Oxidation-Controlled, Strain-Promoted Tellurophene-Alkyne Cycloaddition (OSTAC): A Bioorthogonal Tellurophene-Dependent Conjugation Reaction

Tellurophene-bearing small molecules have emerged as valuable tools for localizing cellular activities in vivo using mass cytometry. To broaden the utility of tellurophenes in chemical biology, we have developed a bioorthogonal reaction to facilitate tagging of tellurophene-bearing conjugates for downstream applications. Using TePhe, a tellurophene-based phenylalanine analogue, labeled recombinant proteins were generated for reaction development. Using these proteins, we demonstrate an oxidation-controlled, strain-promoted tellurophene-alkyne cycloaddition (OSTAC) reaction. Mild oxidation of the tellurophene ring with N-chlorosuccinimide produces a reactive Te(IV) species which undergoes rapid (k > 100 M-1 s-1) cycloaddition with bicyclo[6.1.0]nonyne (BCN) yielding a benzo-fused cyclooctane. Selective labeling of TePhe-containing proteins can be achieved in complex protein mixtures and on fixed cells. OSTAC reactions can be combined with strain-promoted azide alkyne cycloaddition (SPAAC) and copper-catalyzed azide alkyne click (CuAAC) reactions. Demonstrating the versatility of this approach, we observe the expected staining patterns for 5-ethynyl-2'-deoxyuridine (DNA synthesis-CuAAC) and immunohistochemistry targets in combination with TePhe (protein synthesis-OSTAC) in fixed cells. The favorable properties of the OSTAC reaction suggest its broad applicability in chemical biology.

Halogen transfer through halogen bonds in halogen-bound ammonia homodimers

Halogen bond complexes as intermediates in halogen transfer reactions between N-haloamines and ammonia.

Imidazole catalyzes chlorination by unreactive primary chloramines

Hypochlorous acid and simple chloramines (RNHCl) are stable biologically derived chlorinating agents. In general, the chlorination potential of HOCl is much greater than that of RNHCl, allowing it to oxidize or chlorinate a much wider variety of reaction partners. However, in this study we demonstrate by kinetic analysis that the reactivity of RNHCl can be dramatically promoted by imidazole and histidyl model compounds via intermediary formation of the corresponding imidazole chloramines. Two biologically relevant reactions were investigated--loss of imidazole-catalyzed chlorinating capacity and phenolic ring chlorination using fluorescein and the tyrosine analog, 4-hydroxyphenylacetic acid (HPA). HOCl reacted stoichiometrically with imidazole, N-acetylhistidine (NAH), or imidazoleacetic acid to generate the corresponding imidazole chloramines which subsequently decomposed. Chloramine (NH2Cl) also underwent a markedly accelerated loss in chlorinating capacity when NAH was present, although in this case N-α-acetylhistidine chloramine (NAHCl) did not accumulate, indicating that the catalytic intermediate must be highly reactive. Mixing HOCl with 1-methylimidazole (MeIm) led to very rapid loss in chlorinating capacity via formation of a highly reactive chlorinium ion (MeImCl(+)) intermediate; this behavior suggests that the reactive forms of the analogous imidazole chloramines are their conjugate acids, e.g., the imidazolechlorinium ion (HImCl(+)). HOCl-generated imidazole chloramine (ImCl) reacted rapidly with fluorescein in a specific acid-catalyzed second-order reaction to give 3'-monochloro and 3',5'-dichloro products. Equilibrium constants for the transchlorination reactions HOCl + HIm = H2O + ImCl and NH2Cl + HIm = NH3 + ImCl were estimated from the dependence of the rate constants on [HIm]/[HOCl] and literature data. Acid catalysis again suggests that the actual chlorinating agent is HImCl(+); consistent with this interpretation, MeIm markedly catalyzed fluorescein chlorination by HOCl. Time-dependent imidazole-catalyzed HPA chlorination by NH2Cl was also demonstrated by product analyses. Quantitative assessment of the data suggests that physiological levels of histidyl groups will react with primary chloramines to generate a flux of imidazole chloramine sufficient to catalyze biological chlorination via HImCl(+), particularly in environments that generate high concentrations of HOCl such as the neutrophil phagosome.

Oxidation of NADH by chloramines and chloramides and its activation by iodide and by tertiary amines

Irreversible oxidation of reduced nicotinamide nucleotides by neutrophil-derived halogen oxidants (HOCl, chloramines, HOBr, etc.) is likely to be a highly lethal process, because of the essential role of NAD(P)H in important cell functions such as mitochondrial electron transport, and control of the cellular thiol redox state by NADPH-dependent glutathione reductase. Chloramines (chloramine-T, NH(2)Cl, etc.) and N-chloramides (N-chlorinated cyclopeptides) react with NADH to generate the same products as HOCl, i.e., pyridine chlorohydrins, as judged from characteristic changes in the NADH absorption spectrum. Compared with the fast oxidation of NADH by HOCl, k approximately 3 x 10(5) M(-1) s(-1) at pH 7.2, the oxidation by chloramines is about five orders of magnitude slower; that by chloramides is about four orders of magnitude slower. Apparent rate constants for oxidation of NADH by chloramines increase with increasing proton or buffer concentration, consistent with general acid catalysis, but oxidation by chloramides proceeds with pH-independent kinetics. In presence of iodide the oxidation of NADH by chloramines or chloramides is faster by at least two orders of magnitude; this is due to reaction of iodide with the N-halogen to give HOI/I(2), the most reactive and selective oxidant for NADH among HOX species. Quinuclidine derivatives (QN) like 3-chloroquinuclidine and quinine are capable of catalyzing the irreversible degradation of NADH by HOCl and by chloramines; QN(+)Cl, the chain carrier of the catalytic cycle, is even more reactive toward NADH than HOCl/ClO(-) at physiological pH. Oxidation of NADH by NH(2)Br proceeds by fast, but complex, biphasic kinetics. A compilation of rate constants for interactions of reactive halogen species with various substrates is presented and the concept of selective reactivity of N-halogens is discussed.

