6 results on '"Meadows JH"'
Search Results
2. The Photoactive Excited State of the B 12 -Based Photoreceptor CarH.
- Author
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Miller NA, Kaneshiro AK, Konar A, Alonso-Mori R, Britz A, Deb A, Glownia JM, Koralek JD, Mallik L, Meadows JH, Michocki LB, van Driel TB, Koutmos M, Padmanabhan S, Elías-Arnanz M, Kubarych KJ, Marsh ENG, Penner-Hahn JE, and Sension RJ
- Subjects
- Cobalt
- Abstract
We have used transient absorption spectroscopy in the UV-visible and X-ray regions to characterize the excited state of CarH, a protein photoreceptor that uses a form of B
12 , adenosylcobalamin (AdoCbl), to sense light. With visible excitation, a nanosecond-lifetime photoactive excited state is formed with unit quantum yield. The time-resolved X-ray absorption near edge structure difference spectrum of this state demonstrates that the excited state of AdoCbl in CarH undergoes only modest structural expansion around the central cobalt, a behavior similar to that observed for methylcobalamin rather than for AdoCbl free in solution. We propose a new mechanism for CarH photoreactivity involving formation of a triplet excited state. This allows the sensor to operate with high quantum efficiency and without formation of potentially dangerous side products. By stabilizing the excited electronic state, CarH controls reactivity of AdoCbl and enables slow reactions that yield nonreactive products and bypass bond homolysis and reactive radical species formation.- Published
- 2020
- Full Text
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3. Antivitamins B 12 in a Microdrop: The Excited-State Structure of a Precious Sample Using Transient Polarized X-ray Absorption Near-Edge Structure.
- Author
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Miller NA, Michocki LB, Alonso-Mori R, Britz A, Deb A, DePonte DP, Glownia JM, Kaneshiro AK, Kieninger C, Koralek J, Meadows JH, van Driel TB, Kräutler B, Kubarych KJ, Penner-Hahn JE, and Sension RJ
- Subjects
- Carbon chemistry, Cobalt chemistry, Kinetics, Molecular Conformation, Photochemical Processes, Quantum Theory, Thermodynamics, X-Rays, Coordination Complexes chemistry, Models, Molecular, Vitamin B 12 antagonists & inhibitors
- Abstract
Polarized transient X-ray absorption near-edge structure (XANES) was used to probe the excited-state structure of a photostable B
12 antivitamin (Coβ-2-(2,4-difluorophenyl)-ethynylcobalamin, F2 PhEtyCbl). A drop-on-demand delivery system synchronized to the LCLS X-ray free electron laser pulses was implemented and used to measure the XANES difference spectrum 12 ps following excitation, exposing only ∼45 μL of sample. Unlike cyanocobalamin (CNCbl), where the Co-C bond expands 15-20%, the excited state of F2 PhEtyCbl is characterized by little change in the Co-C bond, suggesting that the acetylide linkage raises the barrier for expansion of the Co-C bond. In contrast, the lower axial Co-NDMB bond is elongated in the excited state of F2 PhEtyCbl by ca. 10% or more, comparable to the 10% elongation observed for Co-NDMB in CNCbl.- Published
- 2019
- Full Text
- View/download PDF
4. Probing the Excited State of Methylcobalamin Using Polarized Time-Resolved X-ray Absorption Spectroscopy.
- Author
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Michocki LB, Miller NA, Alonso-Mori R, Britz A, Deb A, Glownia JM, Kaneshiro AK, Konar A, Koralek J, Meadows JH, Sofferman DL, Song S, Toda MJ, van Driel TB, Kozlowski PM, Kubarych KJ, Penner-Hahn JE, and Sension RJ
- Abstract
We use picosecond time-resolved polarized X-ray absorption near-edge structure (XANES) measurements to probe the structure of the long-lived photoexcited state of methylcobalamin (MeCbl) and the cob(II)alamin photoproduct formed following photoexcitation of adenosylcobalamin (AdoCbl, coenzyme B
12 ). For MeCbl, we used 520 nm excitation and a time delay of 100 ps to avoid the formation of cob(II)alamin. We find only small spectral changes in the equatorial and axial directions, which we interpret as arising from small (<∼0.05 Å) changes in both the equatorial and axial distances. This confirms expectations based on prior UV-visible transient absorption measurements and theoretical simulations. We do not find evidence for the significant elongation of the Co-C bond reported by Subramanian [ J. Phys. Chem. Lett. 2018 , 9 , 1542 - 1546 ] following 400 nm excitation. For AdoCbl, we resolve the difference XANES contributions along three unique molecular axes by exciting with both 540 and 365 nm light, demonstrating that the spectral changes are predominantly polarized along the axial direction, consistent with the loss of axial ligation. These data suggest that the microsecond "recombination product" identified by Subramanian et al. is actually the cob(II)alamin photoproduct that is produced following bond homolysis of MeCbl with 400 nm excitation. Our results highlight the pronounced advantage of using polarization-selective transient X-ray absorption for isolating structural dynamics in systems undergoing atomic displacements that are strongly correlated to the exciting optical polarization.- Published
