Relative and absolute kinetic data have been determined for ring closure of methyl-substituted hex-5-enyl radicals: 2-methyl- (10a), 3-methyl-(4a), 4-methyl-(5a), 2,2-dimethyl-(10c), 3,3-dimethyl-(4c) and 4,4-dimethyl-hex-5-enyl (5c) radicals, generated by interaction of tributylstannane with the corresponding bromides (1a)-(3a) and (1c)-(3c). Each radical undergoes regiospecific or highly regioselective 1,5-cyclization more rapidly than does the unsubstituted radical (4d). The rate enhancements, which arise mainly from lowering of the activation energy, can be rationalized in terms of the gem-dimethyl effect. 1,5-Ring closures of monosubstituted species are stereoselective: 2-methyl- and 4-methyl-hex-5-enyl radicals (10a) and (5a) give mainly trans products, whereas 3-methylhex-4- enyl radical gives mainly the cis. This behaviour reflects the effect of the substituent on the stabilities of cyclic transition complexes in chair-like conformations. Ring closure of 2,2-dimethylpent-4-enyl radical or of 3,3-dimethylpent-4-enyl radical (19) could not be detected.

-_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _~_ _~_ _ _ _ _ _ _ _ _ _~~-Six experiments were made in growing pigs to investigate the effects of substituting bran, oatmeal byproduct, pectin and guar gum for barley on gastric emptying rate, pH and dry matter of digesta and on the dry matter content and rate of passage of digesta to the terminal ileum and overall. Twelve pigs with cannulas sited at the pyloric/fundic junction of the stomach were used to measure gastric emptying. Seventy-two pigs were kept in metabolism cages for the total collection of faeces and nine pigs were fitted with simple cannulas 150 mm cranial to the ileo-caecal junction. For the overall transit measurements the barley was ground through a 4.68 mm screen (C) and for the other studies either through this screen or a 1.56 mm screen (F). Gastric emptying rates were significantly faster for F diets than for C diets, but bran and oatmeal by-product substitutions were without significant effect on this variable or on rate of passage to the terminal ileum. Guar gum and pectin significantly speeded up gastric emptying and transit to the terminal ileum in C diets. Guar gum and pectin did not significantly affect overall transit time, but bran accelerated it. The results are discussed in relation to the depressive effects on nutrient utilization which have been recorded for the four polysaccharides: it is postulated that gastric emptying rate and transit time to the terminal ileum are likely to have a minimal effect on overall transit time. Polysaccharides are composed of both structural (cellulose, hemicellulose and pectic substances) and non-structural (gums, mucilages and algal polysaccharides) types. Dietary fibre is usually a mixture of several of these polysaccharides which have different physicochemical properties and different physical characteristics, together with varying proportions of lignin. The aims of the experiments described in the present paper were to study the effects of dietary fibre on gastric emptying rate and rate of passage of digesta to the terminal ileum and through the total gastrointestinal tract, so that an understanding of its mode of action in affecting nutrient digestibility could be furthered. The effects of dietary fibre on gastric emptying rate and rate of passage of digesta in the gastrointestinal tract are likely to be mediated by altering the volumes and physical properties of lumen contents. The emptying rate is also influenced by feedback control from the duodenum via receptors sensitive to the osmolality and acidity of digesta, and to other dietary factors including the particle size of the digesta (Laplace, 1982) which, in barleybased diets, can also affect gastric pH (Lawrence, 1970b). Dietary fibre rich in structural polysaccharides as in bran, and soluble polysaccharides such as guar gum, have been shown to have some effects on gastric emptying rate at various points in time up to 4 h after feeding (Rainbird, 1986; Rainbird & Low, 1986a, 6) V. P O T K I N S A N D O T H E R Semptying the digesta, none were foun...

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