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# What Is Peptide Molecular Weight?

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• Written By: Ray Hawk
• Edited By: E. E. Hubbard
2003-2017
Conjecture Corporation
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Peptide molecular weight is a number without units that is derived from the ratio of an average mass of a peptide molecule divided by one-twelfth the mass of a atom of carbon 12. The peptide molecular weight can also be described as one mole of the peptide in grams, as a mole is a quantity of the peptide that is equal to its molecular weight. Therefore, since the amino acid weight of proline, for instance, is 97.12, one mole of proline is equal to 97.12 grams of the substance.

It is important to know peptide molecular weight numbers in processes such as peptide synthesis, as the weight can be used to determine the concentration of molecular components. For instance, if one component had a molecular weight of 50 and another had a molecular weight of 100, then the molecules in the second compound would weigh twice as much as those in the first compound. If they were both combined in equal grams, the lower weight compound would have twice as many molecules present in the mixture, since they only weighed half as much, and, instead of a 50/50 concentration, there would be a concentration of approximately 66/33.

Molecular weight analyzer software is often used to calculate peptide molecular weight numbers, as the calculations can be complicated otherwise. Not only are there a variety of chemical element molecular weights to account for in amino acid sequences that make up peptides, but there are also other factors to consider. Both hydrophobic and hydrophilic qualities in relation to water come into play, as well as molar absorption coefficients for the proteins themselves. Avogadro's number, which represents how many molecules exist in a mole, must also be used when determining a peptide's volume from the peptide molecular weight.

Amino acids are the building blocks for peptides. They range from a residue molecular weight, which is the actual weight minus water, of 57.05 for the smallest molecule, glycine, up to 156.19 for the largest molecule, arginine. When two or more amino acids are combined, a peptide molecular weight calculator is often used to determine the new molecular weight.

Determining an accurate peptide molecular weight has important applications in a variety of research. Evidence shows, for example, that low molecular weight peptides are important regulators in biology. Hormones that contain these peptides are active in regions of the body, such as the brain, the nervous system, and the gastrointestinal tract.