FWIW Here is a pretty good (IMO) explanation on what is happening to form the "ring".
Nitrous oxide (NO), a gas, is being produced by the combustion of the wood. Nitrous oxide is a relatively unstable molecule and in the presence of heat and oxygen is oxidized to nitrous dioxide (NO₂) As the NO₂ passes over the moist meat, it comes in contact with water forming nitric acid (HN03). The acid dissolves on the moist meat surface and forms a nitrate ion (N03⁺). This nitrate then combines with the myoglobin (see above) forming a pink compound. This myoglobin – nitrate pigment is responsible for the “smoke ring” found in barbequed meats. The smoke ring is generally only ¼ to 3/8 of an inch think. The depth of the smoke ring is limited by the ability of the nitrate ion to penetrate the meat and by the temperature of the meat. Myoglobin proteins begin to denature (break apart) at about 120°F. Denatured myoglobin is no longer able to combine with the nitrate to for the pink pigment. It is important to remember that the formation of the smoke ring occurs early in the barbeque process and its maximum development requires plenty of oxygen (air) at the fire, moisture on the meat and relatively cool meat surface temperatures.
KCBS, NEBS Member