Consecutive halogen transfer between various functional groups induced by reaction of hypohalous acids: NADH oxidation by halogenated amide groups

Cyclic dipeptides (c-Gly(2), c-Ser(2), c-Gly-Phe, etc.) were used as simple protein models to investigate the HOCl-induced generation and reactivity of chlorinated amide groups. The pH dependence of the kinetics of amide chlorination reveals that ClO(-) (not HOCl) is the reactive agent. N-Chlorinated cyclopeptides are stable up to 30 min, they exhibit narrow absorption bands around 215 nm, and they are capable of oxidizing certain biological substrates, the reactivity decreasing in the order GSH > ascorbate > methionine > NADH >> GSSG. The chloroamide is less reactive, but much more selective in its reactions, than HOCl or ClO(-); thus, with formation of the chloroamide prolonged oxidative effects, directed toward specific target molecules, can be expected. Chlorination of NADH, yielding a catalytically inactive species (NAD/Cl), was investigated in most detail because it is likely to be an important and highly lethal process. The chloroamide group is far more reactive toward NADH than chloroamines derived from primary amines. Chloronucleotides formed by reaction of ClO(-) with inosine, GMP, TMP, or UMP are capable of quantitative chlorine transfer to cyclopeptides; however, no chlorine transfer between the amide nitrogen and primary amines is detectable, in either direction. The results presented enable prediction of chlorine transfer cascades induced by HOCl/ClO(-), involving nucleotides, peptide amide groups, and final target molecules. Chlorinated NAD(P)H, as a stable terminal product of consecutive chlorine transfer reactions, might be a useful biological marker for assessing the role of HOCl in inflammatory events. Bromination by BrO(-) of cyclopeptides is more than two orders of magnitude faster than chlorination by ClO(-), and the reactivity of bromoamide with NADH exceeds that of chloroamide by more than four orders of magnitude.

Reactions of hypochlorous acid with biological substrates are activated catalytically by tertiary amines

The activation of reactions of HOCl with a variety of model substrates by tertiary amines was investigated spectroscopically by tandem-mix and stopped-flow techniques. HOCl-induced chlorination of salicylate can be sped up by several orders of magnitude by catalytic amounts of trimethylamine (TMN). The effect is obviously due to the fast generation of reactive quarternary chloramonium ions, TMN+ Cl, which act as chain carrier in a catalytic reaction cycle. Of various catalysts tested, quinine shows the highest activity; this is attributable to the quinuclidine (QN) substituent, a bicyclic tertiary amine, forming a particularly reactive chloro derivative, QN+ Cl, which does not decompose autocatalytically. The rate of catalytic salicylate chlorination as a function of pH (around pH 7) depends not at least on the basicity of the tertiary amine; the rate increases with pH in the cases of TMN and quinuclidine (high basicity), but decreases with pH in the case of MES (low basicity). Tertiary amines also catalyze the interaction between HOCl and alkenes, as shown using sorbate as model. Reaction of HOCl with the nucleotides GMP and CMP is sped up remarkably by catalytic amounts of tertiary amines. In the case of GMP the same product spectrum is produced by HOCl in absence and presence of catalyst, but a change in the product spectra is obtained when AMP and CMP are reacted with HOCl in presence of catalyst. Using poly(dA-dT).poly(dA-dT) as DNA model, it is shown that HOCl primarily induces an absorbance increase at 263 nm, which indicates unfolding of the double strand due to fast chlorination of thymidine; a subsequent secondary absorbance decrease can be explained by slow chlorination of adenosine. Both the primary and secondary processes are activated by catalytic amounts of quinine. No evidence was found for a radical pathway in TMN-mediated oxidation of formate by HOCl. The present results suggest that low concentrations of certain tertiary amines have the potential of modifying the spectrum of target molecules which can be damaged by HOCl in biological systems.

N-halo derivatives III: stabilization of nitrogen-chlorine bond in N-chloroamino acid derivatives

The chlorination of alpha-amino acids and their related derivatives was investigated. A kinetic study of the stability of these N-chlorinated products led to an elucidation of the factors that significantly influence the stability and reactivity of the nitrogen-chlorine bond in these N-chloramines. From the kinetic investigations, a series of low chlorine potential, soft antimicrobial N-chloramines was developed based on derivatives of alpha-aminoisobutyric acid and related compounds.