Table 1.

Rates and activation energies for individual steps of PEG-mediated vesicle and biomembrane fusion

Process	Rate constant, s−1		Activation energy, kcal/mol
Formation of I1 in vesicle fusion	2.43 (LM)	2.16 (PT)	37 (LM)	35 (PT)
Conversion of I1 to I2 in vesicle fusion	0.044 (LM)	0.031 (PT)	27 (LM)	33 (PT)
Formation of FP in vesicle fusion	0.020* (LM)	0.017* (PT)	21 (LM)	22 (PT)

Lipid mixing in viral fusion†	1.77‡ (pH 6.05) 4.21 (pH  5.87) 20.6‡ (pH 5.59)		42
Lipid flip-flop§	≈3.53 × 10−5		24
Pore opening in secretory fusion¶	≈0.010*		23

The rate constants listed for PEG-mediated SUV fusion were determined at 35°C. Rates for biomembrane fusion and flip-flop were interpolated from published data to estimate rates at 35°C. 

^*Fraction of total pores formed per second. 

^†Fusion between vesicular stomatitis virus and erythrocyte ghost (32). 

^‡Rates calculated from values reported for 37°C (2.75 and 32.1 s⁻¹) using the activation energy measured at pH 5.85. 

^§For dimyristoylphosphatidylcholine SUV’s in the fluid phase (26). 

^¶Fusion between secretory granules and mouse mast cell plasma membrane (10).