Mathematical molecules

AP de Silva

In Issue 4 we reported on an amazing molecule that could make Boolean logic decisions on its chemical inputs and produce an 'on-off' light signal depending on whether or not the logic was being satisfied. Now, Prasanna 'AP' de Silva and Nathan McClenaghan at Queen's University Belfast have taken related molecular logic gates on the first step towards a true molecular computer by getting them to do mathematical addition.

	Calcium and hydrogen ions - the sum of their parts

Logic gates are the decision-makers on a computer chip. The AND gate, for example, is 'on', representing a binary 1 when its inputs are both 1s. The OR gate is only on if either one of its inputs is a '1', while the eXclusive OR gate 'XOR' acts as a 'spot the difference' unit being on only if both inputs are different.

The QUB team has now linked their AND molecule - a calcium-trapping chelate linked to a chromophore - to a related molecule that acts as an XOR.

Molecular truth table allows molecules to compute

The link between the two means they can add two numbers together. For instance, when you add binary 1 and 1 (calcium and hydrogen ions both present), the AND gate gives an output of 1. This, explains de Silva, acts as the 'carry over' digit, of kindergarten addition. The XOR output, on the other hand, with the same inputs, is 0. So, we have '0 carry 1' which is 1-0 in binary, in other words '2' So the molecule has answered 1 + 1 = 2! If input is 1 and 0 the answer is '0-1' or 1 and if they're both 0, the answer is 0-0, as one would hope.

The first applications of the QUB chemical calculators will be in biotechnology and combinatorial chemistry, says de Silva, where small volumes are common and these units can act as logical labels for arrays of molecules. Ultimately, though, we will one day see a computer packing molecules instead of silicon chips.

A. P. de Silva et al, J. Am. Chem. Soc., ASAP Paper^*

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