| HIGH TEMPERATURE SIMULATED ANNEALING |
A TOTAL OF 80 STRUCTURES WERE GENERATED STARTING FROM RANDOM COORDINATES BY A HIGH TEMPERATURE SIMULATED ANNEALING PROTOCOL AT 1000K (NILGES ET AL., 1988), USING THE PARALLHDG.PRO FORCE FIELD OF X-PLOR THAT DO NO TAKE INTO ACCOUNT THE ATTRACTIVE TERM OF THE VAN DER WALLS' INTERACTIONS NOR THE ELECTROSTATIC INTERACTIONS. THE STRUCTURES THAT AGREED WITH THE FORCE FIELD AND EXPERIMENTAL RESTRAINTS (NO NOE VIOLATION GRATER THAN 0.2 A) WERE FURTHER REFINED, USING THE FULL CHARMM22 FORCE FIELD OF X-PLOR. AN APPROXIMATE SOLVENT ELECTROSTATIC SCREENING EFFECT WAS INTRODUCED BY USING A DISTANCE-DEPENDENT DIELECTRIC CONSTANT AND BY REDUCING THE ELECTRIC CHARGES OF THE FORMALLY CHARGED AMINO ACID SIDE CHAINS (ARG, LYS AND HIS) AND THE N-TERMINUS TO 20% OF THEIR NOMINAL CHARGES DEFINED IN THE CHARMM22 FORCE FIELD. AFTER 1500 STEPS OF CONJUGATE GRADIENT ENERGY MINIMIZATION, THE DYNAMIC WAS INITIATED AT 750 K, EQUILIBRATED FOR 0.5 PS WITH 1 FS INTEGRATION STEPS, THEN COUPLED TO A HEAT BATH AT 750 K AND THE MOLECULE WAS ALLOWED TO EVOLVE FOR 10 PS BEFORE BEING COOLED SLOWLY TO 300 K ON A PERIOD OF 5.4 PS AND ALLOWED TO EVOLVE AGAIN AT THIS TEMPERATURE FOR 15 PS. AT THE END, STRUCTURES WERE ENERGY MINIMIZED BY 1500 STEPS OF THE CONJUGATE GRADIENT ALGORITHM. THE FORCE CONSTANT USED FOR THE NOE POTENTIAL IN BOTH STEPS WAS 50 KCAL MOL-1 A-2. |
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