The structure of a protein molecule is considered to be primarily determined by the inter-amino-acid nonbonded interactions, such as hydrogen bonds. However, the conformational space of the polypeptide chain should be simultaneously restricted by the intrinsic conformational preferences of the individual amino acids. We present here precise single amino acid potential ( SAAP) surfaces for glycine ( For- Gly- NH2) and alanine ( For- AlaNH(2)) in water (epsilon = 78.39) and ether (epsilon = 4.335), which were calculated at the HF/6- 31+ G( d, p) level applying the self- consistent isodensity polarizable continuum model ( SCIPCM) reaction field with geometry optimization in the corresponding solvents. The obtained Ramachandran potential surfaces in water showed distinct potential wells in the alpha- and beta-regions. The profiles were in almost perfect agreement with the Ramachandran plots of glycine and alanine residues in folded proteins, suggesting the Boltzmann distributions on the SAAP surfaces. Molecular simulations of polyalanines ( For- Ala(n)- NH2; n) 3- 5) by using the SAAP force field equipped with the SCIPCM potentials revealed that the polyalanines readily form 3(10)- helical structures in water but not in vacuo. In ether ( hydrophobic environments), the helical structures were relatively stable, but the most stable structure was assigned to a different one. These results indicated that the intrinsic conformational preferences of the individual amino acids ( i. e., the SAAPs) in water are of significant importance not only for describing conformations of a polypeptide chain in the random coil state but also for understanding the folding to the secondary and tertiary structures.