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differential scanning calorimetry Analytical method that independently measures the rate of heat flow to a sample against a reference standard of the same temperature. Data are obtained by monitoring the differential heat flow as a function of temperature. DSC can measure heat capacities, phase transitions, dehydration, and heats of reaction.

diluent A chemically inert substance added to a solution to increase the volume and reduce the concentration; a diluting agent.

dimer A polymer made up of two identical molecules. When three monomers link up, the resultant polymer is called a trimer. Larger polymers are usually referred to by placing a number before the "-mer" suffix: 4-mer, 5-mer, 6-mer, and so on.

dissociation constant A specific type of equilibrium constant that measures the propensity of a larger object to separate (dissociate) reversibly into smaller components, as when a complex falls apart into its component molecules, or when a salt splits up into its component ions. The dissociation constant is usually denoted Kd and is the inverse of the affinity constant. Dissociation constants are commonly used to describe how tightly a ligand (such as a drug) binds to a protein. Such binding is usually non-covalent, i.e., no chemical bonds are made or broken. Since the binding is usually described by a two-state process P + L = C the corresponding dissociation constant is defined Kd = [P][L]/[C] where [P], [L], and [C] represent the concentrations of the protein, ligand, and bound complex, respectively. The dissociation constant has the units of molar (M), and corresponds to the concentration of ligand [L] at which the binding site on the protein is half occupied, i.e., when the concentration of protein with ligand bound [C] equals the concentration of protein with no ligand bound [P]. The smaller the dissociation constant, the more tightly bound the ligand is; for example, a ligand with a nanomolar (nM) dissociation constant binds more tightly than a ligand with a μmolar (mM) dissociation constant.

disulfide bond A covalent bond formed between sulfur atoms of different cysteines in a protein; such bonds (links, bridges) help hold proteins together.

divalent cations Cations with a net positive charge of +2.

DIW Deionized water, very pure water in which contaminants have been ionized and removed by special filtration.

DMSO Dimethyl sulfoxide; a common cryoprotectant used to cryopreserve cells and tissues.

DNA Deoxyribonudeic acid; the nucleic acid based on deoxyribose (a sugar) and the nucleotides G, A, T, and C. Double-stranded DNA has a corkscrew-ladder shape (the "double helix") and is the primary component of chromosomes, which thus carry inheritable characteristics of life. (See nucleotides, nucleic acids)

DNA array Spots of DNA, oligonucle-otide, or cDNA arranged on a solid support such as a glass or silicon "DNA chip" (or microarray), which is used for screening, sequencing, genetic mapping, and so on.

This is a simplistic model of the DNA structure.
DNA fingerprinting Sequences of nucleic acids in specific areas (loci) on a DNA molecule are polymorphic, meaning that the genes in those locations may differ from person to person. DNA fragments can be cut from those sequences using restriction enzymes. Fragments from various samples can be analyzed to determine whether they are from the same person. The technique of analyzing restriction fragment length polymorphism (RFLP) is called DNA typing or DNA fingerprinting.

DNase Deoxyribonudease, the enzyme that breaks up and destroys DNA sequences (a protective mechanism in higher organisms).

DNA sequencing Determing the order of nucleotide bases in a DNA molecule.

DNA vaccine A nucleic acid vaccine: Genes coding for specific antigenic proteins are injected to produce those antigens and trigger an immune response.

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