Insulin, a simple polypeptide hormone with huge biological importance, has long been known to self-assemble in vitro and form amyloid-like fibrillar aggregates. Utilizing high-resolution NMR, Raman spectroscopy, and computational analysis, we demonstrate that the fluctuation of the carboxyl terminal (C-ter) residues of the insulin B-chain plays a key role in the growth phase of insulin aggregation. By comparing the insulin sourced from bovine, human, and the modified glargine (GI), we observed reduced aggregation propensity in the GI variant, resulting from two additional Arg residues at its C-ter. NMR analysis showed atomic contacts and residue-specific interactions, particularly the salt bridge and H-bond formed among the C-ter residues Arg31(B), Lys29(B), and Glu4(A). These inter-residue interactions were reflected in strong nuclear Overhauser effects among Arg31(B)delta H-Glu4(A)delta H and Lys29(B)delta Hs-Glu4(A)delta H in GI, as well as the associated downfield chemical shift of several A-chain amino terminal (N-ter) residues. The two additional Arg residues of GI, Arg31(B) and Arg32(B), enhanced the stability of the GI native structure by strengthening the Arg31(B), Lys29(B), and Glu4(A) salt bridge, thus reducing extensive thermal distortion and fluctuation of the terminal residues. The high stability of the salt bridge retards tertiary collapse, a crucial biochemical event for oligomerization and subsequent fibril formation. Circular dichroism and Raman spectroscopic measurement also suggest slow structural distortion in the early phase of the aggregation of GI because of the restricted mobility of the C-ter residues as explained by NMR. In addition, the structural and dynamic parameters derived from molecular dynamics simulations of insulin variants highlight the role of residue-specific contacts in aggregation and amyloid-like fibril formation.