Massively parallel biophysical molecular dynamics simulations, coupled with efficient methods, promise to open biologically significant time scales for study. In order to promote efficient fine-grained parallel algorithms with low communication overhead, the fast degrees of freedom in these complex systems can be divided into sets of rigid bodies. Here, a novel Hamiltonian form of a minimal, nonsingular representation of rigid body rotations, the unit quaternion, is derived, and a corresponding reversible, symplectic integrator is presented. The novel technique performs very well on both model and biophysical problems in accord with a formal theoretical analysis given within, which gives an explicit condition for an integrator to possess a conserved quantity, an explicit expression for the conserved quantity of a symplectic integrator, the latter following and in accord with Calvo and Sanz-Sarna, Numerical Hamiltonian Problems ͑1994͒, and extension of the explicit expression to general systems with a flat phase space.

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Molecular dynamics simulations have been performed on a tetramer of the 25-residue (SSDPLVVAASIIGILHLIL-WILDRL) synthetic peptide [1] which contains the transmembrane domain of the influenza A virus M2 coat protein. The peptide bundle was initially assembled as a parallel K K-helix bundle in the octane portion of a phase separated water/octane system, which provided a membrane-mimetic environment. A 4-ns dynamics trajectory identified a left-handed coiled coil state of the neutral bundle, with a water filled funnel-like structural motif at the N-terminus involving the long hydrophobic sequence. The neck of the funnel begins at V27 and terminates at H37, which blocks the channel. The C-terminus is held together by inter-helix hydrogen bonds and contains water below H37. Solvation of the S23 and D24 residues, located at the rim of the funnel, appears to be important for stability of the structure. The calculated average tilt of the helices in the neutral bundle is 27 þ 5³, which agrees well with recent NMR data.z 1998 Federation of European Biochemical Societies.

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