Initial attempts at a theoretical calculation of the conformation of gramicidin-S

Intermolecular anti-parallel beta sheet: Comparison of predicted and observed conformations of gramicidin S

A recently determined x-ray structure of the hydrated gramicidin S-urea complex is compared with a structure predicted by conformational energy minimization. It is shown that the two structures are in good general agreement, including the prediction of a hydrogen bond between the side-chain amino group of ornithine and the backbone carbonyl of phenylalanine. This agreement demonstrates the power of empirical potential energy methods in conformational analysis and illustrates one method for solution of the multiple-minimum problem for small peptides. It is noted that, in the crystal, gramicidin S is a dimer that forms an intermolecular antiparallel four-stranded beta sheet and that differences between the predicted and x-ray structures can be explained by this intermolecular interaction. The residual conformational asymmetry of the x-ray structure is shown to be due to the formation of the complex with urea.

Atomically detailed folding simulation of the B domain of staphylococcal protein A from random structures

The conformational space of the 10-55 fragment of the B-domain of staphylococcal protein A has been investigated by using the electrostatically driven Monte Carlo (EDMC) method. The ECEPP/3 (empirical conformational energy program for peptides) force-field plus two different continuum solvation models, namely SRFOPT (Solvent Radii Fixed with atomic solvation parameters OPTimized) and OONS (Ooi, Oobatake, Némethy, and Scheraga solvation model), were used to describe the conformational energy of the chain. After an exhaustive search, starting from two different random conformations, three of four runs led to native-like conformations. Boltzmann-averaged root-mean-square deviations (RMSD) for all of the backbone heavy atoms with respect to the native structure of 3.35 A and 4.54 A were obtained with SRFOPT and OONS, respectively. These results show that the protein-folding problem can be solved at the atomic detail level by an ab initio procedure, starting from random conformations, with no knowledge except the amino acid sequence. To our knowledge, the results reported here correspond to the largest protein ever folded from a random conformation by an initial-value formulation with a full atomic potential, without resort to knowledge-based information.

Calculations of the conformations of Iturin A in relation with NMR studies

Iturin A is an antifungal antibiotic which was isolated from a strain of Bacillus subtilis, and contains a lipophilic beta amino acid closing an heptapeptide cycle with polar L and D residues. Iturin A belongs to a lipopeptide family of which the LDDLLDL sequence is kept constant. NMR spectroscopy and semi-empirical energy calculations are combined to design the conformations of Iturin A in pyridine solution. J coupling constants and nOes (nuclear Overhauser enhancements) are used as guiding line for energy calculations. This preliminary study shows that Iturin A in pyridine appears as rather rigid, especially in the L Pro 5-D Asn 6 region, probably involved in a beta turn. The polar side chains can form different networks of intramolecular hydrogen bonds. The Tyr side chain, relatively mobile, could be involved in interactions with an hydrophobic environment as the beta amino acid side chain found away from the peptide cycle.

Analysis of the conformation of polypeptides : the combined use of energy computations and nuclear magnetic resonance studies

This review surveys current approaches to the problem of determining the solution conformation of polypeptides. The basic principles of energy computations are described. The utility, problems, and limitations of various theoretical methods are summarized: conformational energy mapping, energy minimization, scanning of selected local conformations, statistical predictive schemes. The need for combining the calculations with experimental studies is pointed out. The information content of various physico-chemical methods is compared for this purpose. The analysis of nmr coupling constants is discussed in more detail. In combination with energy computations, it can furnish specific information on local aspects of the conformation. Examples of such combined studies on small peptides are summarized.

Conformations at local energy minimums for gramicidin S: optical calculations

Energy minimization techniques applied to an arbitrary peptide primary sequence result in a still large number of hypothetical structures of similar energy. Optical activity calculations can, in principle, be used to screen the low-energy conformations. Those conformations which yield a spectrum different from the observed spectrum by a factor greater than the current reliability of the calculation can be excluded. The method is applied to gramicidin S.