A decapeptide is a peptide composed of 10 amino acids. On complete hydrolysis, it breaks down into its constituent amino acids. Given that glycine contributes 47.0% to the total weight of the hydrolyzed products, we need to find the number of glycine units in the decapeptide.

Let the number of glycine units be $n$. Since it is a decapeptide, the total number of amino acids is 10. Let the number of alanine and phenylalanine units combined be $10-n$.

Upon hydrolysis, the molecular weight of the decapeptide (796) is equal to the sum of the molecular weights of the individual amino acids minus the weight of water molecules released during hydrolysis. For a decapeptide, hydrolysis releases 9 water molecules (since each peptide bond cleavage consumes one water molecule). The molecular weight of water is 18, so the total weight of water released is $9\times 18=162$.

Therefore, the total weight of hydrolyzed products (amino acids) is:

$\text{Weight of hydrolyzed products}=796+162=958$

Glycine contributes 47.0% to this total weight. So, the weight contributed by glycine is:

$\text{Weight of glycine}=\frac{47.0}{100}\times 958=450.26$

Since glycine has a molecular weight of 75, the number of glycine units $n$ is:

$n=\frac{450.26}{75}\approx 6.003$

This is approximately 6. Since the number of amino acids must be an integer, we conclude that there are 6 glycine units in the decapeptide.

Related Topics

Peptides and Proteins: Peptides are chains of amino acids linked by peptide bonds. Hydrolysis breaks these bonds, releasing the individual amino acids. The molecular weight calculation must account for water added during hydrolysis.

Percentage Composition: Understanding how to calculate the contribution of a component to the total mass is crucial. Here, glycine's mass percentage is given, which helps find its quantity.

Formulae

For a peptide with $k$ amino acids, hydrolysis releases $k-1$ water molecules. The total weight of hydrolyzed products is:

$\text{Weight of amino acids}=\text{Molecular weight of peptide}+(k-1)\times 18$

The number of a specific amino acid unit $n$ can be found from its mass contribution:

$n=\frac{\text{Mass of the amino acid in hydrolyzed products}}{\text{Molecular weight of the amino acid}}$