Is the parallel or antiparallel beta-sheet more stable? A semiempirical study

Area of Science:

• Computational chemistry
• Molecular modeling
• Biophysics

Background:

• Peptidic beta-sheets are fundamental secondary structures in proteins.
• Understanding the stability of parallel versus antiparallel beta-sheet arrangements is crucial for predicting protein folding and function.
• Previous studies have explored beta-sheet energetics, but precise computational analysis of small model systems is valuable.

Purpose of the Study:

• To calculate and compare the geometric and energetic properties of parallel and antiparallel peptidic beta-sheets.
• To validate computational methods against experimental data using cyclooctapeptide dimers.
• To investigate the influence of amino acid composition (Alanine and Glycine) on beta-sheet stability.

Main Methods:

• Utilized the Austin Model 1 (AM1) semi-empirical quantum mechanical method for calculations.
• Modeled beta-sheets using two peptide chains with up to 11 amino acid residues (Alanine and Glycine).
• Employed cyclooctapeptide dimers as model systems for structural and energetic analysis.

Main Results:

• Calculated a very small enthalpic difference between parallel and antiparallel beta-sheet arrangements.
• Observed good agreement between calculated and experimental data for cyclic systems.
• Achieved a root-mean-square deviation (RMSD) of 0.223 Å for the cyclooctapeptide dimer 1 (cyclo-D,L-(Ala)8) compared to X-ray crystallographic data.

Conclusions:

• The AM1 method accurately predicts the relative stability of parallel and antiparallel beta-sheet conformations.
• Computational modeling provides a reliable approach for studying peptide structure and energetics.
• The findings support the experimental observation of minimal energetic preference between parallel and antiparallel arrangements in cyclic peptides.