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We analyzed the correlations between molecular volume, solvent-accessible surface, and folding state (secondary structure content) for unfolded conformers of (holo- and apomyoglobin) and (retinal-binding protein) proteins and a small water-soluble alanine-rich -helical peptide. Conformers with different degrees of folding were obtained using molecular dynamics at constant temperature and pressure with implicit solvent (dielectric constant adjustment) for all four systems and with explicit solvent for the single helix peptide. Our results support the view that unfolded conformations are not necessary extended, that volume variation is not a good indication of folding state and that the simple model of water penetrating the interior of the protein does not explain the increase in volume upon unfolding. [ABSTRACT FROM AUTHOR]