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Polyxanthylic acid: structures of the ordered forms
Author(s) -
Roy Kunal B.,
Frazier Joe,
Miles H. Todd
Publication year - 1979
Publication title -
biopolymers
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.556
H-Index - 125
eISSN - 1097-0282
pISSN - 0006-3525
DOI - 10.1002/bip.1979.360181213
Subject(s) - chemistry , counterion , alkali metal , crystallography , helix (gastropod) , ion , infrared spectroscopy , ionization , hydrogen bond , inorganic chemistry , molecule , organic chemistry , ecology , snail , biology
Abstract We present evidence for structures of two ordered forms of polyxanthylic acid based on ir spectroscopy, pH titrations, and thermal transitions. Over the pH range ∼6–9.5, the structure is a four‐stranded helix with alkali metal ions specifically complexed in the central channel. These internal counterions stabilize the structure by complexing with carbonyl oxygens and by partial screening of electrostatic repulsion caused by ionization of the xanthine residues in this pH range. Below pH 5, the structure is quite different and much more stable. Our data are consistent with a six‐stranded helix in which both carbonyl oxygens and both NH protons are hydrogen bonded.