Good's buffers (also Good buffers) are twenty buffering agents for biochemical and biological research selected and described by Norman Good and colleagues during 1966–1980. Most of the buffers were new zwitterionic compounds prepared and tested by Good and coworkers for the first time, some (MES, ADA, BES, Bicine) were known compounds previously overlooked by biologists. Before Good's work, few hydrogen ion buffers between pH 6 and 8 had been accessible to biologists, and very inappropriate, toxic, reactive and inefficient buffers had often been used. Many Good's buffers became and remain crucial tools in modern biological laboratories.
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Selection criteria
Good sought to identify buffering compounds which met several criteria likely to be of value in biological research.
- pKa. Because most biological reactions take place near-neutral pH between 6 and 8, ideal buffers would have pKa values in this region to provide maximum buffering capacity there.
- Solubility. For ease in handling and because biological systems are in aqueous systems, good solubility in water was required. Low solubility in nonpolar solvents (fats, oils, and organic solvents) was also considered beneficial, as this would tend to prevent the buffer compound from accumulating in nonpolar compartments in biological systems: cell membranes and other cell compartments.
- Membrane impermeability. Ideally, a buffer will not readily pass through cell membranes, this will also reduce the accumulation of buffer compound within cells.
- Minimal salt effects. Highly ionic buffers may cause problems or complications in some biological systems.
- Influences on dissociation. There should be a minimum influence of buffer concentration, temperature, and ionic composition of the medium on the dissociation of the buffer.
- Well-behaved cation interactions. If the buffers form complexes with cationic ligands, the complexes formed should remain soluble. Ideally, at least some of the buffering compounds will not form complexes.
- Stability. The buffers should be chemically stable, resisting enzymatic and non-enzymatic degradation.
- Biochemical inertness. The buffers should not influence or participate in any biochemical reactions.
- Optical absorbance. Buffers should not absorb visible or ultraviolet light at wavelengths longer than 230 nm so as not to interfere with commonly used spectrophotometric assays.
- Ease of preparation. Buffers should be easily prepared and purified from inexpensive materials.
List of Good's buffers
The Good's buffers are presented in the table below. The table presents “apparent” pKas measured by Good's group at 20oC and concentrations of about 100mM. The apparent pKa is equal to the buffer's pH when the concentrations of two buffering species are equal, and the buffer solution has the maximum buffering capacity. The apparent pKa, and therefore the pH, of any buffer are temperature and concentration dependent. Consequently, the pH of prepared concentrated buffer solutions will change on dilution and with temperature. The apparent pKa at various concentrations and temperatures may be predicted using online calculators.
Different Good's buffers fulfill the selection criteria to various degrees. No one of the buffers is truly and completely inert in biological systems. In Good's own words, "it may be that the quest for universal biological inertness is futile." Only three of Good's buffers do not form noticeable complexes with any metals: MES, MOPS and PIPES. Piperazine-containing buffers (PIPES, HEPES, POPSO and EPPS) can form radicals and should be avoided in studies of redox processes in biochemistry.
Tricine is photo-oxidised by flavins, and therefore reduces the activity of flavone enzymes at daylight. Free acids of ADA, POPSO and PIPES are poorly soluble in water, but they are very soluble as monosodium salts. ADA absorbs UV light below 260 nm, and ACES absorbs it at 230 nm and below.
Over the years, pKas and other thermodynamic values of many Good's buffers have been thoroughly investigated and re-evaluated. In general, Norman Good and his co-workers attracted attention of the scientific community to the possibility and benefits of using zwitterionic buffers in biological research. Since then, other zwitterionic compounds, such as AMPSO, CABS, CHES, CAPS, CAPSO, etc., were evaluated and accepted as biological buffers. Not surprisingly, while these buffers were not directly introduced by Good's research group, they are often referred to as "Good's buffers."