Conformational studies of poly((S)-beta-aminobutyric acid)

Hydrazino peptides as foldamers: an extension of the beta-peptide concept

Replacing the C(beta) atoms in the beta-amino acid constituents of beta-peptides by nitrogen atoms leads to hydrazino peptides. A systematic conformation analysis of blocked hydrazino peptide oligomers of the general type I at the HF/6-31G, MP2/6-31G, and DFT/B3LYP/6-31G levels of ab initio MO theory and on the basis of molecular mechanics reveals a wide variety of secondary structures, as for instance various helices and sheet- and turnlike conformers. Some of them are closely related to secondary structure types found in beta-peptides; others represent novel types. Thus, a very stable, novel helix with 14-membered hydrogen-bonded pseudocycles, which occupies a conformation space different from that of helices with 14-membered rings found among the most stable conformers in beta-peptides, is indicated. The most important secondary structure elements are characterized by interactions between peptidic NH and CO groups. The additional hydrazino N(alpha)H group takes part in special structuring effects but is of lesser importance for secondary structure formation. The influence of environmental effects on the existence and stability of the various structure types is discussed. Due to the wide variety of structural possibilities, hydrazino peptides might be a useful tool for peptide and protein design.

Basic conformers in beta-peptides

The conformation of oligomers of beta-amino acids of the general type Ac-[beta-Xaa]n-NHMe (beta-Xaa = beta-Ala, beta-Aib, and beta-Abu; n = 1-4) was systematically examined at different levels of ab initio molecular orbital theory (HF/6-31G*, HF/3-21G). The solvent influence was considered employing two quantum-mechanical self-consistent reaction field models. The results show a wide variety of possibilities for the formation of characteristic elements of secondary structure in beta-peptides. Most of them can be derived from the monomer units of blocked beta-peptides with n = 1. The stability and geometries of the beta-peptide structures are considerably influenced by the side-chain positions, by the configurations at the C alpha- and C beta-atoms of the beta-amino acid constituents, and especially by environmental effects. Structure peculiarities of beta-peptides, in particular those of various helix alternatives, are discussed in relation to typical elements of secondary structure in alpha-peptides.

Formation of a molten-globule-like state of myoglobin in aqueous hexafluoroisopropanol

The effects of aqueous hexafluoroisopropanol (HFIP) media on the structure of myoglobin are reported. Circular dichroism (CD) spectra of this alpha-helical protein in as little as 4% (v/v) HFIP indicate that native-like amounts of secondary structure remain while rigid tertiary structure is lost. However, thermal studies suggest some residual cooperativity of unfolding in this state. At much higher HFIP concentrations, the helicity exceeds the native value and the protein behaves as a series of independent helices which do not interact with each other. We did not observe cold denaturation of myoglobin, even though this phenomenon has been observed for molten globule states of myoglobin, as well as for monomeric amphipathic alpha-helices when moderate quantities of HFIP are present. The pH dependence of trifluoroethanol-induced disruption of tertiary structure revealed that the degree of disruption increases as the enthalpic advantage of the folded state is diminished at low pH.