- 2019
- Full Text
- View/download PDF
5. The estimation of dietary protein intake in chronic renal failure.
- Author
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Coles GA, Meadows JH, Bright C, and Tomlinson K
- Subjects
- Adult, Aged, Ambulatory Care, Circadian Rhythm, Diet Records, Evaluation Studies as Topic, Female, Humans, Kidney Failure, Chronic urine, Male, Middle Aged, Nitrogen urine, Urea urine, Dietary Proteins administration & dosage, Kidney Failure, Chronic diet therapy
- Abstract
Forty-five adult clinic patients with chronic renal failure each supplied a 4-day weighed dietary record, a 24-h urine collection, and a nocturnal spot urine sample. Total nitrogen (N) losses derived from the urines were corrected for proteinuria and non-urea nitrogen excretion. Individual estimates of N intake were compared by correlation and assessing the level of agreement. Daily urea N excretion derived from the spot sample correlated well with the 24-h collection P less than 0.001, but the degree of agreement was poor, mean difference being +1.62 g with 95% limits of +5.7 to -2.47 g. The correlation between the spot-sample-derived N loss and dietary N intake was poor, r = 0.42; P less than 0.05. For 12 patients taking a low-protein diet, N intake correlated well with 24-h urine derived N losses, P less than 0.001, mean difference being +0.59 g, 95% limits +2.39 to -1.21 g. The correlation and agreement was less satisfactory for the subjects who had not received dietary instruction, due largely to individual variation in day-to-day protein intake. Use of spot urine samples is too inaccurate for routine clinical practice. Single 24-h urine derived estimates of N intake are only of value for assessing patients previously prescribed a low-protein intake.
- Published
- 1989
- Full Text
- View/download PDF
6. The risks and benefits of a low protein-essential amino acid-keto acid diet.
- Author
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Lucas PA, Meadows JH, Roberts DE, and Coles GA
- Subjects
- Adult, Body Weight, Creatinine blood, Female, Glomerular Filtration Rate, Humans, Kidney physiopathology, Kidney Failure, Chronic physiopathology, Male, Middle Aged, Muscles anatomy & histology, Risk, Skinfold Thickness, Time Factors, Amino Acids, Essential administration & dosage, Dietary Proteins administration & dosage, Keto Acids administration & dosage, Kidney Failure, Chronic diet therapy
- Abstract
Twelve patients with progressive renal failure were placed on a very low protein diet supplemented by an essential amino acid-keto acid mixture for six to twelve months. Total daily intake was 0.04 g nitrogen/kg and 50 kcal/kg. Eight subjects had a significant change in the slope of reciprocal plasma creatinine, becoming less steep and in two cases positive. GFR did not improve, but in four patients the decline over twelve months was less than 0.5 mliter/min. There were significant falls in blood and urinary urea, serum phosphate PTH and calcium X phosphate product. Body wt decreased during the first three months. Arm muscle circumference fell by 0.9 cm (P less than 0.005). Serum albumin and transferrin levels did not change significantly. Muscle mass and plasma creatinine fell simultaneously in several patients. Creatinine excretion per kg muscle mass, assessed anthropometrically, declined by 21% in the first three months. This diet may slow the decline in renal function in a proportion of patients. However, muscle mass can be lost. Serum protein levels do not accurately reflect nutritional changes. A fall in plasma creatinine may not be due to improved GFR but instead to altered creatinine metabolism.
- Published
- 1986
- Full Text
- View/download PDF
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