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G Porter

Publications and source records attributed to G Porter.

At least 19 recordsLinked to original sources
PubMed1979

Picosecond time-resolved fluorescence study of chlorophyll organisation and excitation energy distribution in chloroplasts from wild-type barley and a mutant lacking chlorophyll b.

Picosecond time-resolved fluorescence spectroscopy has been used to investigate the fluorescence emission from wild-type barley chloroplasts and from chloroplasts of the barley mutant, chlorina f-2, which lacks the light-harvesting chlorophyll a/b-protein complex. Cation-controlled regulation of the distribution of excitation energy was studied in isolated chloroplasts at the Fo and Fm levels. It was found that: (a) The fluorescence decay curves were distinctly non-exponential, even at low excitation intensities (less than 2 x 10(14) photons . cm(-2). (b) The fluorescence decay curves could, however, be described by a dual exponential decay law. The wild-type barley chloroplasts gave a short-lived fluorescence component of approximately 140 ps and a long-lived component of 600 ps (Fo) or 1300 ps (Fm) in the presence of Mg2+; in comparison, the mutant barley yielded a short-lived fluorescence component of approx. 50 ps and a long-lived component of 194 ps (Fo) and 424 ps (Fm). (c) The absence of the light-harvesting chlorophyll a/b-protein complex in the mutant results in a low fluorescence quantum yield which is unaffected by the cation composition of the medium. (d) The fluorescence yield changes seen in steady-state experiments on closing Photosystem II reaction centres (Fm/Fo) or on the addition of MgCl2 (+Mg2+/-Mg2+) were in overall agreement with those calculated from the time-resolved fluorescence measurements. The results suggest that the short-lived fluorescence component is partly attributable to the chlorophyll a antenna of Photosystem I, and, in part, to those light-harvesting-Photosystem II pigment combinations which are strongly coupled to the Photosystem I antenna chlorophyll. The long-lived fluorescence component can be ascribed to the light-harvesting-Photosystem II pigment combinations not coupled with the antenna of Photosystem I. In the case of the mutant, the two components appear to be the separate emissions from the Photosystem I and Photosystem II antenna chlorophylls.

Chlorophyll↗
PubMed1979

The fluorescence decay kinetics of in vivo chlorophyll measured using low intensity excitation.

We report fluorescence lifetimes for in vivo chlorophyll a using a time-correlated single-photon counting technique with tunable dye laser excitation. The fluorescence decay of dark-adapted chlorella is almost exponential with a lifetime of 490 ps, which is independent of excitation from 570 nm to 640 nm. Chloroplasts show a two-component decay of 410 ps and approximately 1.4 ns, the proportion of long component depending upon the fluorescence state of the chloroplasts. The fluorescence lifetime of Photosystem I was determined to be 110 ps from measurements on fragments enriched in Photosystem I prepared from chloroplasts with digitonin.

Chlorella↗
PubMed1979

Reduced T cell reactivity in vasectomized rhesus monkeys: association with histocompatibility type.

The capacity of peripheral lymphocytes from rhesus monkeys which had been vasectomized for 7 or 11 years to stimulate and respond to normal rhesus lymphocytes in mixed lymphocyte cultures (MLC) was tested to determine whether vasectomy affects immunologic reactivity. The ability to respond in MLC, a T cell function, was significantly reduced in the 11-year vasectomized animals and in two 7-year vasectomized animals. The ability to stimulate in MLC, a B cell function, was significantly increased in the 11-year vasectomized group. MLC reactivity of normal lymphocytes cultured in plasma from vasectomized animals and lymphocytes from vasectomized animals cultured in normal plasma was not altered, ruling out serum effects in the reduction of MLC responsiveness in these vasectomized animals. Seventy-five per cent of the vasectomized animals with markedly reduced MLC reactivity had the RhL-A determinates 19 and 24, indicating an association between the tendency toward reduced MLC reactivity after vasectomy and histocompatibility type.

Animals↗
PubMed1978

Picosecond time-resolved energy transfer in Porphyridium cruentum. Part I. In the intact alga.

