Question

(S)- (+)-Monosodium glutamate (MSG) is a flavour enhancer used in many foods. Some people have an allergic reaction to MSG (including headache, chest pains, and an overall feeling of weakness). Fast food often contains substantial amounts of MSG, which is widely used in Chinese food as well. (S)-(+)-MSG has a specific rotation of +24.

a. What is the specific rotation of (R)- (-)-monosodium glutamate?

b. What is the specific rotation of a racemic mixture of MSG?

Short answer

a. -24˚

b. 0

Step-by-step solution

Definition of specific rotation

The observed rotation can be used to calculate the specific rotation at a given temperature and wavelength using the formula-

[𝞪] = 𝞪 / lx c

where, [𝞪] is the specific rotation,

T is the temperature in degrees Celsius,

l is the wavelength of the incident light (when the sodium D-line is used, l is indicated as D),

ɑ is the observed rotation,

l is the length of the sample tube in decimetres, and

c is the sample concentration in grams per 100 millilitres.

Given quantities

Specific rotation of MSG = +24.

Calculation of specific rotation of (R)- (-)-monosodium glutamate

Specific rotation of (-) MSG

C = 169gm / 845 ml l = 2m

θ degree = θ / cl = 9.6/ 169 / 845x2 = -24 degree.

Step 4: Calculation of the specific rotation of a racemic mixture of MSG

A racemic mixture is an equal mix of two enantiomers with a specific rotation of 0 degrees (optically inactive).

Therefore, the specific rotation of the racemic mixture of MSG is 0.