  The cobalt glycyl-leucine dipeptide is a model system for studying the
effects of Karplus equation calibration, molecular mechanics accuracy, backbone
conformation, and thermal motions on the measurability of side chain rotational
isomer populations. We analyze measurements of 8 vicinal coupling constants
about the alpha to beta-carbon and beta to gamma-carbon bonds of the leucine
side chain and of 10 NOESY cross relaxation rates across these bonds. Molecular
mechanics and peptide and protein crystallographic databases are an essential
part of this analysis because they independently suggest that the trans
gauche-plus and gauche-minus trans rotational isomers of the leucine side chain
predominate. They also both suggest that puckering of the cobalt dipeptide ring
system reduces the gauche-plus gauche-plus rotational isomer population to less
than about 10%. At the present +/- 1 Hz calibration accuracy of Karplus
equations for vicinal coupling constants, the predominant trans gauche-plus and
gauche-minus trans rotational isomer populations can be measured with about 5%
accuracy, but the population of the gauche-plus gauche-plus rotational isomer
is probably very near or just below the limit of measurability. These estimates
also depend upon qualitative assessments of the accuracy of the molecular
mechanics energy wells. We introduce gel graphics that are ideally suited to
presenting qualitative error and measurability estimates.
