Indication of 3₁₀-Helix Structure in Gas-Phase Neutral Pentaalanine.

Area of Science:

• Chemical Physics
• Molecular Spectroscopy
• Computational Chemistry

Background:

• Understanding peptide structure is crucial for biochemistry and drug design.
• Gas-phase studies provide insights into intrinsic peptide conformations, free from solvent effects.

Purpose of the Study:

• To determine the gas-phase structure of neutral pentaalanine.
• To identify stable conformations and their relative energies.
• To validate computational models with experimental spectroscopic data.

Main Methods:

• Supersonic jet cooling for sample preparation.
• Infrared multiphoton dissociation (IRMPD) spectroscopy.
• Vacuum-ultraviolet (VUV) action spectroscopy.
• Quantum chemical spectral calculations.
• Born-Oppenheimer molecular dynamics simulations.

Main Results:

• Experimental IR spectrum was obtained in the 340–1820 cm^-1 range.
• Multiple stable conformations were identified, primarily of two types.
• The most stable conformation features an N-terminal C5 ring and a backbone resembling an 3₁₀-helix with two β-turns.
• Molecular dynamics simulations revealed a systematic frequency shift due to non-zero simulation time steps.

Conclusions:

• Pentaalanine adopts diverse stable conformations in the gas phase.
• The observed structures provide a benchmark for theoretical peptide modeling.
• Accurate simulation of peptide dynamics requires careful consideration of time step parameters.