异取决于分子量。
