The average molecular weight of the 20 standard amino acids is 138, but biochemists use 110 when estimating the number of amino acids in a protein of known molecular weight: Why? The number 110 is based on the fact that the average molecular weight of a protein is 110,000 with an average of 1,000 amino acids_ 2_ The number 110 reflects the number of amino acids found in the typical small protein, and only small proteins have their molecular weight estimated this way: 3_ The number 110 takes into account the relatively small size of nonstandard amino acids_ 4_ The number 138 represents the molecular weight of conjugated amino acids. 5_ The number 110 reflects the higher proportion f small amino acids in proteins, as well as the loss of water when the peptide bond forms_ The most important contribution to the stability of a protein's conformation appears to be the: Entropy increase from the decrease in ordered water molecules forming a solvent shell around it 2 Maximum entropy increase from ionic interactions between the ionized amino acids in a protein. 3_ Sum of free energies of formation of many weak interactions among the hundreds of amino acids in a protein. 4 Sum of free energies of formation of many weak interactions between its polar amino acids and surrounding water: 5_ Stabilizing effect of hydrogen bonding between the carbonyl group of one peptide bond and the amino group of another: