Trying to identify a very old story that involved human colonists who land on a distant earth-like planet and find human survivors of a much earlier colonizing attempt. The new colonists discover that while their own enzymes are based on common L-isomers, the survivors have evolved and mutated so that their enzymes are R-isomers (L- and R- denote opposite chiralities). It turns out that the food brought by the new colonists cannot sustain the survivors and the planet's natural food supply cannot sustain the new colonists.
In A Small Colonial War (1989) by Robert Frezza, the protagonists are soldiers trying to keep the peace between the second and third wave of colonists on such a planet and meddling corporations. The first wave of colonists has mostly died by the time of the novel. The intelligence officer explained how successive waves of colonists were better prepared to deal with the alien biochemistry.