AB-INITIO MOLECULAR-DYNAMICS STUDY OF PROTON-TRANSFER IN A POLYGLYCINE ANALOG OF THE ION-CHANNEL GRAMICIDIN-A

Proton transfer in biological systems is thought to often proceed thro ugh hydrogen-bonded chains of water molecules. The ion channel, gramic idin A (gA), houses within its helical structure just such a chain, Us ing the density functional theory based ab initio molecular dynamics C ar-Parrinello method, the structure and dynamics of proton diffusion t hrough a polyglycine analog of the gA ion channel has been investigate d, In the channel, a proton, which is initially present as hydronium ( H3O+), rapidly forms a strong hydrogen bond with a nearest neighbor wa ter, yielding a transient H5O2+ complex, As in bulk water, strong hydr ogen bonding of this complex to a second neighbor solvation shell is r equired for proton transfer to occur. Within gA, this second neighbor shell included not only a channel water molecule but also a carbonyl o f the channel backbone, The present calculations suggest a transport m echanism in which a priori carbonyl solvation is a requirement for pro ton transfer.