Diazo groups have broad and tunable reactivity. That and other attributes endow diazo compounds with the potential to be valuable reagents for chemical biologists. The presence of diazo groups in natural products underscores their metabolic stability and anticipates their utility in a biological context. The chemoselectivity of diazo groups, even in the presence of azido groups, presents many opportunities. Already, diazo compounds have served as chemical probes and elicited novel modifications of proteins and nucleic acids. Here, we review advances that have facilitated the chemical synthesis of diazo compounds, and we highlight applications of diazo compounds in the detection and modification of biomolecules.

We introduce a stabilized diazo group as a reporter for chemical biology. ManDiaz, which is a diazo derivative of N-acetylmannosamine, is found to endure cellular metabolism and label the surface of a mammalian cell. There, its diazo group can undergo a 1,3-dipolar cycloaddition with a strained alkyne, providing a signal comparable to that from the azido congener, ManNAz. The chemoselectivity of diazo and alkynyl groups enables dual labeling of cells that is not possible with azido and alkynyl groups. Thus, the diazo group, which is approximately half the size of an azido group, provides unique opportunities for orthogonal labeling of cellular components.

The diazo group has untapped utility in chemical biology. The tolerance of stabilized diazo groups to cellular metabolism is comparable to that of azido groups. Yet, chemoselectivity has been elusive, as both groups undergo 1,3-dipolar cycloadditions with strained alkynes. Removing strain and tuning dipolarophile electronics yields diazo group-selective 1,3-dipolar cycloadditions that can be performed in the presence of an azido group. For example, diazoacetamide but not its azido congener react with dehydroalanine residues, as in the natural product nisin.

Nature utilizes large biomolecules to fulfill tasks that require spatially well-defined arrangements at the molecular level such as electron transfer, ligand-receptor interactions, or catalysis. The creation of synthetic molecules that enable precise control over spacing and functionalization provides opportunities across diverse disciplines. Key requirements of functionalizable oligomeric scaffolds include the specific control of their molecular properties where the correct balance of flexibility and rigidity must be maintained in addition to the prerequisite of defined length. These molecules must ideally be equally applicable in aqueous and organic environments, they must be easy to synthesize in a controlled stepwise fashion, and they must be easily modified with a palette of chemical appendages having diverse functionalities. Oligoproline, a peptidic polymer comprised of repeating units of the amino acid proline, is an ideal platform to meet such challenges. Oligoproline derives its characteristic rigidity and well-defined secondary structure from the innate features of proline. It is the only naturally occurring amino acid that has its side-chain cyclized to its α-amino group, generating often-populated trans and cis conformers around the tertiary amide bonds formed in proline oligomers. Oligoprolines are widely applied to define distance on the molecular level as they are capable of serving as both a "molecular ruler" with a defined length and as a "molecular scaffold" with precisely located and predictably oriented substitutions along the polymeric backbone. Our investigations focus on the use of oligoproline as a molecular scaffold. Toward this end, we have investigated the role of solvent upon helical structure of oligoproline, and the effect that substituents on the pyrrolidine ring and the oligomer termini have on the stability of the helix. We have also further explored the molecular characteristics of oligoproline through spectroscopic and crystallographic methods. All of these structural insights laid the basis for implementation of oligoproline in materials science and chemical biology. Within this Account, we highlight the value of oligoprolines for applications in distinctly different research areas. Toward materials chemistry, we have utilized oligoprolines for the size-controlled generation of noble metal nanoparticles, and to probe the role of spatial preorganization of π-systems for molecular self-assembly. Within the biological realm, we have applied oligoprolines to probe the role of distance on G-protein coupled receptor-mediated ligand uptake by cancerous cells and to investigate the effects of charge preorganization on the efficacy of cationic cell-penetrating peptides.

Proline derivatives bearing substituents at Cγ are valuable tools for biological and materials investigations. However, the stereochemistry at Cγ can produce undesired steric or stereoelectronic interactions. Here, we introduce γ‐azaproline (γ‐azPro), which lacks a stereogenic center at Cγ, as a pH‐responsive and functionalizable proline analogue that can adapt to its environment. Conformational analyses by NMR spectroscopy and DFT calculations revealed that the imidazolidine ring of γ‐azPro is flexible. Incorporation of γ‐azPro into collagen model peptides (CMPs) produced pH‐responsive triples helices and triple helices that can be easily functionalized.