Kleinmolekulare

Small molecules, in German often referred to as kleinmolekulare, are low-molecular-weight chemical substances. The term is used to describe organic or inorganic compounds with definite, discrete molecular structures and relatively low molar masses, typically well below those of macromolecules such as polymers or biopolymers. In practice, the threshold varies by field, but many sources define small molecules as having molecular weights roughly under 900–1000 daltons.

Kleinmolekulare are characterized by defined stoichiometry and individual molecular identities. They tend to be isolable as pure substances or simple mixtures and exhibit properties governed by their specific functional groups and three-dimensional shapes. This contrasts with macromolecules, which are large, often polydisperse assemblies like proteins, nucleic acids, or synthetic polymers.

Examples span a wide range, including hydrocarbons (benzene), simple solvents (ethanol), and more complex but still discrete compounds such as caffeine, acetaminophen, aspirin, and glucose. Small molecules are central to many areas of chemistry and related disciplines due to their tunable reactivity, accessibility, and well-understood physicochemical behavior.

Analytical methods commonly used to study kleinmolekulare include mass spectrometry, nuclear magnetic resonance (NMR) spectroscopy, infrared (IR) spectroscopy, UV–visible spectroscopy, and chromatographic techniques. In pharmacology and chemical biology, small molecules serve as drugs, probes, ligands, or reagents that interact with biological targets in a defined and reversible manner.

The concept distinguishes small-molecule chemistry from macromolecular chemistry and underpins much of synthesis, drug discovery, materials science, and metabolomics, where molecular weight and discrete structure influence reactivity, solubility, and bioavailability.