We employed Monte Carlo simulated annealing to classify the low-energy conformations of Met-enkephalin in the gas phase and in aqueous solution. In order to include the free energy of cavity formation in aqueous solution, we used the method of extended scaled particle theory. This is the first attempt to combine the Monte Carlo simulated annealing method and the extended scaled particle theory. We conducted 20 Monte Carlo simulated annealing runs of 10000 Monte Carlo sweeps both in the gas phase and in aqueous solution. It was found that the obtained conformations (20 each) can be classified into 3 groups of similar structure both in the gas phase and in aqueous solution. We studied in detail the structural characteristics of the classified conformations. It was found that the cavity-formation effects of aqueous solution do not drastically change the backbone structures obtained in the gas phase. The relation between the solvent-accessible surface area and the cavity-formation free energy was studied in detail. The results show unambiguously that the cavity-formation free energy is not neccessarily proportional to the accessible surface area, at least for a small peptide.
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