Determination of crude protein in animal feed, forage, grain, and oilseeds by using block digestion with a copper catalyst and steam distillation into boric acid: collaborative study

A collaborative study was conducted to evaluate the repeatability and reproducibility of an extension of AOAC Official Method 991.20, Nitrogen (Crude) in Milk, to animal feed, forage (plant tissue), grain, and oilseed materials. Test portions are digested in an aluminum block at 420 degrees C in sulfuric acid with potassium sulfate and a copper catalyst. Digests are cooled and diluted, and concentrated sodium hydroxide is added to neutralize the acid and make the digest basic; the liberated ammonia is distilled by using steam distillation. The liberated ammonia is trapped in a weak boric acid solution and titrated with a stronger standardized acid, hydrochloric acid; colorimetric endpoint detection is used. Fourteen blind samples were sent to 13 collaborators in the United States, Denmark, Sweden, Germany, and the United Kingdom. Recoveries of nitrogen from lysine, tryptophan, and acetanilide were 86.8, 98.8, and 100.1%, respectively. The within-laboratory relative standard deviation (RSDr, repeatability) ranged from 0.40 to 2.38% for crude protein. The among-laboratories (including within-) relative standard deviation (RSD(R), reproducibility) ranged from 0.44 to 2.38%. It is recommended that the method be adopted First Action by AOAC INTERNATIONAL. A lower concentration (1% H3BO3) of trapping solution was compared with the concentration specified in the original protocol (4% H3BO3) and was found comparable for use in an automatic titration system in which titration begins automatically as soon as distillation starts. The Study Directors recommend that 1% H3BO3 as an optional alternative to 4% boric acid trapping solution be allowed for automatic titrators that titrate throughout the distillation.     

Off-resonance R(1rho) NMR studies of exchange dynamics in proteins with low spin-lock fields: an application to a Fyn SH3 domain

An (15)N NMR R(1rho) relaxation experiment is presented for the measurement of millisecond time scale exchange processes in proteins. On- and off-resonance R(1rho) relaxation profiles are recorded one residue at a time using a series of one-dimensional experiments in concert with selective Hartmann-Hahn polarization transfers. The experiment can be performed using low spin-lock field strengths (values as low as 25 Hz have been tested), with excellent alignment of magnetization along the effective field achieved. Additionally, suppression of the effects of cross-correlated relaxation between dipolar and chemical shift anisotropy interactions and (1)H-(15)N scalar coupled evolution is straightforward to implement, independent of the strength of the (15)N spin-locking field. The methodology is applied to study the folding of a G48M mutant of the Fyn SH3 domain that has been characterized previously by CPMG dispersion experiments. It is demonstrated through experiment that off-resonance R(1rho) data measured at a single magnetic field and one or more spin-lock field strengths, with amplitudes on the order of the rate of exchange, allow a complete characterization of a two-site exchange process. This is possible even in the case of slow exchange on the NMR time scale, where complementary approaches involving CPMG-based experiments fail. Advantages of this methodology in relation to other approaches are described.     

NEW PHTHALOCYANINE PHOTOSENSITIZERS FOR PHOTODYNAMIC THERAPY

Six new aluminum and silicon phthalocyanines have been synthesized and their photocytotoxicity toward V79 cells has been studied. The compounds that have been prepared are: AIPcOSi(CH₃)₂(CH₂),N(CH₃)₂, I; AIPcOSi(CH₃)₂(CH₂)₃N(CH₃)₃⁺I^?, II; CH₃SiPcOSi(CH₃)₂(CH₂)₃N(CH₃)₂, III; HOSiPcOSi(CH₃)₂(CH₂)₃N(CH₃)₂, IV; HOSiPcOSi(CH₃)₂(CH₂)₃)₃(CH₃)₃⁺I^?, V; and SiPc[OSi(CH₃)₂(CH₂)₃N(CH₃)₃⁺I^?]₂, VI. Relative growth delay values for compounds I-VI and relative cytotoxicity values for compounds I, II, IV, V and VI have been determined. Compounds I and II have been shown to be comparable in photocytotoxicity to what is presumed to be AIPcOH.xH₂O, and compound IV has been shown to have greater activity. The classes of compounds to which these six compounds belong appear to have potential for photodynamic therapy.     

Melt‐state polymer chain dimensions as a function of temperature

The unperturbed chain dimensions (〈R²〉_o/M) of cis/trans‐1,4‐polyisoprene, a near‐atactic poly(methyl methacrylate), and atactic polyolefins were measured as a function of temperature in the melt state via small‐angle neutron scattering (SANS). The polyolefinic materials were derived from polydienes or polystyrene via hydrogenation or deuteration and represent structures not encountered commercially. The parent polymers were prepared via lithium‐based anionic polymerizations in cyclohexane with, in some cases, a polymer microstructure modifier present. The polyolefins retained the near‐monodisperse molecular weight distributions exhibited by the precursor materials. The melt SANS‐based chain dimension data allowed the evaluation of the temperature coefficients [dln 〈R²〉_o/dT(κ)] for these polymers. The evaluated polymers obeyed the packing length (p)‐based expressions of the plateau modulus, G = kT/np³ (MPa), and the entanglement molecular weight, M_e = ρN_anp³ (g mol^?1), where n_t denotes the number (～21) of entanglement strands in a cube with the dimensions of the reptation tube diameter (d_t) and ρ is the chain density. The product np³ is the displaced volume (V_e) of an entanglement that is also expressible as pd or kT/G. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1768–1776, 2002     

The peripheral benzodiazepine receptor in photodynamic therapy with the phthalocyanine photosensitizer Pc 4

