O-17, N-15, C-13, and H-1 NMR chemical shieldings are calculated using density functional theory to differentiate among the three primarily helical forms, 3(10), alpha, and pi in polyalanine peptides under periodic boundary conditions. This study suggests O-17 as the best observable, as it has been demonstrated to be sensitive to hydrogen bonding and highly affected by small changes in the polypeptide in helix conformations. This theoretical study seeks to characterize the subtle conformational differences of helical structures by NMR chemical shift observables which may lead to important questions in experimental structure determination on the basis of using chemical shifts to identify protein secondary structures.