A pH buffer that controls acidity regardless of temperature should help the countless scientists, from chemists to biophysicists, who store and study fragile samples at cryogenic temperatures.

Yi Lu and colleagues at the University of Illinois, Urbana-Champaign, US, have designed the first buffer system that maintains the pH independently of temperature. Buffers are used to keep a sample at a particular pH, important for studying sensitive molecules, such as enzymes, that would decompose under acidic or alkaline conditions. But buffered solutions usually change pH at low temperatures, potentially damaging the sample as it is cooled to be studied.

Lu developed his buffer system simply by combining, in the right ratio, one buffer that increases in pH on freezing with one that decreases. The two common buffers, HEPES and potassium phosphate, form pH 7 solutions at room temperature but when cooled below -100°C reach pH 8.5 and pH 6.5, respectively. Using pH-indicator dyes, Lu's team showed that a 60 to 40 mixture of the two, which they named a temperature independent pH (TIP) buffer, maintains pH 7 at temperatures as low as -180°C.

Indicator dyes show that the temperature-independent buffer (centre) stays at a steady pH 7 as it cools

Matthew Lloyd, who studies enzymes at the University of Bath, UK, said: 'It has been known for a long time that buffer pH varies with temperature, but it isn't always appreciated that these changes are buffer-dependent and can be relatively large. This work provides a simple but elegant solution to the problem. Experiments using variable temperature are often used in structure-function studies of enzymes, proteins and other molecules, so the number of potential applications of the study is large.'

'We'll next apply the method to make TIP buffers at other pH values,' said Lu. 'We then plan to make the method widely available, perhaps through a website, so that if anyone wants a TIP buffer at a particular pH they can follow our recipes.'

'The biggest challenge is the theoretical understanding of combined buffers at different temperatures, to be able to calculate what buffer components are needed to prepare TIP buffers at any desired pH,' added Lu.

James Mitchell Crow