The wavelength-resolved fluorescence emission kinetics of the accessory pigments and chlorophyll a in Porphyridium cruentum have been studied by pico-second laser spectroscopy. Direct excitation of the pigment B-phycoerythrin with a 530 nm, 6 ps pulse produced fluorescence emission from all of the pigments as a result of energy transfer between the pigments to the reaction centre of Photosystem II. The emission from B-phycoerythrin at 576 nm follows a nonexponential decay law with a mean fluorescence lifetime of 70 ps, whereas the fluorescence from R-phycocyanin (640 nm), allophycocyanin (660 nm) and chlorophyll a (685 nm) all appeared to follow an exponential decay law with lifetimes of 90 ps, 118 ps and 175 ps respectively. Upon closure of the Photosystem II reaction centres with 3-(3,4-dichlorophenyl)-1,1-dimethylurea and preillumination the chlorophyll a decay became non-exponential, having a long component with an apparent lifetime of 840 ps. The fluorescence from the latter three pigments all showed finite risetimes to the maximum emission intensity of 12 ps for R-phycocyanin, 24 ps for allophycocyanin and 50 ps for chlorophyll a. A kinetic analysis of these results indicates that energy transfer between the pigments is at least 99% efficient and is governed by an exp --At1/2 transfer function. The apparent exponential behaviour of the fluorescence decay functions of the latter three pigments is shown to be a direct result of the energy transfer kinetics, as are the observed risetimes in the fluorescence emissions.

Chlorophyll↗
PubMed1978

Picosecond time-resolved energy transfer in Porphyridium cruentum. Part II. In the isolated light harvesting complex (phycobilisomes).

The transfer of excitation energy between phycobiliproteins in isolated phycobilisomes has been observed on a picosecond time scale. The photon density of the excitation pulse has been carefully varied so as to control the level of exciton interactions induced in the pigment bed. The 530 nm light pulse is absorbed predominantly by B-phycoerythrin, and the fluorescence of this component rises within the pulse duration and shows a mean 1/e decay time of 70 ps. The main emission band, centred at 672 nm, is due to allophycocyanin and is prominent because of the absence of energy transfer to chlorophyll. Energy transfer to this pigment from B-phycoerythrin via R-phycocyanin produces a risetime of 120 ps to the fluorescence maximum. The lifetime of the allophycocyanin fluorescence is found to be about 4 ns using excitation pulses of low photon densities (10(13) photons.cm-2), but decreases to about 2 ns at higher photon densities. The relative quantum yield of the allophycocyanin fluorescence decreases almost 10 fold over the range of laser pulse intensities, 10(13)--10(16) photons-cm-2. Fluorescence quenching by exciton-exciton annihilation is only observed in allophycocyanin and could be a consequence of the long lifetime of the single exciton in this pigment.

Energy Transfer↗
PubMed1978

Comparative nephrotoxicity of gentamicin and tobramycin in rats.

A rat model was utilized to compare the nephrotoxic potential of gentamicin and tobramycin. Gentamicin, 40 mg/kg per day, predictably produced renal failure and morphological evidence of proximal tubular necrosis over 14 days of treatment. An identical dosage of tobramycin was associated with only minimal morphological changes and normal concentrations of serum creatinine and blood urea nitrogen. Similar results were obtained even after the tobramycin dosage was tripled to 120 mg/kg per day. A decrease in urine osmolality, mechanism unknown, was observed in all aminoglycoside-treated rats, but the lowest osmolalities were found in the gentamicin-treated rats. According to both histological criteria and renal function measurements, gentamicin was more nephrotoxic than tobramycin in this animal model.

Animals↗
PubMed1978

Gentamicin and tobramycin nephrotoxicity. A morphologic and functional comparison in the rat.

Fischer 344 rats were treated with tobramycin or gentamicin, 40 mg/kg/day, for up to 10 days or with tobramycin, 120 mg/kg/day, for up to 14 days. Serum creatinine and BUN at the time of sacrifice were determined, and kidney tissues were examined by light and electron microscopy. Rats receiving gentamicin demonstrated progressive renal proximal tubular necrosis which was nearly universal at the end of 10 days. Their BUN and creatinine levels rose progressively over the same period. Even at the higher dosage, tobramycin therapy resulted in only rare foci of proximal tubular necrosis and minimal elevation of BUN and creatinine. Although they occurred later and were substantially less severe, the ultrastructural changes induced by tobramycin were the same as those seen following gentamicin administration. These results indicate that the mechanism of tobramycin-induced renal injury is probably similar to that of gentamicin and that tobramycin is significantly less nephrotoxic in this experimental model.

Animals↗
PubMed1977

Excited state annihilation in the photosynthetic unit.

The kinetics of the in vivo fluorescence decays and fluorescence yields, as a function of excitation intensity, have been analysed with a model using excited state annihilation and time-dependent quenching processes. Triplet states, formed in the singlet-singlet annihilation processes, account for additional quenching of singlet states and the persistence of annihilation at longer times than the fluorescence life-time. Together these processes give a satisfactory account of existing experimental data of the intensity dependence of fluorescence in vivo.

