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Tandem mass spectral effects of the replacement of alanine by proline in pentaalanine were studied. The principal difference when residue 3 or 4 is proline is the great enhancement of the y3 or y2 ion, respectively. This is precisely the principal difference which thermochemical arguments predict, since a y-type ion with N-terminal proline is estimated to be 32 kJ mol-1 more stable than with N-terminal alanine. When this proline effect and the effect of neutral diketopiperazine loss occur simultaneously, the y-type ion is not observed, as predicted from the additivity of substituent effects by the kinetic approach. No interpretation is offered for observations of b-type ion intensities, since the pattern is not obvious to us.

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