Coupling experimental procedures

Winge et al. [730] have investigated the determination of twenty or more trace elements in saline waters by the inductively coupled plasma technique. They give details of experimental procedures, detection limits, and precision and accuracy data. The technique when applied directly to the sample is not sufficiently sensitive for the determination of many of the elements at the low concentrations at which they occur in seawater, and for these samples preconcentration techniques are required. However, it has the advantages of being amenable to automation and capable of analyzing several elements simultaneously. 

Further simplifications can be made by assuming the ofT-diagonal elements of Vi, which represent the covariance between the measured quantities, are zero this effectively means that there is no coupling between the different experimental determinations in the experimental procedure, which is reasonable. 

Indeed, there were those who described the azide coupling method as racemization-free. [15l However, this viewpoint proved to be overly optimistic. In 1970, Sieber reported that during a synthesis of calcitonin M by the azide method, significant epimerization occurred during two of the segment condensation steps in one of these reactions 40% of the epimerized product was observed. 16 There is a crucial detail in the experimental procedure here. The workers used tert-butyl nitrite to convert a peptide hydrazide into a peptide azide, but did not isolate the azide as was typical for research at that time. Instead, they neutralized the active intermediate in situ with DIPEA and added the amino segment for acylation. This demonstrates another important theme in the control of epimerization, the presence of a tertiary amine in the reaction mixture, even if only as a neutralization equivalent, can result in the formation of epimerized products. Indeed, most observations of racemization during... 

Dupuy and coworkers have reported a direct gas chromatographic procedure for the examination of volatiles in vegetable oils (11). peanuts and peanut butters (12, 13), and rice and com products (14). When the procedure was appTTed to the analysis of flavor-scored samples, the instrumental data correlated well with sensory data (15, 16, 17), showing that food flavor can be measured by instrvmental means. Our present report provides additional evidence that the direct gas chromatographic method, when coupled with mass spectrometry for the identification of the compounds, can supply valid information about the flavor quality of certain food products. Such information can then be used to understand the mechanisms that affect flavor quality. Experimental Procedures... 

The p-sulfanyl amides 28 are synthesized from N-protected amino acids 24 via amino alcohols 25, which are converted into (5-acetylsulfanyl amides 26 by a Mitsunobu reaction. The (5-amine disulfide 27 is subsequently coupled with a variety of carboxylic acids, followed by reduction with tributylphosphine in aqueous THF in the presence of pyridine to produce the free thiol 28 (Scheme 5).1211 Detailed experimental procedures for these compounds have not been reported. 

Testing. Studies of the catalytic activity and selectivity were conducted in a tubular unit reactor. Since this is a reactor consisting of a catalyst-packed tube of dimensions identical with those of a single tube inside a large-scale reactor, it becomes possible to reproduce the industrial conditions A thermocouple was placed in the catalyst bed, A detailed description of the experimental procedure has been presented elsewhere [4]. The tube was packed with two layers of catalyst (0.6 dm3 each). The total bed length was about 2.65 m. The feed was o-xylene of purity 98.4 wt.%. The air flow rate was 3.5 m3(STP)/h. The calcined catalyst samples were tested under conditions of continuous operation over a period of a couple of weeks during this period the concentration of o-xylene was varied from 20 to 70 g/m3(STP). 

The coupling reaction is important in the industrial preparation of azo dyes as well as in the analytical determination of diazonium compounds. The reaction has been reviewed, and experimental procedures have been given,... 

Since the discovery by Carpino et al.f l that A -Pbf-protected amino acids can be activated even as acid chlorides, such derivatives have been found to be well suited for the acylation of sterically demanding amino components (for details and experimental procedures, see Section 3.3.1). Oxazolone formation is prevented by this type of N -protection, racemization is not observed during activation and coupling.In line with these findings, further sulfonamide derivatives have been proposed such as l,3-benzothiazol-2-ylsulfonyl (Bts, 1Q4),[659,660] 5-methyl-l,3,4-thiadiazol-2-ylsulfonyl (105),f l 2-nitrophenylsulfonyl (oNbs, 4-nitrophenylsulfonyl (107),t l and 2,4-dinitrophenylsulfonyl group (108) (Scheme 49).f l... 

Mixed anhydrides are also useful reagents for the generation of A-hydroxysuccinimide esters,as well as HOBt and HOAt esters. After formation of the mixed anhydride, as described in the experimental procedures below, typically, 2 equivalents of the A-hydroxy component are added followed by 2 equivalents of NMM. The active ester is rapidly formed and in about 10 min the amine component to be coupled is added. The procedure is useful for the condensation of hindered amines where wrong-way opening would otherwise be a problem. Prolylprolyl dipeptides have been obtained rapidly and in high yield with less than 2% wrong opening. 

For the coupling of racenoization-prone amino acid derivatives and in fragment condensations involving protected peptides not having a C-terminal Gly or Pro unit, the use of 1-1.5 equivalents of DIPEAP or a weaker and more-hindered base, such as 2,4,6-tri-methylpyridine (collidine) is reconunended.P The favorable effect of coUidine on the suppression of epimerization is demonstrated in Table 3 for couplings carried out in solution (see also the experimental procedures for peptides 57, 60, and 63, Schemes 6 and 7). 

The following experimental procedures describe the solid-phase synthesis of peptides by means of Fmoc chemistry and TBTU activation (Scheme 8), as well as by HBTU-mediated coupling of Boc amino acid derivatives and in situ neutralization (Scheme 9). Segment condensations on solid phase with C-ternoinal glycine or proline residues can be easily achieved, even on a multigram scale (Scheme 10). Segment condensations involving the activation of other amino acids require the careful optimization of reaction conditions in order to suppress extensive epimerization (see Scheme 11 and Section 3.8.1.2). 

The coupling of carboxyl-containing compounds to amino groups of the polypeptide carrier is usually performed with an excess of carboxyl groups and an equivalent amount of the water-soluble carbodiimide around pH 5 at room temperature. In some cases where the hapten is not water soluble, it is usually dissolved in DMF or dioxane. In such cases it is also possible to prepare the hydroxysuccinimide ester using dicyclo-hexylcarbodiimide and couple this ester directly to the carrier. The experimental procedure will be demonstrated by two examples. 

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