Question

(6 pts total: 2 pts each) Proteins that catalyze chemical reactions in living systems are called enzymes. The basic building blocks of proteins are called amino acids and have the general structure pictured below, left, where R is a substituent containing one of many organic functional groups. Some representative examples of amino acids found in nature are pictured here as well (R groups only) O . CH, HO-C-H H-C-H HẠC-C-CH, CH,SH HNT-CO,H Cysteine Valine Threonine Aspartic Acid An Amino Acid Based on your knowledge of organic structure, identify (by naming) which of the amino acids pictured above contains an 'R' group that corresponds to the following. Place your answer in the space provided pka - 15 pka - 7 pka - 5 2. (4 pts total, 2 pts each) Rank the following compounds in order of increasing Bronsted acidity (1 = strongest acid, 3 = weakest acid and 2 = the one in the middle) CHÚCH CHO CHCH2OH a. CH3COH CH.CH.SH b. CH3CH2NH2 CH3CH2OH

Answer 1

Solution:

1) pka = 15 ( Valine)

pka = 7 ( Threonine)

pka = 5 ( Cystein)

The presence of two alkyl side chain decreases the acidity of valine, hence its pka value maximum.

Cystein is highly acidic than threonine due to presence of sulfur.

2) Strength of acids:

CH3CO2H (1) , CH3CHO (3), CH3CH2OH (2)

CH3CH2NH2 (3), CH3CH2OH (2) , CH3CH2SH (1)

3) Name of functional groups:

A) -OH. = Alcohol

B) -NH2 = Amine

C) CO2H = Carboxylic acid

D) = (double bond) = Alkene

E) -OCH3 = ether

F) CHO = aldehyde

G) R-COOR = ester

H) - Br = halide

I) Triple bond = Alkyne