Insulin Assembly Damps Conformational Fluctuations: Raman Analysis of Amide I Linewidths in Native States and Fibrils

The crystal structure of insulin has been investigated in a variety of dimeric and hexameric assemblies. Interest in dynamics has been stimulated by conformational variability among crystal forms and evidence suggesting that the functional monomer undergoes a conformational change on receptor binding. Here, we employ Raman spectroscopy and Raman microscopy to investigate well-defined oligomeric species: monomeric and dimeric analogs in solution, native T6 and R6 hexamers in solution and corresponding polycrystalline samples. Remarkably, linewidths of Raman bands associated with the polypeptide backbone (amide I) exhibit progressive narrowing with successive self-assembly. Whereas dimerization damps fluctuations at an intermolecular β-sheet, deconvolution of the amide I band indicates that formation of hexamers stabilizes both helical and non-helical elements. Although the structure of a monomer in solution resembles a crystallographic protomer, its encagement in a native assembly damps main-chain fluctuations. Further narrowing of a β-sheet-specific amide I band is observed on reorganization of insulin in a cross-β fibril. Enhanced flexibility of the native insulin monomer is in accord with molecular dynamics simulations. Such conformational fluctuations may initiate formation of an amyloidogenic nucleus and enable induced fit on receptor binding.