A polypeptide is a sequence of amino acids linked by peptide bonds, which are formed between the amino and carboxyl group. Thus, the peptide bonds form the backbone of polypeptides, and the side chain of each amino acid sticks out from the side of the chain. By convention, if the number of amino acids in a polypeptide is 50 or less, the molecule is known as a peptide; if the sequence is more than 50 amino acid units, it is known as a protein. The number 50 is somewhat arbitrary but is useful in distinguishing among large and small polypeptides. Small peptides have certain chemical properties that differ from proteins, e.g., peptides are generally soluble in acid, while proteins generally are not.