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At the majority of mutants in the region Glu181‐Val200 incorporating a conserved AsnPheThrΦΦxLys motif cysteine substitution had no effect on sensitivity to ATP, partial agonists, or methanethiosulfonate (MTS) compounds. For the F185C mutant the efficacy of partial agonists was reduced by ∼ 90% but there was no effect on ATP potency or the actions of MTS reagents. At T186C, F188C and K190C mutants ATP potency and partial agonists responses were reduced. The ATP sensitivity of the K190C mutant was rescued towards WT levels by positively charged (2‐aminoethyl)methanethiosulfonate hydrobromide and reduced by negatively charged sodium (2‐sulfonatoethyl) methanethiosulfonate. Both MTS reagents decreased ATP potency at the T186C mutant, and abolished responses at the F195C mutant. 32P‐2‐azido ATP binding to the mutants T186C and K190C was sensitive to MTS reagents consistent with an effect on binding, however binding at F195C was unaffected indicating an effect on gating. The accessibility of the introduced cysteines was probed with (2‐aminoethyl)methanethiosulfonate hydrobromide‐biotin, this showed that the region Thr186‐Ser192 is likely to form a beta sheet and that accessibility is blocked by ATP. Taken together these results suggest that Thr186, Phe188 and Lys190 are involved in ATP binding to the receptor and Phe185 and Phe195 contribute to agonist evoked conformational changes.

Experimental projected ranges of electrons between 20 eV and 10 keV have been correlated by a simple scaling factor to yield 'best-fit' expressions applicable to all media. Extrapolated ranges, R,, (in pg cm-2) are given by In((Z/A)R,,) = -4.546 7 +0.311 04 In E +0.077 73 (In E)' valid for 20 eV < E < lo4 eV with a precision of 25% in ( Z / A ) R , , or-0.59 andwhere Z / A is the ratio of charge to mass numbers for the absorbing medium. From an analysis of the transmission curves, the following relation between median or mean projected ranges and the extrapolated ranges is obtained R5,, = (0.3655 *0.05)R:,024, 20 e~ < E < io4 eV.Differentiation of the extrapolated range-energy expression yields an effective stopping power along the projected track of the electron and is given by d E / d R . , = E / [ R , , ( 0 . 1 5 5 4 6 I n ( E ) + 0 . 3 1 1 0 4 ) ] e V c m 2 ~g ~' , 2 0 e V ~E ~l O 4 e V with a precision of 30%. Improved accuracy of the experimental data is required for track structure applications and to resolve phase effects.

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