H14 Proteins and DNA DNA is one of the most important compounds known and its importance and uses are increasing rapidly. Proteins that make up so much of living organisms are made using the genetic information stored in the DNA molecule. Proteins Naturally occurring condensation polymers of amino acids. There are two main types of protein Fibrous proteins including o Keratin o Collagen o Fibroin Globular proteins including o Insulin o Haemoglobin o Myoglobin Amino acids Compounds that contain both an amine (-NH2) group and a carboxylic acid (-COOH) functional group. There are 20 amino acids commonly found in proteins and these fall into three groups: Non-polar R groups Polar R groups capable of being charged Polar R groups that do not tend to become charged. Zwitterions Amino acids exist primarily as dipolar ions because the acidic –COOH group loses a proton to the basic –NH2 group. Amino acids are consequently crystalline solids with relatively high melting points and they are soluble in water. Dipeptide Compound formed when two amino acids react together. Water is also formed from this condensation reaction. Peptide bond The amide linkage ( - CO – NH - ) between two amino acids that combine to form a dipeptide. Determination of protein structure The chemist needs to determine the Identity of all the amino acids present Relative molar amounts in which these amino acids are present The sequence in which the amino acids are linked together. Tests for proteins and/ or amino acids These tests indicate that a protein or amino acids are present: Ninhydrin turns purple when heated with either a protein or amino acid. Biuret turns purple when a few drops of copper sulfate are added to an alkaline solution of protein or polypeptide. Biuret does not give a positive result with amino acids. Chromatography Method of separating substances based on their differential distribution between two phases, one stationary and the other mobile. Electrophoresis Method of separating substances based on the different signs of their charge and on their different mobilities. The speed at which particles move depends on their size and charge. Neutral molecules do not migrate during electrophoresis, positively charged ions move in one direction and negatively charged ions move in the opposite direction. Small ions move faster than large ions (given the same charge) Doubly charged ions move faster than singly charged ions of the same size. Isoelectric point The pH at which an amino acid exists as an over-all neutral zwitterion. Chromatography and electrophoresis Both separate mixtures of amino acids Chromatography separates amino acids based on their solubilities in polar and non-polar solvents. The degree of separation can be improved by changing the solvent. Electrophoresis separates amino acids based on their charges and size. Altering the pH of the solution can vary the charge on the amino acid. Electrophoresis gives more effective separation than chromatography but involves the use of more expensive equipment. DNA Abbreviation for deoxyribonucleic acid. Acid because of the many phosphoric acid groups Nucleic because the molecule occurs in the nucleus of cells Deoxyribo- because the molecule contains the sugar deoxyribose. DNA is a condensation polymer in which the monomer units are nucleotides. DNA occurs as a double helix in which the two helixes are held together by hydrogen bonds. The hydrogen bonds holding the two helixes together easily break and reform. Nucleotide Consists of a sugar molecule (deoxyribose in DNA) attached to a phosphate unit and to a base. Four bases are used in DNA Adenine Guanine Cytosine Thymine The bases form complementary pairs in the DNA molecule Adenine with thymine Guanine with cytosine Genes DNA carries the genetic code that allows living things to function and reproduce themselves. Genes are sections of DNA molecules. Each DNA molecule contains thousands of genes. The sequence of bases along the section determines the sequence of amino acids in the protein that the gene synthesises. Exons are coding sequences along the DNA strand (genes). Introns are the non-coding sequences and vary significantly from one person to another. Introns are used for DNA analysis by forensic chemists. Steps in DNA analysis Uses for DNA analysis Separate the DNA from other material in the sample Make multiple copies of selected segments of the DNA in intron regions by using the polymerase chain reaction. Determine the length of these copied segments by electrophoresis. Rather than recording the number of base pairs in each fragment the practice is to record the number of repeat units, called short tandem repeats (STRs) Compare the samples to see if they match by comparing the DNA profile (the number of STRs on each DNA molecule for 10 different segments from identical positions). Identify the person who produced a biological sample at a crime scene. Identify the father of a child in disputed paternity cases Establish familial links