Studies of naturally occurring and chemically modified insulins indica
te that relatively few of the 51 amino acid residues may be assigned s
pecific roles in insulin-receptor interactions. Most of the insulin X-
ray structural information is derived from aggregated species (notably
hexamers). Because insulin exerts its physiological effect as a 5808
Dalton monomeric species, it is necessary to consider whether crystal-
packing forces have modified the structure from that required for biol
ogical action. Insulin aggregation in solution complicates high resolu
tion NMR studies of the monomer. However, site-directed mutagenesis ca
n be used to generate biologically active mutants (e.g., B16-Tyr --> H
is) that remain monomeric at millimolar concentrations in aqueous solu
tion at low pH. The resulting homogeneous and monomeric samples are su
itable for structure determination by NMR methods. The high resolution
solution structure of B16-Tyr --> His insulin resembles crystal struc
tures, notably molecule 1 of T-6 insulin. Side-chain conformation in s
ome biologically important motifs, however, shows subtle differences b
etween solution and crystal structures.