NMR and CD studies are reported for two length series of solubilized, spaced, highly helical polyalanines that are N-capped by the optimal helix stabilizer (beta)Asp-Hel and C-capped by P-aminoalanine beta and that are studied in water at 2degreesC, PH 1-8. NMR analysis yields a structural characterization of the peptide Ac(beta)AspHelAla(8)betaNH(2) and selected members of one (beta)AspHelAla(n)beta series. At pH > 4.5 the (beta)AspHel cap provides a preorganized triad of carboxylate anion and two amide residues that is complementary to the helical polyalanine N-terminus. The C-terminal beta-aminoalanine assumes a helix-stabilizing conformation consistent with literature precedents. H(N)CO NMR experiments applied to capped, uniformly C-13- and N-15-labeled Ala(8) and Ala(12) Peptices define Ala(n) hydrogen bonding signatures as alpha-helical without detectable 310 character. Relative NH-->ND exchange rates yield site protection factors PFi, that define uniquely high fractional helicities FH for the peptide Ala(n) regions. These Ala(n) calibration series, studied in water and lacking helix-stabilizing tertiary structure, yield the first C-13 NMR chemical shifts, 3J(HNHalpha) coupling constants, and CD ellipticities [theta(Molar)](lambda.n) characteristic of a fully helical alanine within an Ala(n) context. CD data are used to assign parameters X and [theta]lambda.(infinity), required for rigorous calculation of FH values from CD ellipticities.