期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2002
卷号：99
期号：14
页码：9190-9195
DOI：10.1073/pnas.112193999
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：A sequence of seven alanine residues--too short to form an -helix and whose side chains do not interact with each other--is a particularly simple model for testing the common description of denatured proteins as structureless random coils. The 3JHN coupling constants of individual alanine residues have been measured from 2 to 56{degrees}C by using isotopically labeled samples. The results display a thermal transition between different backbone conformations, which is confirmed by CD spectra. The NMR results suggest that polyproline II is the dominant conformation at 2{degrees}C and the content of {beta} strand is increased by approximately 10% at 55{degrees}C relative to that at 2{degrees}C. The polyproline II conformation is consistent with recent studies of short alanine peptides, including structure prediction by ab initio quantum mechanics and solution structures for both a blocked alanine dipeptide and an alanine tripeptide. CD and other optical spectroscopies have found structure in longer "random coil" peptides and have implicated polyproline II, which is a major backbone conformation in residues within loop regions of protein structures. Our result suggests that the backbone conformational entropy in alanine peptides is considerably smaller than estimated by the random coil model. New thermodynamic data confirm this suggestion: the entropy loss on alanine helix formation is only 2.2 entropy units per residue.
