Muonium Addition Reactions and Kinetic Isotope Effects in the Gas Phase: k∞ Rate Constants for Mu + C2H2.

The kinetics of the addition reaction of muonium (Mu) to acetylene have been studied in the gas phase at N2 moderator pressures mainly from ∼800 to 1000 Torr and over the temperature range from 168 to 446 K, but also down to 200 Torr at 168 K and over a much higher range of pressures, from 10 to 44 bar at 295 K, demonstrating pressure-independent rate constants, kMu(T). Even at 200 Torr moderator pressure, the kinetics for Mu + C2H2 addition behave as if effectively in the high-pressure limit, giving k∞ = kMu due to depolarization of the muon spin in the MuC2H2 radical formed in the addition step. The rate constants kMu(T) exhibit modest Arrhenius curvature over the range of measured temperatures. Comparisons with data and with calculations for the corresponding H(D) + C2H2 addition reactions reveal a much faster rate for the Mu reaction at the lowest temperatures, by 2 orders of magnitude, in accord with the propensity of Mu to undergo quantum tunneling. Moreover, isotopic atom exchange, which contributes in a major way to the analogous D atom reaction, forming C2HD + H, is expected to be unimportant in the case of Mu addition, a consequence of the much higher zero-point energy and hence weaker C-Mu bond that would form, meaning that the present report of the Mu + C2H2 reaction is effectively the only experimental study of kinetic isotope effects in the high-pressure limit for H-atom addition to acetylene.

Muon implantation of metallocenes: ferrocene.

Muon Spin Relaxation and Avoided Level Crossing (ALC) measurements of ferrocene are reported. The main features observed are five high field resonances in the ALC spectrum at about 3.26, 2.44, 2.04, 1.19 and 1.17 T, for the low-temperature phase at 18 K. The high-temperature phase at 295 K shows that only the last feature shifted down to about 0.49 T and a muon spin relaxation peak at about 0.106 T which approaches zero field when reaching the phase transition temperature of 164 K. A model involving three muoniated radicals, two with muonium addition to the cyclopentadienyl ring and the other to the metal atom, is postulated to rationalise these observations. A theoretical treatment involving spin-orbit coupling is found to be required to understand the Fe-Mu adduct, where an interesting interplay between the ferrocene ring dynamics and the spin-orbit coupling of the unpaired electron is shown to be important. The limiting temperature above which the full effect of spin-orbit interaction is observable in the muSR spectra of ferrocene was estimated to be 584 K. Correlation time for the ring rotation dynamics of the Fe-Mu radical at this temperature is 3.2 ps. Estimated electron g values and the changes in zero-field splittings for this temperature range are also reported.

[Serotypes and antimicrobial susceptibility of Streptococcus agalactiae]. / Serotipos y susceptibilidad antimicrobiana de Streptococcus agalactiae.

BACKGROUND: Streptococcus agalactiae or group B streptococcus, GBS, is the leading cause of neonatal and maternal infections and an opportunistic pathogen in adults with underlying disease. In the last decade, a dramatic increase in the resistance of this microorganism to erythromycin and clindamycin has been observed. AIM: To determine the serotype distribution and antimicrobial susceptibility of isolates of S. agalactiae collected from infections and colonization and to assess the genetic mechanisms of macrolide and clindamycin resistance. MATERIAL AND METHODS: A total of 100 GBS isolates were collected between 1998 and 2002, in Santiago, Chile. They were isolated from the amniotic fluid from patients with premature rupture of membranes (7 isolates), blood from neonatal sepsis (10 isolates), neonate colonizations (2 strains), skin and soft tissue infections (7 isolates), urinary tract infections (5 isolates), genital infections (3 isolates), articular fluid (one isolate), and 65 strains were recovered from vaginal colonization55. RESULTS: Serotypes Ia, II and III were the predominant serotypes identified in our study, accounting for 90 (90%) of the strains. Five isolates belonged to serotypes Ib (5%) and two (2%) to serotype V respectively; no strains belonging to serotype IV were found. All strains were susceptible to penicillin G, ampicillin and cefotaxime, and four isolates (4%) were resistant to both erythromycin (MIC >64 microg/ml) and clindamycin (MIC >64 microg/ml). The strains had a constitutive macrolide-lincosamide-streptogramin (cMLSB) resistance phenotype and the erm(A) gene was present in the four isolates. CONCLUSIONS: Serotypes Ia, II and III were the predominant serotypes in this study. All strains were susceptible to penicillin G, ampicillin and cefotaxime, and four (4%) strains were resistant to both erythromycin and clindamycin. The cMLSB resistance phenotype, and the erm(A) gene was detected in resistant strains.

Muonium addition to DMPO and PBN sorbed in silica-gel.

Nitroxyl radicals were formed by adding the light hydrogen isotope, muonium to the spin traps DMPO (5,5-dimethyl-1-pyrroline-N-oxide) and PBN (N-tert-butyl-alpha-phenylnitrone) sorbed as 30 wt% ethanol solutions in silica-gel; evidence is presented for a specific hydrogen-bonded interaction between the DMPO adduct and the silica surface; longitudinal-field muon spin relaxation measurements (LF-MuSRx) were performed which identified two distinct motional regimes in both samples.

A radiolabelling study of radicals derived from benzene, toluene and benzaldehyde sorbed in model environmental carbon.

Radiolabelled free radicals were formed by the addition of muonium--a radioactive hydrogen atom with a positive muon as its nucleus--to benzene, toluene and benzaldehyde, as sorbed in porous carbon. The activation parameters associated with their reorientational motion were measured using longitudinal-field muon spin relaxation (LF-MuSRx). Two distinct sorbed fractions were detected in each sample, characterised by molecular reorientational activation energies of 5.9/25.8 kJ/mol for benzene, 2.5/5.9 kJ/mol for toluene and 2.9/11.5 kJ/mol for benzaldehyde.

Mycoplasma hominis y ureaplasma urealyticum en embarazadas e infértiles: diferencias en patologías tubaria / Mycoplasma hominis and ureaplasma urealyticum in pregnant and infertile women: differences in tube pathology

Estudiamos la frecuencia de gérmenes cérvico-vaginales en 47 pacientes embarazadas (entre 16 a 41 semanas de gestación), en 47 pacientes infértiles y en 4 pacientes ginecológicas. Al comparar la frecuencia de presentación de todos los gérmenes estudiados entre las embarazadas y las mujeres infértiles, la mayor diferencia se encontró para u. urealyticum. En efecto, la frecuencia de u. urealyticum fue de 61 por ciento en forma global, 72 por ciento en las embarazadas y 47 por ciento en las infértiles, a diferencia de lo que ocurrió con m. hominis que se presentó con una frecuencia de 18 por ciento para el total de las pacientes estudiadas, 13 por ciento en las embarazadas y 21 por ciento en las infértiles. La mayor frecuencia con que se presentó u. urealyticum en las embarazadas fue estadísticamente significativa y probablemente se deba a un efecto de los estrógenos que facilitan la proliferación de esta bacteria. Dentro del grupo de pacientes infértiles, las que tuvieron daño tubario, presentaron una frecuencia de gérmenes mayor que las sin daño tubario, a diferencia de las con daño tubario en que se presentó con una frecuencia de 25 por ciento