The Dalton, symbolized as Da, is a specialized unit of mass used across chemistry and biology to quantify the microscopic components that make up matter. This unit provides a standard language for discussing the mass of atoms, molecules, and subatomic particles, which are far too small to be conveniently measured using standard metric units like grams or kilograms. By employing the Dalton, scientists can assign precise and manageable mass values to the fundamental building blocks of life. The unit simplifies complex calculations and facilitates clear communication regarding the composition and structure of molecular systems.
Defining the Dalton
The Dalton is officially defined by referencing a particular atomic isotope, establishing a fixed standard of measurement. One Dalton is precisely one-twelfth the mass of an unbound, neutral atom of carbon-12 in its nuclear and electronic ground state. This definition fixes the mass of a single carbon-12 atom at exactly 12 Daltons, providing a stable, universal reference point for measuring all other atomic and molecular masses.
This definition is synonymous with the unified atomic mass unit, or ‘u’, which is why the terms are often used interchangeably in scientific literature. The mass of a single proton or a single neutron is approximately one Dalton, which makes calculating the approximate mass of any atom straightforward by summing the number of protons and neutrons in its nucleus. While the Dalton is an extremely small value, it is equivalent to about \(1.66 times 10^{-27}\) kilograms.
Measuring Macromolecules
The primary practical application of the Dalton occurs in biochemistry, where it is used to measure the mass of macromolecules like proteins, DNA, and synthetic polymers. These large biological structures are composed of thousands of atoms, resulting in masses that are too cumbersome to express with the Dalton unit alone. The mass measurement is fundamental because a molecule’s mass is directly related to its size and structural complexity, which dictates its function within a living system.
For example, the hormone human insulin, which is composed of 51 amino acids across two peptide chains, has a molecular mass of approximately 5,808 Daltons. A molecule like hemoglobin, the oxygen-carrying protein in red blood cells, is substantially larger, existing as a complex of four subunits with a total mass of about 64,500 Daltons. Even larger structures, such as a single base pair in a strand of DNA, possess a mass of about 650 Daltons, meaning an entire human chromosome can reach into the gigadalton range. Using the Dalton provides a direct, relative comparison of size and complexity among these diverse components.
The Kilodalton Connection
When working with large molecules, scientists commonly use the Kilodalton (kDa) to simplify the notation of molecular masses. The prefix “kilo-” indicates a thousand, meaning one Kilodalton is equal to 1,000 Daltons. This unit is a practical necessity because many biologically relevant molecules have masses reaching tens or hundreds of thousands of Daltons.
For instance, expressing the mass of the hemoglobin protein as 64,500 Daltons is less concise in a research paper than stating its mass is 64.5 kDa. This conversion simplifies large, unwieldy numbers into more manageable figures, which is the standard practice in published literature and laboratory reports. The use of the Kilodalton streamlines communication and data analysis in fields like proteomics and structural biology.