What is peptide bond?

Peptide bond-

Formation of peptide bond-  Amino acids are joined together  in a sequence by peptide bond . When α-amino group of 1 amino acid reacts with a α- carboxyl group of another amino acid a peptide bond is formed with elimination of a molecule of H2O to yield a dipeptide.

Sequence  -   - C-N- 

Characteristics of peptide bond-

• It can be broken by amide hydrolysis ; (metastable) in the presence of H2O  they will break spontaneously .
• It has a partial double bond character..  - C-N- (Rotation is allowed in the axis.   - C and  N-. But the rotation is not possible in C-N axis so behaves like a partial double bond character.) 

Image source -

http://iweb.langara.bc.ca/biology/mario/Biol2315notes/biol2315chap3.html

• It is rigid and planar (all the molecules are in a same plane).Planar as resonance between the carboxylic group and the amino nitrogen leads the C-N bond a partial double bond character. 
• Like all amide linkages the -C=O and  -NH groups of the peptide bond are uncharged.
• Neither accept nor release protons over the pH range of 2-12.
•  The -C=O and  -NH groups of the peptide bond are  polar ( both hydrophobic and hydrophillic nature) and involved in H-bonding.

Q1. Three amino acids of molecular weights 75,89 and 105 daltons each have a tripeptide . The molecular wt. of tripeptide in dalton will be -
a. 251                           b.269                                   
c.cannot be predicted.  d.233

Ans. 233 how ?
 Well tripeptide means 2 (H2O)water molecules are expelled i,e 18+18 =36
Total mol wt . is 75+89+105- 36= 233.

2 .What is true about the peptide bond of a protein -
a. Planar,but rotates to three preferred dihedral angles.
b.Non-polar but rotates to three preferred dihedral angles
c.Polar and fixed in a trans confirmation
d.Planar and usually found in a trans conformation 

Ans.  d

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