A)Structure:- *as the name impliescontain:-

*carboxylic acid and amino groups areattached to the α carbon atom(carbon 2).

*aside chain (R group ) also attached to theα carbon ,R group differ for each of the 20common amino acid(protein).

*functions of the protein are related to thechemical properties of R group.

ids, thus the (R) instead of (S).

An amino acid in its (1) un-ionized and (2) zwitterionic forms

B)Classification:-

Based on R group :-1)Non polar ,Aliphatic side chains amino acid:*Glycine) Gly(-is not asymmetric.-greatest structural flexibility.*proline(Pro):--called imino acid.*Alanine(Ala);-*Branched chain amino acid:-Valine(Val),leucine(leu),iso leucine(lle).

2)Aromatic side chain amino acid:-

*Phenyl alanine(Phe):---phenyl group.-Very hydrophobic.*tyrosine(Tyr) and Tryptophan(Trp);--hydrophobic (more polar)-tyrosine contain phenolic group.-tryptophan contain N in its indol group.

3)Polar ,Uncharged side chains amino acid:-

-hydrophilic (water loving)-found on the surface of globular protein(interact with water).

*Serine(Ser) and Threonine (Tre):--contain polar hydroxyl group so form H bond water or other polar compounds that bind to protein.

*Asparagine (Asn),Glutamine (Gln):--are amides of amino acid aspartate and glutamate.-they are polar because of the carbonyle and nitrogen atoms in their amide groups.

*Cystein(Cys) and Methionine(Met):--polar because they each contain asulfur atom-More hydrophobic because of its sulfhydryl group interact with other sulfhydrl group to form disulfides.

4)Charge side chain amino acid:-

*The negative amino acid:-

*Aspartic(Asp) and Glutamate(Glu):--contain carboxylic acids.-called acidic amino acids

4)Charge side chain amino acid:-

*The negative amino acid:-

*Aspartic(Asp) and Glutamate(Glu):--contain carboxylic acids.-called acidic amino acids

*The positive amino acid:-

*Arginine(Arg) and Lysine(Lys) and Histidine(His):

-side chain contain nitrogen.-ph is 7.4.-Arg side chain it is guanidinium.-Lys side chain it is amino on the € epsilon carbon.-His has an imidazole.

GlycineAlanine

ValineLeucine

Isoleucine Proline

Phenylalanine

Tryptophan

Tyrosine

Glutamine

Asparagine

Serine

Threonine

Cysteine

Methionine

Aspartic

Acid

Glutamic

Acid

Arginine

Lysine

Histidine

C)Optical Properties of the amino acid:-

-The one amino acid not exhibiting is glycinesince its R group is a hydrogen atom.

-The ability of a molecule to rotate the plane of polarized lighteither to the right (dextrorotatory) or to the left(levorotatory).

-All amino acid are L-α-amino acids

-D-amino acids found in polypeptide antibiotics

-D-amino acid are never found in proteins

-The ability of protein to absorb ultravioletlight is predominantly due to the presence of thetryptophan which strongly absorbs U.V.

D)ACID –base Properties:-

*amino acids that do not ionizable side chains are titrated,two pKа values are observed:-

-At low ph ,both groups(α-carboxyl/pk₁,α-amino/pk₂) carry protons .amino group has appositive charge ,carboxyl has zero charge{pkа=1}.

-pH is increased by the addition of alkali(oH),amino group positivecharge,carboxyl is negative{pkа=0}.

-pH is increased by the addition of alkali(oH),amino group positivecharge,carboxyl is negative{pkа=0}.

-Isoelectric point(pI):-the pH at which the net charge on themolecules in solution is zero is called pI.zero the pH will be

equivalent to the isoelectric point: pI.

-At this pH the molecules called zwitterions will not migrate in anelectric field because negative charge equal positive charge., Electricallyneutral at this pH.

-Association constant(kа)is acidic strength of the carboxyl, amino andionizable R-group in amino acids.

-pkа = -log kа.

-Net charge is sum of all charge in amino acid , peptide or protein. itsdepend on ph.

-During the titration of any amino acid and protein the Net chargedepend on pH(Net charge=0 ,the pH is PI.

E)Functional significance of amino acid R.group:-

In solution it is the nature of amino acid R groups that dictate structure-function relationships of peptides and protein.

-the hydrophobic amino acids will be found on the interior of protein(un contact with water).

-The hydrophilic amino acids are found on the exterior(active side ofenzymatic ally active protein).

-The imidazole ring of histidine act proton donor or acceptor atphysiologicalPH.found in the reactive side of enzyme.important in hemoglobin to buffer the H ions fromcarbonic acid ionization in RBCs .(Hb exchange O₂ &CO₂ at the tissuesor lungs ,respectively.

-primary alcohol of serine & threonine as well as the thiol(-SH) ofcysteine act as nucleophiles during enzymatic catalysis.the thiol of cysteine is able to form adisulfidebond with othercysteines(cysteine-S-S-cysteine)this simple disulfideisidentified as cystine.

-Disulfide bond present in a large number of proteins.(formation activestructural domains).

-Disulfide bond in cysteines in different polypeptide chain of oligomericprotein play a crucial role in ordering the structure of complex proteins(insulin receptor).