RESUMO
The evidence that reverse turns frequently occur as structural components of proteins, as well as of linear and cyclic peptides, is overwhelming. This review summarizes and examines critically the experimental evidence derived from physical methods such as 1H and 13C nuclear magnetic resonance spectroscopy, spin-lattice relaxation time, circular dichroism, IR spectroscopy, and X-ray crystallography. Secondly, theoretical evidence obtained from energy calculations, which rely on empirical energy functions, and correlative methods, which rely on algorithms based on the frequency of occurrence of amino acids, is evaluated. In particular, those theoretical studies for which complementary physical studies have been completed are emphasized. Finally, examples of structure-function relationships involving reverse turns and their biological recognition are demonstrated.
Assuntos
Peptídeos , Conformação Proteica , Aminas , Antibacterianos , Antitoxinas , Fenômenos Químicos , Físico-Química , Dicroísmo Circular , Colágeno/metabolismo , Elastina/metabolismo , Encefalinas , Glicoproteínas/metabolismo , Hormônios , Ligação de Hidrogênio , Hidroxilação , Ionóforos , Espectroscopia de Ressonância Magnética , Muramidase/imunologia , Fosforilação , Prolina/metabolismo , Proteínas/imunologia , Espectrofotometria Infravermelho , Análise Espectral Raman , Toxinas Biológicas , Tuftsina/análise , Difração de Raios XRESUMO
A 270-MHz 1H nuclear magnetic resonance investigation of an ion-binding cyclic peptide analogue of valinomycin, cyclo(L-Val-Gly-Gly-L-Pro)3, and its cation complexes is reported. In CD2Cl2 and CDCl3, the peptide is proposed to occur in a C3-symmetric conformer with the N--H's of all six glycine residues intramolecularly hydrogen bonded. This conformation is different from the familiar valinomycin bracelet structure and lacks any "cavity". Cations do not bind, or bind only weakly, to the peptide in these solvents. Uncomplexed cyclo(L-Val-Gly-Gly-L-Pro)3 in acetonitrile appears to be averaging among several conformations with no evidence found for any preferred intramolecular hydrogen bonds. The strong 1:1 complexes of cyclo(L-Val-Gly-Gly-L-Pro)3 with K+ ANd Ba2+ in acetonitrile are structurally analogous to the bracelet conformation of valinomycin and involve the N--H's of the Val residues and of the Gly's preceding Pro in intramolecular hydrogen bonding. Tl+ was also found to form strong 1:1 complexes with the dodecapeptide.
