Characterization of sodium alginate and its block fractions by surface‐enhanced Raman spectroscopy

The surface‐enhanced Raman scattering (SERS) of sodium alginates and their hetero‐ and homopolymeric fractions obtained from four seaweeds of the Chilean coast was studied. Alginic acid is a copolymer of β‐D‐mannuronic acid (M) and α‐L guluronic acid (G), linked 1 → 4, forming two homopolymeric fractions (MM and GG) and a heteropolymeric fraction (MG). The SERS spectra were registered on silver colloid with the 632.8 nm line of a HeNe laser. The SERS spectra of sodium alginate and the polyguluronate fraction present various carboxylate bands which are probably due to the coexistence of different molecular conformations. SERS allows to differentiate the hetero‐ and homopolymeric fractions of alginic acid by characteristic bands. In the fingerprint region, all the poly‐D‐mannuronate samples present a band around 946 cm−1 assigned to CO stretching, and CCH and COH deformation vibrations, a band at 863 cm−1 assigned to deformation vibration of β‐C1H group, and one at 799–788 cm−1 due to the contributions of various vibration modes. Poly‐L‐guluronate spectra show three characteristic bands, at 928–913 cm−1 assigned to symmetric stretching vibration of COC group, at 890–889 cm−1 due to CCH, skeletal CC, and CO vibrations, and at 797 cm−1 assigned to α C1H deformation vibration. The heteropolymeric fractions present two characteristic bands in the region with the more important one being an intense band at 730 cm−1 due to ring breathing vibration mode.