The peripheral benzodiazepine receptor (PBR) is an 18 kDa protein of the outer mitochondrial membrane that interacts with the voltage-dependent anion channel and may participate in formation of the permeability transition pore. The physiological role of PBR is reflected in the high-affinity binding of endogenous ligands that are metabolites of both cholesterol and heme. Certain porphyrin precursors of heme can be photosensitizers for photodynamic therapy (PDT), which depends on visible light activation of porphyrin-related macrocycles. Because the apparent binding affinity of a series of porphyrin analogs for PBR paralleled their ability to photoinactivate cells, PBR has been proposed as the molecular target for porphyrin-derived photocytotoxicity. The phthalocyanine (Pc) photosensitizer Pc 4 accumulates in mitochondria and structurally resembles porphyrins. Therefore, we tested the relevance of PBR binding on Pc 4-PDT. Binding affinity was measured by competition with 3H-PK11195, a high-affinity ligand of PBR, for binding to rat kidney mitochondria (RKM) or intact Chinese hamster ovary (CHO) cells. To assess the binding of the Pc directly, we synthesized 14C-labeled Pc 4 and found that whereas Pc 4 was a competitive inhibitor of 3H-PK11195 binding to the PBR, PK11195 did not inhibit the binding of 14C-Pc 4 to RKM. Further, 14C-Pc 4 binding to RKM showed no evidence of saturation up to 10 microM. Finally, when Pc 4-loaded CHO cells were exposed to activating red light, apoptosis was induced; Pc 4-PDT was less effective in causing apoptosis in a companion cell line overexpressing the antiapoptotic protein Bcl-2. For both cell lines, PK11195 inhibited PDT-induced apoptosis; however, the inhibition was transient and did not extend to overall cell death, as determined by clonogenic assay. The results demonstrate (1) the presence of low-affinity binding sites for Pc 4 on PBR; (2) the presence of multiple binding sites for Pc 4 in RKM and CHO cells other than those that influence PK11195 binding; and (3) the ability of high supersaturating levels of PK11195 to transiently inhibit apoptosis initiated by Pc 4-PDT, with less influence on overall cell killing. We conclude that the binding of Pc 4 to PBR is less relevant to the photocytotoxicity of Pc 4-PDT than are other mitochondrial events, such as photodamage to Bcl-2 and that the observed inhibition of Pc 4-PDT-induced apoptosis by PK11195 likely occurs through a mechanism independent of PBR.     

Enzymatic oxidation of tert-butylcatechol in the presence of sulfhydryl compounds: Application to the amperometric detection of penicillamine

The high sensitivity that can be attained using an enzymatic system and mediated by 4-tert-butylcatechol (4-TBC) has been verified by on-line interfacing of a rotating biosensor and continuous flow/stopped-flow/continuous-flow processing. Horseradish peroxidase, HRP, [EC 1.11.1.7], immobilized on a rotating disk, in presence of hydrogen peroxide catalyzed the oxidation of 4-TBC, whose back electrochemical reduction was detected on glassy carbon electrode surface at −150 mV. Thus, when penicillamine (PA) was added to the solution, these thiol-containing compounds participate in Michael type addition reactions with 4-TBC to form the corresponding thioquinone derivatives, decreasing the peak current obtained proportionally to the increase of its concentration. The highest response for PA was obtained around pH 7. This method could be used to determine PA concentration in the range 0.02-80 μM (r = 0.998). The determination of PA was possible with a limit of detection of 7 nM, in the processing of as many as 50 samples per hour. The HRP-rotating biosensor was successfully applied to the determination of PA in pharmaceutical formulations.     

Synthesis of pyrido[2,3-d]pyridazines and pyrazino[2,3-d]-pyridazines—novel classes of GABAA receptor benzodiazepine binding site ligands

Novel syntheses of 2,3,8-trisubstituted pyrido[2,3-d]pyridazines and 2,3,5-trisubstituted pyrazino[2,3-d]pyridazines are described. Two complementary routes to pyrido[2,3-d]pyridazines were developed, the first of which began by constructing the pyridine ring, and the second of which started by constructing the pyridazine ring. Pyrazino[2,3-d]pyridazines were prepared in a route employing an aza-Wadsworth-Emmons cyclization as the key step. The resulting compounds were found to be high affinity ligands for the GABA_A receptor benzodiazepine binding site.     

High-resolution Mn EXAFS of the oxygen-evolving complex in photosystem II: structural implications for the Mn4Ca cluster

The biological generation of oxygen by the oxygen-evolving complex in photosystem II (PS II) is one of nature's most important reactions. The recent X-ray crystal structures, while limited by resolutions of 3.2-3.5 A, have located the electron density associated with the Mn4Ca cluster within the multiprotein PS II complex. Detailed structures critically depend on input from spectroscopic techniques, such as EXAFS and EPR/ENDOR, as the XRD resolution does not allow for accurate determination of the position of Mn/Ca or the bridging and terminal ligand atoms. The number and distances of Mn-Mn/Ca/ligand interactions determined from EXAFS provide important constraints for the structure of the Mn4Ca cluster. Here, we present data from a high-resolution EXAFS method using a novel multicrystal monochromator that show three short Mn-Mn distances between 2.7 and 2.8 A and, hence, the presence of three di-mu-oxo-bridged units in the Mn4Ca cluster. This result imposes clear limitations on the proposed structures based on spectroscopic and diffraction data and provides input for refining such structures.     

Reversible formation of DNA G-quadruplex hairpin dimers from stilbenediether conjugates

Rapid and reversible G-quadruplex hairpin dimer formation is observed for bis(oligonucleotide) conjugates possessing stilbenediether (Sd) linkers connecting two short poly(G) sequences.