What is the ideal concentration of MGCI2 in a PCR reaction?

For many uses, a concentration of 1.5-4.5 mM MgCl2 typically yields optimal results in PCR. However, higher concentrations for certain reactions (up to 7 mM) may be necessary. This additional MgCl2 can enhance the reaction in two main ways. One way is by providing more magnesium cofactor for the enzyme. The second way is by the attachment of magnesium ions to the backbone of your oligos which reduces their negative charge. This typically results in higher polymerase activity and improved primer annealing efficiency.