The molecular weight of Cysteine [HOOCCH(NH₂)CH₂SH] is 121.15964.

Cysteine (HOOCCH(NH₂)CH₂SH) is an organic compound of five elements: Carbon, Hydrogen, Nitrogen, Oxygen, and Sulfur. The molecular weight of Cysteine is 121.15964 which can be calculated by adding up the total weight (atomic weight multiplied by their number) of all its elements.

CALCULATION PROCEDURE: Cysteine  [HOOCCH(NH₂)CH₂SH] Molecular Weight Calculation

Step 1: Find out the chemical formula and determine constituent atoms and their number in a Cysteine molecule.
From the chemical formula, you will know different atoms and their number in a Cysteine molecule. The chemical formula of Cysteine is HOOCCH(NH₂)CH₂SH. From the chemical formula, you can find that one molecule of Cysteine consists of three Carbon (C) atoms, seven Hydrogen (H) atoms, one Nitrogen (N) atom, two Oxygen atoms, and one Sulfur atom.

Step 2: Find out the atomic weights of each atom (from the periodic table).

Atomic weight of Carbon (C): 12.0107 (Ref: Jlab-ele006)
Atomic weight of Hydrogen (H): 1.008 (Ref: Lanl-1)
Atomic weight of Nitrogen (N): 14.00674 (Ref: Jlab-ele007)
Atomic weight of Oxygen (O): 15.9994 (Ref: Jlab-ele008)
Atomic weight of Sulfur (S): 32.066 (Ref: Jlab-ele016)

Step 3: Calculate the total weight of each atom in a Cysteine molecule by multiplying its atomic weight by its number.

Number of Carbon atoms in Cysteine: 3
Atomic weight of Carbon (C): 12.0107
Total weight of Carbon atoms in Cysteine: 12.0107 x 3 = 36.0321

Number of Hydrogen atoms in Cysteine: 7
Atomic weight of Hydrogen (H): 1.008
Total weight of Hydrogen atoms in Cysteine: 1.008 x 7 = 7.056

Number of Nitrogen atoms in Cysteine: 1
Atomic weight of Nitrogen (N): 14.00674
Total weight of Nitrogen atoms in Cysteine: 14.00674 x 1 = 14.00674

Number of Oxygen atoms in Cysteine: 2
Atomic weight of Oxygen (O): 15.9994
Total weight of Oxygen atoms in Cysteine: 15.9994 x 2 = 31.9988

Number of Sulfur atoms in Cysteine: 1
Atomic weight of Sulfur (S): 32.066
Total weight of Sulfur atoms in Cysteine: 32.066 x 1 = 32.066

Step 4: Calculate the molecular weight of Cysteine by adding up the total weight of all atoms.

Molecular weight of Cysteine: 36.0321 (Carbon) + 7.056 (Hydrogen) + 14.00674 (Nitrogen) + 31.9988 (Oxygen) + 32.066 (Sulfur) = 121.15964

So the molecular weight of Cysteine is 121.15964.

Cysteine [HOOCCH(NH₂)CH₂SH] Molecular Weight Calculation

REFERENCES:

• Lanl-1: https://periodic.lanl.gov/1.shtml 
• Jlab-ele006: https://education.jlab.org/itselemental/ele006.html 
• Jlab-ele007: https://education.jlab.org/itselemental/ele007.html
• Jlab-ele008: https://education.jlab.org/itselemental/ele008.html
  
• Jlab-ele016: https://education.jlab.org/itselemental/ele016.html

See also:

• Hydrogen chloride molecular weight calculation