Abstract

Interactions of alginate with univalent cations in solution have been investigated by circular dichroism (c.d.) and rheological measurements. Poly- l-guluronate chain-segments show substantial enhancement (∼ 50%) of c.d. ellipticity in the presence of excess of K +, with smaller changes for other univalent cations: Li + < Na + < K + > Rb + > Cs + > NH 4 +. The maximum c.d. change is attained by 0.3 m, with no further increase at higher concentrations of cation. No significant dependence on polymer concentration is observed. Spectral changes for poly- d-mannuronate and heterotypic chain-sequences are much smaller. For intact alginates, the magnitude of c.d. change varies almost linearly with poly- l-guluronate content. Difference spectra (c.d. with excess of univalent counterion minus c.d. in distilled water) can be fitted accurately to two Gaussian bands at 211 and 198 nm, assigned to carboxyl n → π * and π → π * transitions, respectively. The perturbations induced by Li +, K +, Rb +, Cs +, and NH 4 + show a clear family relationship, and are mainly in the π → π * spectral region. With Na +, by contrast, c.d. change is largely confined to the n → π * transition, and is similar to that previously reported for intermolecular (“egg-box”) binding of divalent cations, consistent with results of rheological studies which indicate Na +-induced association of poly- l-guluronate chain-sequences. These associations are further enhanced on freezing and thawing. This combined evidence is interpreted in terms of three modes of interaction between univalent cations and alginate chains in solution: ( a) ion-pair formation with carboxyl groups of mannuronate and isolated guluronate residues; ( b) specific site-binding to contiguous guluronate residues; and ( c) co-operative “egg-box” binding, particularly of Na +, between poly- l-guluronate chain-sequences.

Talk to us

Join us for a 30 min session where you can share your feedback and ask us any queries you have

Schedule a call

Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.