Walter Lwowski

Microwave rotational spectrum of methyl azidoformate

The R branch, a dipole microwave rotational spectra of four isotopic species of methyl azidoformate have been assigned over the range 18–40 GHz. The assignment includes vibrational satellite series as well as the vibrational ground state transitions. The methyl barrier to internal rotation as determined from the E–A doubling data for the first excited state of the methyl torsion is V3=400±10 cm−1. The methyl barrier for the first excited state of another vibrational mode, methyl ground state, is determined to be V3=370±10 cm−1. Only one conformation, that with the azido group and the methyl group cis to the C=O group, was found to be measurably populated at room temperature. A partial vibration–rotation anaylsis has been performed with extremely limited success.

Nitrenes in Photoaffinity Labeling: Speculations of an Organic Chemist

Recent experience shows that nitrenes’ are powerful tools for photoaffinity labeling2 No single nitrene seems to be best for all the different situations encountered, however. Of the nitrenes known to date, arylnitrenes (including the heteroarylnitrenes) have been used almost exclusively.’ Their precursors have always been the corresponding azides, which indeed have many advantages. Aryl azides are readily available and methods by which to attach the aryl azide groups to structures with the required biochemical affinities are known. The photochemical behavior of properly substituted aryl azides is quite favorable: They absorb light of wavelengths well above 300 or even 400 nm, with extinction coefficients high enough to capture most of the incident light. This results in short irradiation times as well as in protection of the biomolecules from unrelated photochemical damage-the high absorption by the azide shields the biomolecules, a t least for the earlier span of the irradiation time. Nitro-substituted aryl and heteroaryl azides are particularly good in this respect. The reactivities of arylnitrenes are relatively low, but they can be enhanced by electron-withdrawing substituents on the aryl ring, especially Fand 02N--. However, arylnitrenes are not altogether ideal: their lifetimes are short and intersystem crossing to the more stable triplet states is usually rapid. (See the article by N. Turro in this volume.) Furthermore, singlet nitrenes undergo rapid intramolecular bond reorganization^,^^^ all of which might lead both to the formation of several different kinds of covalent bonds and to the depletion of the reactive species by several reaction paths. Depending on their goals, investigators might wish for other nitrenes, combining the virtues of arylnitrenes with those of higher reactivity and longer singlet lifetimes, or lower reactivity and greater selectivity. In cases in which escape of the nitrene from the site of its formation seems unlikely, a selective nitrene of long lifetime might be attractive. In such a situation, a selective nitrene might form just one type of covalent bond, i.e., one product, whose identification in just one fraction of a degradation or sequencing scheme would be relatively easy. A very unselective nitrene might form covalent bonds to many or all of the residues it can reach in the original biomolecule adduct, thus spreading the label over several fractions after degradation. In cases in which escape of the nitrene might occur, or in which the nearby residues are of low reactivity towards electrophiles, reactivities higher than those that can be achieved with arylnitrenes might be needed. One might hope that cooperation between biochemists and organic chemists eventually will make it possible both to select a nitrene for a given task and to be able to predict in advance what residues in the biomolecule might be attacked and how the