Adsorption of biomolecules to ice and their effects upon ice growth. 1. Measuring adsorption orientations and initial results

Different fish, insects, and plants have evolved a range of peptides and glycopeptides that adsorb to ice from water solution and dramatically influence ice growth in ways that benefit the organisms. Understanding the adsorption itself is fundamental but difficult to study experimentally. One fundamental attribute of the adsorption is the orientation relationship between the adsorbing molecules and the ice surface. A method that has been used to determine the ice interface orientations at which adsorption of AFs leads to engulfment into growing ice (Knight, C. A.; Cheng, C. C.; DeVries, A. L. Biophys. J. 1991, 59, 409-418) is extended here to reveal orientations at which the solute molecules are adsorbed but not engulfed. Adsorption without engulfment is found to be quite common, and some examples show a surprising sensitivity of adsorption of this kind to small changes of interface orientation. Solutes that prevent ice growth must adsorb permanently, fixed to the ice itself at the ice/water interface and unable to migrate with the interface. The new technique provides a way to detect adsorption that is either impermanent or allows the adsorbate to advance with the interface but does not distinguish between these two possibilities. Peptides that contain predominantly or exclusively lysine and alanine are used for this first study and demonstration of the technique. They all have a strong tendency to adsorb at {21 (1) over bar0}, secondary prism plane orientations, with alignment deduced to be approximately parallel to <01<(1)over bar>2>. However, many of the materials examined here are not engulfed as the ice grows and do not stop ice crystal growth. Polymers and peptides of lysine and alanine could be an interesting, relatively simple system to study in a systematic way to elucidate the mechanisms of action of nonequilibrium antifreezes.