Chlorophyll↗
PubMed1977

Intensity effects on the fluorescence of in vivo chlorophyll.

A technique for measuring relative quantum yields of fluorescence with a picosecond streak camera is described. We show that Chlorella pyrenoidosa exhibit an intensity dependent quantum yield when irradiated with single picosecond light pulses. This effect also occurs under conditions that inhibit the activity of the reaction centres, which can therefore be excluded as the cause. When a pulse train (pulse separation 6.9 ns) was used, the quantum yield was further reduced by the light absorbed from previous pulses, which indicates the formation of a quenching species having a relatively long lifetime. Absolute quantum yields calculated from the fluorescence decay show that single excitation pulses of 3 - 10(13) photons/cm2 give results comparable to those obtained by very low intensity methods.

Chlorella↗
PubMed1977

Picosecond laser study of fluorescence lifetimes in spinach chloroplast photosytem I and photosystem II preparations.

Fractions enriched in either Photosystem I or Photosystem II have been prepared from chloroplasts with digitonin. A more detailed analysis of the decay kinetics of fluorescence excited by a picosecond laser pulse has been possible compared to experiments with unfractionated systems. The Photosystem I fractions show a very short component (less than or equal to 100 ps) at room temperature which is apparently independent of pulse intensity over the range of photon densities used (5 - 10(13)--1 - 10(16) photons cm-2). The Photosystem II fraction has a short initial lifetime at room temperature which is strongly intensity-dependent approaching 500 ps at low photon densities, but decreasing to close to 150 ps at the highest photon densities. All of these room temperature decays appear to be non-exponential, and may possibly be fitted by at t1/2 expression, expected from a random diffusion of excitations via Förster energy transfer. On cooling to 77K, lifetimes of both Photosystem I and Photosytem II increase, the lengthening with Photosystem I being more striking. The Photosystem I decays become intensity dependent like the Photosystem II, and at the lowest photon densities decays which are more nearly exponential within the experimental error give initial lifetimes of about 2 ns. The non-exponential decays seen at high photon densities appear to fit a t1/2 expression.

Chloroplasts↗
PubMed1977

Cell-mediated immunity in vasectomized rhesus monkeys.

Cell-mediated immunity in rhesus monkeys that had been vasectomized for 2, 4, 7, or 11 years was measured by lymphocyte blastogenesis following stimulation with concanavalin A, phytohemagglutinin (PHA), and pokeweed mitogens. Several of the 7- and 11-year vasectomized animals had significantly reduced PHA reactivity when compared with control animals, and the percentage of animals with reduced PHA reactivity increased with time after vasectomy.

Alpha-Globulins↗
PubMed1977

Kinetic differences between uterine and renal renins in the dog.

It has previously been reported that in the dog the Michaelis constant (Km) of renal renin with respect to homologous plasma substrate is the same as that for the angiotensin-generating enzyme extractable from the myometrium during pregnancy. We have reexamined the kinetics with a different methodology. Enzyme preparations from dog kidneys, the uterus of a nonnephrectomized pregnant bitch, and the uterus of a nephrectomized pregnant bitch were used. The substrate, prepared by diluting plasma from a nephrectomized, hysterectomized pregnant bitch, had a maximum endogenous substrate concentration of 1,750 ng/ml angiotensin I equivalents. Lineweaver-Burk kinetic analysis was used to calculate the Michaelis constants at pH 6.2. The Km of the renal enzyme was 720 ng/ml, of the enzyme prepared from the nonnephrectomized uterus 1,570 ng/ml, and of the nephrectomized dog uterus 1,500 ng/ml. The uterine enzyme was not activated by transient acid treatment, in contrast to renins reported from other sources. The data support the concept of biochemical heterogeneity of the two enzymes in the dog.

Angiotensinogen↗
PubMed1976

Fluorescence lifetimes of Chlorella pyrenoidosa.

The flucrescence decay of Chlorella pyrenoidosa has been investigated under a variety of conditions in the picosecond and nanosecond time regions. Most of the fluorescence is accounted for by an expression of the form I(t) = I0exp-(At+Bt1/2) though an additional exponential term is required to include a weak component of lifetime 32 ps observable only at the higher pulse intensities. This interpretation reconciles earlier and apparently conflicting data. The weak 32 ps component may be associated with Photosystem I, although the possibility that it is an artefact of the high intensity pulses used cannot be excluded at present. The main fluorescence, described by the equation above is attributed to the antenna chlorophyll and is of the form which would be expected from a single light harvesting array with trapping at randomly distributed sites.

Chlorella↗