No abstract

Following the observation that allyl radicals trapped in an argon matrix can be photolytically converted into cyclopropyl radicals (λ = 410 nm, 18 K), the IR spectrum of the cyclopropyl radical was recorded for the first time and interpreted. Bicyclopropane and cyclopropane are formed when the photolysed argon matrix is warmed from 18 to 35 K. The identification of these new species unequivocally proves the presence of cyclopropyl radicals in the photolysed matrix. This radical is shown to be a σ‐type (Cs symmetry) and not a π‐type (C2v symmetry) radical; of the 18 normal frequencies of the Cs cyclopropyl radical, all active in the IR, 16 were observed and were assigned to their corresponding normal modes. For this assignment advantage was taken of ab initio frequency computations reported in the literature and performed by the authors.

Electrostatic calculations of the p K, of ionizable groups in the reaction center of Rhodopseudomonas (Rps.) viridis were carried out to investigate three possible mechanisms for proton transfer to the singly reduced acceptor ubiquinone QB. The program Delphi, which solves the Poisson-Boltzmann equation given the distribution of charges and dielectric boundaries, was used to determine the electrostatic potential. The shift in pK, of the titratable residues in the Qs binding pocket in response to the one-electron reduction and following protonation of QB was obtained from calculated interactions with the reaction field, background protein dipoles, charged cofactors, and other ionizable residues. A limited number of bound waters was also included in the computations as titrating sites. Their titration behavior was shown to be strongly coupled to neighboring ionizable sites. The results show that strong electrostatic interaction between the radical anion QB' and a neighboring serine residue (SER L 223) as well as the protein environment stabilize a system in which the incoming proton is localized on serine and only shared in a hydrogen-bonding relationship with QB' . These results hint to the possibility that actual proton transfer to QB' only occurs after a second negative charge has been added to the system through transfer of a second electron either to the menaquinone QA, with formation ofthe QB'QA' system, or to QB' , leading to the doubly reduced Qi-. This preposition is consistent with spectroscopical and electron nuclear double resonance (ENWR) experimental results for bacterial reaction centers (RCS) failing to find evidence for the existence of the protonated semiquinone QBH.. 0 1993 John Wiley & Sons, Inc.

A pharmacophore for p-opioid receptor recognition based on a study of the fentanyl class of opioids has recently been characterized in our laboratories. To validate this pharmacophore, we have extended our theoretical studies to include four opiate analogs from structurally different classes and with high affinity but varying selectivity for the p-opiate receptor. An extensive conformational search of the flexible regions of these compounds has been carried out at two levels of approximations, using the CHARMm force field and the semiempirical molecular orbital method AM^. In a subsequent step, we have determined a series of structural, environmental, and electronic properties for each low-energy conformer of the analogs studied. All four analogs studied can assume a low-energy conformation in which at least three of the four stereoelectronic properties identified as modulators of recognition in the fentanyls are present in the same spatial arrangement. These results provide additional evidence for the plausibility of the proposed pharmacophore for p-opioid receptor recognition. 8